Remember last week, when we were talking about glass houses? Well, here’s another Title 24 case study on a 4,500 SF house, also from Swatt|Miers Architects. This house had almost 60% glazing to floor area, much of it custom built on site: 564 square feet of single paned butt glazed corner windows, 540 square feet of frameless glazing, a steel framed window, a 30 foot tall translucent window in a stair tower, 300 square feet of skylights, and a custom built wood screen interspersed with glass panels. That’s almost 2,700 square feet of glass.

And, to make the challenge that much more… piquant… it was in California climate zone 2 (Sonoma – HOT)… AND, they needed to beat California’s Title 24 energy standard by 15% because of local ordinances. It was the combination of all that single glazed area with the climate zone that concerned us the most. But, we had a reputation to maintain, and our motto to designers was, “We’ll never tell you that you have to shrink your windows.”

(Above image courtesy Swatt|Miers Architects.)

Well, we did end up telling them that they would have to find a way to make the butt glazing work with double panes (which we described in our article on window performance). So I suppose now we have broken our cardinal rule of never impacting the visible design.

Complex Shapes

Although the design was entirely based on rectangular planes, the volumes didn’t always line up one stacked directly over the other. This meant that there were some extra floor and roof areas to account for, and there were some subtle variations in building height, too. Most of the time, this can be generalized, but in this case we wanted to be as exact with every surface area as we could, so that we could claim the maximum thermal mass credit. I knew the planners might be reviewing the report against the drawings with a fine-toothed comb, and we needed to be prepared to respond to any comments with a solid grasp of facts.

Wood framed overhanging floor areas were modeled separately from the slab flooring on the main level.

Start at the Beginning, Grasshopper

As we mentioned in the previous case study, we try to start at a basic level with whatever systems information we have from the designer, and then work up from there. The main heating system was radiant, with A/C. On the plus side, the design called for slab flooring, with gypcrete on the upper level – this thermal mass gave us a ray of hope. Even so, the first trial was dismal. 73% below the standard. Heating was missing by 75% and cooling was missing by a whopping -136%. Only pride kept me from throwing in the towel – pride and curiosity.

“Patience, Grasshopper,” I said to myself. “Just do what you usually do, and don’t say anything until you have some good news to report.” So, here’s what we did, and what worked the best.

Cool Roof/Radiant Barrier. Although this seems like a minor place to start, we’d have to try a cool roof at some point, and it might actually help in Sonoma. The cool roof did make a difference (down to -67%), although the cooling improvements were offset by a small detriment on the heating side. What we really needed was a selectively cool roof, that changes color based on outside temperature – maybe someday soon there will be such a thing. The radiant barrier helped less, and they would have had to change the roof construction to include it. The gains from the barrier didn’t justify including it – unless we absolutely had to.

Wall insulation. Next, we upped the wall insulation from the requisite R13 to R21. That pushed us from -67% to -55%. Pretty good, but still way behind. The designer had thoughtfully provided us with the wall assemblies, so we knew that the cavities were 2 x 6 – large enough to fit R21.

The designer provided us with complete details, which helped us to ascertain how much insulation we could specify in the energy model. Image courtesy Swatt|Miers Architects.

Ducts. There would be both heating and cooling ducts in this project. Although the main heating system was radiant, there would be a forced air backup. We couldn’t model both systems, but we had to keep the heating ducts in the energy model, which cost us. We did verify with the designer that the ducts would be located within conditioned space, which gave a credit. And, we added the HERS test for duct leakage, which brought us from -55% to -40%. (Actually, we tried eliminating the heating ducts and it didn’t help as much as it had on other projects.)

Blower Door Test. With heating and cooling ducts within conditioned space and the duct test taking us to -40%, we added another HERS test – the blower door test, which measures the airtightness of the entire home. Doing these HERS tests on a sizeable house such as this was bound to be challenging, so we stressed to the designer that they, and their builder, should read our article on HERS tests so that they knew what was involved. The builder in particular would need to know that the project was required to pass these tests. The blower door took us from -40% to -37%, not that much. Well, we’d keep it in for now, since it was looking like we’d need every last inch of compliance.

A/C Verifications. Next, we tried adding the HERS tests that apply to air conditioning systems: test for refrigerant charge, airflow, fan watt draw. These took us from -37% to -32%. I had hoped for more. We tried upping the A/C SEER from the standard 13 up to 18 SEER, which, together with the HERS tests, brought us to -29.5%. One thing to note is that often, the HERS tests have more of an impact on compliance than simply upping the SEER. But, all of this was simply postponing the day of reckoning, which was to attack the windows.

Window Performance

As with the other Swatt|Miers case study, we divided up all the window areas by type: Butt glazed corners, frameless wall insets, the stair tower, the custom steel window, various sliding pocket doors, operable casements, the 40 foot long wood screen window on the upper gallery, and the skylights. The design called for various overhangs, including a large canopy extending over the main house and a separate guest house. Most of the window framing was metal, which is not as good an insulator as wood.

Initially, I used the performance specs from Efficient Windows as a starting point for estimating all the custom areas, assuming that all windows with the exception of the corner butt glazing would be double paned, low e glass. There was a lot of back and forth with the designer to establish the composition and construction of the various custom windows. We couldn’t go any better than the standard on most of these (we were lucky to get the standard). The casements and the sliding pocket doors were Fleetwood window products, with numbers that we could look up.

I asked around and searched for information on whether any sort of single glazed window could ever be “high performing”. Alas, there was no magic glass. The experts all informed me that the main factor in window performance is 1) multiple panes with insulating layers of air or gas fill 2) airtightness of the frame itself and 3) insulating properties of the framing material. There was no such thing as a thermally  broken, metal framed window with single glazing, because why? There wouldn’t be any demand for it.

But we were still at -29%. Something had to give. So, I broke our rule and made all those butt glazed corner windows double glazed. That took us to -11%. And then, I modeled the Fleetwood windows using the best numbers they had available for each type. That took us to -6%. At the same time, I put out the word to see if anyone had successfully built a corner glazed window with double paned glass, because I knew that the designer really wanted to keep that transparent appearance, and putting a spacer bar on the corners would be a major disappointment, to say the least.

And it still wasn’t enough. Even adding the dreaded QII test (a HERS test where every bit of insulation is inspected as it’s installed during construction) wasn’t enough, although the QII did bring us from -11% to -0.8%. So close – and yet not close enough, considering that we had another 15% to go.

Interior Mass Surfaces

At this point I dragged Mark English over and made him review the entire drawing set plus all the details. As an experienced architect who’s been designing and building homes for 25 years, I figured he’d see a few things that I had missed, and he would make sure we didn’t take too many liberties with the wall and roof cavities.

Based on consultations with Mark and numerous exchanges with the designer to verify the exact location of every wall and floor finish, we added the thermal mass of a dramatic 2-story stone veneer wall, over 1475 SF of thermal mass. Additionally we included all the tile flooring in the bathrooms, countertops, and the gypcrete from the second floor’s suspended slab floor. Even though this floor was largely covered by wood or carpet, it still yielded some credit. That took us from -0.8% to +12%.

Upon our request, the designer provided us with the location of vertical thermal mass surfaces - stone veneer walls - which we could then include in the energy model. Image courtesy Swatt|Miers Architects.

And it still wasn’t quite enough. I felt like a magician reaching into a hat for another rabbit and coming up with a hamster instead.

What the Designer Said

I figured it was time to fill in the designer with our progress to date, and test the waters about making the butt glazed windows double paned. It might be a good time to insist on an uber-efficient water heater. We’d actually started with a reasonably efficient one, a .80 energy factor, but without further information, I was hesitant to commit to anything extreme. Eventually we would have to include the actual models they were using, and there would likely be more than one with a house that size anyway. I don’t think they had worked out the mechanical systems to that detail, so now was a great time to test and suggest a few things.

But before we reached into our top hat for that last rabbit (the water heater itself), we tried a few more things just to see what would happen.

Solar water heating credit. The design hadn’t specified solar equipment of any kind. Well so what? Maybe it would let them keep that single glazing, although I doubted that.  Even though there’s no credit for the use of renewable energy for electricity or heat, there is Title 24 credit for solar hot water. It’s based on the percentage of hot water that the home is expected to get from solar, and sure enough, set this percentage high enough and the compliance score improved.  So, by pushing this number to an unrealistically high 50% we were able to inch our compliance from +12% over to 17% over, although I doubted that this would actually work.

And why not? Well, the problem with it is that you still need some kind of indirect storage tank to ensure hot water in the evening, unless you only plan to shower at high noon. In addition, this home would have extra water heating demand because of the radiant heating. But hey… we reached our goal, in theory at least.

Higher Solar Heat Gain on Some Windows

On the last case study, I had, purely out of curiosity, tested a series of window performance combinations just to see what would happen. Although a low U value window was always the best choice, because it provided thermal insulation for both hot and cold temperatures, what could we do if the best U value we could find was average – if that? Since we had so many custom windows that would be built in the field, we couldn’t make aggressive assumptions about them.

So, I inched up just the solar heat gain on the Fleetwood windows, while keeping the U value the same. This would allow at least some glazing areas to keep up their insulating value, while allowing a little more solar heat. Although originally the design had lagged more on cooling, it was now the heating side that had all the shortfall. We were actually ahead on cooling. This latest change brought us from 17% over to 18% over, so it didn’t make a huge difference. Heating was better, cooling lost a bit.

Skylights

The skylights were another big unknown. There were a lot of them, and the designer indicated they didn’t want wood framed because they were concerned about leaks and such. Then they selected a manufacturer who actually had pretty good numbers, which brought us from +18% to +21% over.

Highest Efficiency Boiler

It was time to pull out our very last rabbit, which was to boost the water heater performance as far as it would go. By specifying a boiler with a 95% efficiency, and keeping a solar hot water credit at 25% (still probably too aggressive), we got the house to exceed Title 24 by 32%. At this point, we dialed back the Fleetwood windows to the actual numbers (Westwood and Norwood product lines, dual glazed low E, thermally broken, but no argon – air fill), and removed the solar hot water credit altogether. We kept the HERS testing for credits: the three A/C tests and the duct test.

The final score? It came out as exceeding Title 24 by 25%. This would give them a little margin if by some chance it didn’t pass every HERS test. Some of those tests were worth GreenPoints, too. So, chances are this house will earn a respectable GreenPoint Rated score as well once construction is complete.

A few final notes follow, on what the designer did to help us, and a note on modeling multiple zones.

Detailed Wall and Floor Assemblies

This was one of the few projects where the designer provided us with detailed wall and floor assemblies, 3 drawing sheets of it. This was great, because we could see exactly where the gypcrete was, which portions of the floor were covered (and with what) and which walls had interior stone veneers.

Although we don't always have to model every layer in the floor, in some cases it helps to know exactly what's in there. Image courtesy Swatt|Miers Architects.

Modeling Multiple Heating and Cooling Zones

This was something that we would have resorted to only at gunpoint, because it’s very time-consuming. Essentially, you model each zone as a separate volume – including floors, walls, and ceilings – as if it were its own little house. Title 24 gives credit for this, but I’ve only ever done it once on a private home, and we had no way of knowing how much it would help unless we tried it.

Actually the real question is whether an all-glass pavilion can still comply with the new version of California’s Title 24 energy code. Although Title 24 has been around since the 1970s, it is only now that designers are feeling the pinch. Given the increasing strictness of the energy code, what can an architect do if he (or she) wants to create designs with dramatic glass curtain walls?

The “glass house” shown on the cover image is, of course, Philip Johnson’s famous Modernist masterpiece, also called the Glass House. Even that house could, with the right high-performing window system, comply with Title 24 requirements – I tested it out. But, let’s talk about some more current designs for our case study.

When we got our first Title 24 project from Swatt|Miers Architects, I didn’t really know if we’d be able to finesse it. We had never had a project quite like it before. But then I remember seeing an award-winning Net Zero home in Truro, Massachusetts (by Zero Energy Design) which had acres of glass everywhere. If they could make it work in a cold place like Massachusetts, surely we could do the same in California!

This Net Zero home in Truro, Massachusetts, was designed by Zero Energy Design. And yet, this house still manages to be efficient even with all this glass. Photos: Eric Roth.

Our first Title 24 project from Swatt|Miers Architects was a 6,000 SF all-glass pavilion with a generous roof,  a custom window wall on one side, and heated slab floors. On the plus side, the roof included large shading overhangs, and we had ample time and flexibility to select high-efficiency heating and water heating systems. There were no large skylights or ductwork to worry about, either. The house was located in a moderate climate zone, so they weren’t even going to install any cooling systems at all.

Another big plus was they didn’t have any “beyond compliance” goals such as GreenPoints. So, we didn’t have to beat the standard by 15% or more. As you’ll see in a minute, that would have been possible, but very expensive.

On the challenge side, there was almost 5,000 square feet of glazing – 75% of the conditioned floor area. In addition, the design called for metal framed windows, which are inherently less efficient than windows framed in wood or vinyl. Even thermally broken metal windows can’t always match the performance of wood.

This house had 75% glazing to floor area and yet, it still managed to comply with California's Title 24 energy requirements. Design Courtesy Swatt|Miers Architects.

Strategy: Start Low, Increment Up

We usually start by running each project with baseline assumptions, usually the same ones used for the prescriptive Title 24 method. The Title 24 prescriptive method sets forth minimum requirements for things like heating and cooling performance, insulation levels, maximum allowable glazing areas, and window performance – but it’s all or nothing. Either you meet every requirement, or you have to use the performance modeling method to satisfy Title 24. The software model has some built-in generic inputs for things like water heaters that also assumes a very basic level of compliance.

Using the software modeling method of Title 24 energy compliance, a house can fall below the standard in one area as long as it makes up for that shortfall in another area.

In this case, the first trial missed by -76%, with shortfalls on heating and water heating. Surprisingly, cooling was not a problem – at least, not in the model. Still -76% seems like a big, scary number – one which we attacked incrementally.

Preliminary Trials with Insulation and Water Heating

Increasing insulation levels from R13 to R21 brought it to -67%. There wasn’t that much wall to work with, the walls being mostly glass, which is measured by window performance rather than insulation. The roof already had plenty of insulation, and adding more past a certain point had a negligible impact.

Just as a test, adding an overhang over the South window wall (which didn’t have one) actually made it worse! That’s one thing about trade-offs. Sometimes a measure that helps with cooling will create more load on the heating side, and heating was what we needed to fix. But this design didn’t specify an overhang there, and even when there are overhangs, modeling them is optional.

Next, we tried upping the water heater performance. Originally we’d specified a generic setting, because the mech hadn’t been worked out yet. For a house this size, over 6,000 SF, you probably wouldn’t have a single heater anyway, and you definitely wouldn’t be running all the hot water AND the radiant through one heater. So we had started with a 75 gallon storage tank with a .75 energy factor – legacy numbers.

A tankless heater with a storage tank, with 2 separate systems, and a higher energy factor, got us down to only -49% below Title 24 requirements.

Main Trials: Windows

But all this was simply prep, because the windows were going to be the biggest challenge. The window performance factors that are important for Title 24 are the U value, which measures thermal performance (keeps winter heat in), and the Solar Heat Gain Coefficient (keeps summer sun’s heat out). We had all been thinking that with all that glass, cooling would be the concern, and we’d have to get the lowest SHGC we could find. Not so. It was the U factor on all those windows that would make or break our compliance.

In fact, having a higher SHGC might help because we could afford to lose some cooling margin if the solar heat gain would actually help on the heating side. Passive solar designs in far North latitudes try to leverage the sun’s heat, especially in the wintertime when there’s so much less of it. We don’t normally think that way in California, but occasionally a higher SHGC can actually help, especially on the East wall. The reasoning is that solar energy on the east, when the sun is just rising, can help the house to recover from cooler nighttime temperatures.

Typical test results from Fleetwood Windows, available online, show just how many options there can be even within one product line.

The Title 24 prescriptive minimum for window performance is .40 U/.35 SHGC, lower being better for both these numbers. That’s pretty tough, considering that most ordinary metal framed windows run around .50 or .60 U factor, even for double glazed, “Low E” glass. We had started with the generic “metal double glazed low e” input which is around .65/.40. This project specified Fleetwood windows, which has several product lines with a wide array of options for glazing, thermal breaks, gas fills, and even glass types. All of their test results are easy to find, too.

We also modeled each window opening separately (over 77 of them), so that we could use different performance numbers for each window type: there were Casements, Awnings, Fixed, Sliding, and a couple of custom settings. That way if a better performance could be had from the fixed windows but not from the operable ones, we could include it. Sometimes clients have also asked us to use different settings for different cardinal directions, for example, using a more expensive but spectrally selective glass on a western or southern wall where it might matter the most.

For the Swatt|Miers design, Green Compliance Plus modeled over 70 different glazing areas in order to account for small performance variations among window types.

U Factor or SHGC – Which Is More Important?

Although we were more concerned with the U factor, it was possible that testing different combinations of U and SHGC factors might yield some interesting information. So, we spent a lot of time trying very low U/low SHGC combinations. We also tried some average U with very high SHGC windows, as well as trials where we kept one number the same and varying the other one up and down to see what would happen.

A low U factor was the best choice, regardless of SHGC – but, this might not be achievable, even if they used the more expensive, thermally broken frames. Of course, triple glazing with argon gas fill would help a lot, too – I’m not sure what the quote was for 4,500 square feet of windows with triple glazing/argon/thermal breaks, but it would have killed the project budget. We had to find some way to make it work with double glazing.

A U value of .35 or under would be better overall. A U value of .25 would be awesome. If the U value was low enough, like .30, it didn’t matter as much what the SHGC was. In fact, with a U value of .25 some of the runs came out almost 40% over compliance! Some interesting window results: With a higher U value, a moderate solar heat gain acted to compensate somewhat for heating loss. It was almost as if there existed a second, smaller “peak” of outliers which, if I had a lot more time, I’d try to graph in more detail.

Of course, it’s all very well to say that we need a window with a U value under .30. The U value was the one thing that most limited us, because with the window products and framing materials selected, it simply wasn’t possible or cost-effective to get down that low. And, specifying an artificially high SHGC just to “make the numbers” didn’t make sense. But then again, neither did specifying triple glazed, argon filled windows in a mild California climate zone.

Custom Window Wall

The custom window wall on the South wall was another challenge. Fortunately, this area was wood framed – not aluminum like the rest of the windows. The wood framing would help with thermal performance, and the plans already called for double glazing with Low E glass. However, since there were no formal NFRC test results for it, what numbers could we safely assume for performance? As it turns out, we checked with the local Planning department for the project’s location, and they indicated that using the Title 24 prescriptive standard, .40 U/.35 SHGC, would be acceptable. This standard already assumes a wood framed, double glazed, low E glass window – so it made sense to use it for the custom window areas.

We might not be so lucky in another jurisdiction, though. The plan checkers have some leeway to use their own judgment and, if we’d been in another area, we could have been asked a lot more questions about materials, glass types, etc. – and possibly we might have had to use less forgiving numbers.

Additional Measures

In the actual project, we tried a few more things. One was a super-efficient water heater. Most water heaters are in the .65-.80 range for energy efficiency, but there are some out there that go almost to .95, so by creatively assuming that they would use the best systems available, we could at least present that as an option for compliance.

We called out each area of interior exposed thermal mass – exposed stone, tile, and concrete – because usually this helps with Title 24 calculations.

And, as a last resort, we could have called for some HERS verifications for extra compliance credit. Our article on HERS testing describes each of these tests in detail: duct blaster, blower door, QII, and various tests on the A/C system. Of course, not all these tests were available on this project. With radiant heating and no A/C, there were no ducts to test. And, the various A/C tests – refrigerant charge, fan watt draw, airflow, and verified EER – couldn’t be used, either.

Would Cooling Be an Issue in the Real World?

Initially we were surprised that cooling was not a problem, and we were all wondering how realistic that would be once the house was actually built and inhabited. Title 24 intentionally ignores some real-world conditions such as landforms and shade trees, but it does apply location-specific climate data on top of the broader “climate zone” designation. The designer expressed concern that with a high SHGC window, the house would be too warm during the summer, especially along the South side. And really, to us it didn’t make sense to use an artificially obtained number when the most straightforward thing to do would be to use California standard Low-E, low SHGC glazing.

Would Philip Johnson’s Glass House Meet Title 24 Energy Requirements?

Just for fun, I did a quick test of Philip Johnson’s Glass House, using floor plans and dimensions that I was able to find on Google… no guarantees on accuracy, but it seemed like it was worth a try. The trials included three California climate zones: Woodside (Zone 3), Tahoe (Zone 16) and Livermore (Zone 2) and a few compass orientations for 0, 45, and 90. For starters, I assumed double glazing and Low E glass, with the same numbers as a wood frame would be. And, for heating I assumed radiant heated slab flooring… which I think is the actual method.

Title 24 ignores shade trees, which in the case of Philip Johnson's Glass House are a major environmental feature.

The compass orientation didn’t matter that much – it mattered a little – but, the location had a bigger effect, mainly in the balance between the shortfalls between heating and cooling. No surprise, Livermore had the biggest cooling problem initially.

Setting the glazing to something more like the actual (metal framing, clear glazing) was disastrous for Title 24 compliance. However, when I changed the windows to the “best available” performance numbers – something around .20 U/.20 SHGC – the house complied in all three zones without any further changes.

I've never been inside the actual Glass House. I wonder how comfortable it really is in the winter? There are almost no wintertime images of it.

The Catch – Custom Built Windows Can’t Use NFRC Test Results

In reality, it may not be possible to even obtain pre-manufactured windows of the size that are used in Johnson’s Glass House. And, in order for performance numbers to be valid and acceptable for Title 24, the window units have to be NFRC rated. We’ve discussed this in a previous article, so I can’t assume that the Glass House design as it is now would ever be able to fully comply with Title 24. And, of course, the Title 24 modeling software only has climate data for California, not Connecticut where the house is actually located. Nonetheless, it could comply a lot more easily than some other projects we’ve worked on.

Another Glass House: Olson Kundig’s Glass Farmhouse

As I was looking around for any accounts of the actual comfort of Johnson’s Glass House, I found another, much more recent take on the all-glass house from Olson Kundig Architects, located in Oregon. Alas, no reports on actual energy use or comfort here, either – but Kundig did employ solar design features and high-performing glass and took some care to adapt the house to the local climate. You can see some nice images and a description at The Contemporist.

Many architects have yet to realize how much a window’s energy performance can impact their projects, especially under the new Title 24 energy code. Ordinary glass is great at letting in daylight but it’s a terrible insulator. It also does little to block the sun’s heat in the summertime. Windows lose heat through the glass, and they can also leak air around the edges of the frame.

Here are a few analogies to understand the different ways that windows can lose both heating and cooling energy. If you wear a big holey sweater in the wind, it doesn’t keep you very warm. That’s air leakage. Now, imagine just wearing a single sheet of clear plastic on a winter day. It’s a better windbreaker than that holey sweater, but you’ll still feel pretty darn cold. That’s because a thin sheet of plastic, like a single sheet of glass, is a poor insulator. And remember what happens to your car parked in the summer sun? It gets 20 degrees hotter than the outside, or more – if you have black vinyl seats, you’ll scream when you sit on them in your summer shorts. That’s solar heat gain.

When it comes to efficient windows, it's the entire window unit that matters, especially for air leakage. To understand the importance of air flow on energy needs, think of a sweater as your own personal "building envelope". Wool is a great insulator, but not if it's a holey sweater worn outdoors on a windy day.

Windows Lose More Energy Than Solid Walls

The more windows a house has, the more energy that house will require for both heating and cooling. And if those windows are on the South or West side of the house in direct sun, they can turn the home into a summertime toaster oven.

Windows can leak energy in several ways:

• Air leakage through and around the frame
• Heat transfer directly through the glass
• Thermal bridging through the frame, especially with metal frames
• Radiant heat from direct sunlight

Which building will be easier to heat in winter and cool in the summer, the saltbox on the left with modestly sized window openings, or the all-glass modern office building on the right?

What Kind of Windows Are More Energy-Efficient?

Title 24 requires that windows meet minimum performance standards. Most legacy windows don’t meet this standard, and a lot of fancy metal-framed windows don’t meet it, either, due to metal’s heat conductivity. There are metal windows that meet the Title 24 standard, if you’re willing to pay for triple-glazing, thermally broken frames, and argon gas fill. For a large house that’s got acres of glass, this is very expensive and it’s really overkill, too, in mild California. If you don’t want to break the bank, you’ll have to make it up with additional energy measures elsewhere.

By blocking infrared radiation, low-emissivity glass works both to keep the Sun's heat out, and indoor heat in.

There are many ways to make windows more energy-efficient:

• Special “low-e” glass that blocks solar heat radiation without blocking visible light
• Double or even triple paned construction with air layers sandwiched between the layers of glass
• Filling that air gap with argon gas instead of regular air
• Airtight frame construction
• Use of framing materials such as wood that are less conductive of heat
• Thermally broken frames to  further limit heat loss through the frame

Metal is a good conductor of heat, which is why metal cookware works so well. But that conductivity is the last thing you want in the walls of your house. Thermal bridging is another term for this type of unintended heat loss. To prevent it, some metal window frames include thermal breaks as shown on the right.

Windows framed in stone, poured concrete, or similar material might have similar properties to a masonry wall – but they’re less common and I’ve never had to model any.

Window Placement Can Make a Difference

In a moderate northern latitude, the sun's path through the sky summer and winter traces an arc across the southern portion of the horizon. This affects the amount of sunlight that reaches windows on east, south, and west facing walls of a building.

It’s probably not news that windows on the South and West walls let in more solar heat, but let’s consider windows on each cardinal direction. (Southern hemisphere folks will have the sunniest side on the North, but let’s stick with California for the moment.)

• East facing windows receive morning sun, at a low angle. Because the house is presumed to be still cool from the night, the solar heat gain from the eastern sun can actually be helpful in warming the house in the morning.

• South facing windows receive noonday sun, at a higher angle. How high exactly depends on your latitude and the time of year. The right size overhang can block sun in the summer, but admit it in the winter. Still, large areas of South facing glazing can be a liability for cooling.

• West facing windows admit the setting sun at a lower angle, but it’s after the heat of the day when you don’t want more solar heat gain. Large areas of West facing glazing can be a cooling problem – and, shading overhangs won’t help.

• North facing windows will never experience direct sun in California. Overhangs don’t do much for shading, although they’ll still keep the rain off. However, heat loss through the glass can be more pronounced on North facing windows.

Clockwise from upper left: In the morning, the sun is lower on the eastern sky and shines into an east facing window at a low angle. By noon, the sun is higher overhead in the south (although still at an angle). The setting sun in the west again shines through any west facing window at a low angle. North facing windows will never see direct sunlight.

Overhangs and Window Shape

Overhangs help with shading on the South, but a few other design features are important too: overhead clearance and whether the window has a more horizontal or more vertical shape.

Left: When shading a tall skinny window, much of the window area is still exposed. Middle: This horizontal wall of glass is situated immediately beneath a long overhang to shade a greater proportion of the glazing area. Design by Wendell Burnette Architects. Bottom: In this recent design from Swatt|Miers Architects, the horizontal windows, deep overhangs, and reduced amount of South facing windows all helped with Title 24 energy compliance.

Measuring Window Performance

There are two, and only two measures that matter for Title 24 energy compliance: the U value and the Solar Heat Gain Coefficient (SHGC). It’s important to understand the difference between the two.

• The U value helps with heating AND cooling.
• The Solar Heat Gain Coefficient only helps with cooling.

It’s true that I often ask people whether their windows are double glazed, single glazed, and whether the frame is wood or metal. That’s just when I need to come up with generic numbers for the U/SHGC. When it comes to Title 24, these two numbers are really all that matter, and the window products have to be officially tested and rated by the National Fenestration Rating Council (NFRC). Title 24 now requires that the windows be tagged, and that this NFRC rating be etched directly onto the glass.

(There are other important window performance measures, such as visible light transmittance and also the glass’ ability to block ultraviolet rays, but those aren’t considered in Title 24 energy performance.)

U Value

U value measures rate of heat transfer through the window. Lower is better. Both the framing material and the type of glass have a big impact. A window with a U value of .40 is whole a lot better at keeping the heat in on a cold day than a window with a U value of .75. Title 24 expects a maximum U value of .40, although you can find windows with a U value of under .20. Just for comparison, an ordinary window with two panes of clear glass might have a U value of .65.

Solar Heat Gain

Solar Heat Gain Coefficient (SHGC) is about how hot the interior of the house will get on a sunny day when that window is hit by direct sunlight. The hotter it gets, the more A/C you’ll be using. Well, actually it measures the amount of solar heat coming through the window. Think infrared heat lamp. In California, lower is usually, but not always, better. Title 24 usually expects a maximum SHGC of .35.

In far Northern latitudes you might want a higher solar heat gain because you’ll want to capture more of the sun’s heat in a passive solar design – but you’d still want a low U value.

Title 24 Window Performance Standard

The Title 24 baseline standard of .40 U/.35 SHGC assumes that you’re using a wood or vinyl framed, double paned, airtight window unit with specially coated “low-emissivity” glass. It’s generally better at keeping heat in, and it also filters out some of the sun’s more heat-inducing rays. Because sunlight has a range of frequencies visible to the human eye, some frequencies of visible light transmit less heat than others. So we can now obtain spectrally selective glass that looks relatively clear, but filters out a lot of the hotter rays.

Here's why metal framed windows are a problem. Even the better products don't always meet Title 24 minimum standards.

Single, Double, Triple Glazing

We occasionally get questions about whether there’s such a thing as a high performing single glazed window, and the answer is NO. There may be some variation in the thickness of the glass panes, but that alone won’t help much, nor will having a fancier glass type help all that much either. It’s the layer of air between panes of glass that does most of the work, and if you take away this layer, you take away 90% of that window’s insulating ability.

Remember that holey sweater? Well, assuming that you’re indoors out of a draft, that sweater might do a better job of trapping a layer of air next to your skin which warms to your body temperature and helps you feel warmer in a cool room. As long as nothing strips that air away, you’ll feel warmer.

Adding layer of air helps a window's heat insulating ability. Even more so if that layer is a gas fill like argon rather than regular air. For extreme climates, triple glazing may be needed.

Getting NFRC Test Results from Window Manufacturers

What we see in our Title 24 work comes from other small design firms doing custom residential design – firms like us. The architects who come to us for Title 24 consulting care a lot more than average about aesthetics, about design integrity, and about pleasing individual client tastes.

Top: This French style home designed by Taylor Lombardo Architecture used wood frame, double glazed, low e windows to achieve a traditional look. Bottom: This Modern design from Swatt|Miers Architects specified Fleetwood aluminum framed windows for most of the glazing. The grouping in the center of the illustration is actually a custom-built window wall. Green Compliance Plus provided the Title 24 energy compliance reports for both these projects.

Some window manufacturers are really good about posting their numbers, others are not. If you’re a design-oriented architect and prefer certain styles, you might have to get used to looking at the numbers first, and the appearance second. We’d hate to have to tell you to abandon some of your favorites, but if we can’t find the numbers, and the window manufacturers can’t tell us, we have to assume the worst when doing our Title 24 analysis.

In terms of window manufacturers for our various Title 24 projects, the names we see the most are for Milgard, Loewen (wood or vinyl); Fleetwood, Bonelli, Milgard (metal or metal clad); and Royalite or Velux (skylights) – but there are many more quality makers out there.

Sometimes we don’t see products by name, we just tell the designer what numbers they have to meet or beat. For those who really want metal, Fleetwood has a wide range of styles, including sliding doors, casements, awnings, fixed – as well as energy saving levels.

Here's an example of NFRC ratings for one window manufacturer, Fleetwood. They're one of the few metal frame window products offering triple glazed, argon filled, thermally broken frames.

Ultra High Performing Windows

Most of our Title 24 clients aren’t extremists in this regard, although they all want to get the best window they can within their budgets. The most efficient windows available are typically used either in Passive House certified designs, or in climates with more extreme cold – and sometimes those areas require a different formula with a higher solar heat gain – especially passive solar designs in extreme northern or southern latitudes.

Sorpetaler, Pazen (sold in California through Quantum Builders) or local manufacturer Serious Windows in the South Bay, seem to be the best. And they’re not cheap. The Sorpetaler windows that I saw at Quantum’s offices are as hermetically sealed as an airlock on a spaceship. They’ve got triple glazing, low-e glass, frames with thermal breaks, super low air leakage, argon gas fill – the works. Some of them perform almost as well as a wall in terms of limiting heat loss.

I’ve also heard a few people say that such extremes are wasted in the Bay Area where it’s so temperate that you’re better off investing in other areas rather than windows. Be that as it may, the window’s rated performance does make a huge difference in Title 24 energy compliance – deserved or not.

High-efficiency windows can be executed in a variety of styles as well as custom-built units. Clockwise from left are a modern embedded frame style from Pazen, a double hung unit from Serious Windows, and two traditional examples from Sorpetaler. Pazen and Sorpetaler are German manufacturers, while Serious Windows is located in Mountain View, CA.

NFRC Rating

Well what’s to prevent window manufacturers from making outrageous claims about their products? Regulations and a lot of cumbersome and expensive testing, that’s what. To ensure that people don’t fudge their own test results, window manufacturers are required to send all their stuff to a lab accredited by the National Fenestration Rating Council for physical testing. At the lab, they take the window units and test them, over and over, to measure how much heat they lose, how much of the sun’s heat they let through, and how much air leakage occurs. This process takes up to a year and costs $30,000.

The NFRC rating must be posted on each window unit at the construction site. In addition, California energy code requires that this same rating be etched right onto the glass.

To be considered 100% kosher, these ratings are for the entire manufactured window unit, frame and all. It’s not just the glass. There is something called a “center of glass” measurement that can be used for site-fabricated units such as curtain walls, but officials that we’ve checked with at various local Building Departments have all indicated to us that center-of-glass performance results are not really acceptable for Title 24.

Custom Field Built Windows, Frameless Butt Glazing, and Energy Efficiency

This is a thorny problem because field-built windows don’t have NFRC ratings.

You can forget about single glazed butt joined corner windows, unless they’re small. We recently had a couple of very ambitious glass wall designs, and to our astonishment the one with 67% glass to floor area did OK – eventually – that was because it was all double glazed. Even so, we originally thought it’d need to use triple glazing. Our design client almost had a heart attack. Way too expensive. Eventually we did make it work with double, but they had to make up for it with a lot of HERS tests.

Then we got another design from the same architect that called for miles of single glazed frameless windows including a lot of corner glazing. All single glazed. I thought, “Oh no, we’ll finally have to break our commitment to preserving design intent and tell them we can’t make it work unless they make the windows smaller! But we’ve promised to never, ever do that! They’ll never call us again.” Even there, by counting every possible square inch of interior thermal mass, and pushing them to go to double glazing even for the butt joints, we were able to get it to pass – by 15%, which was a requirement for that particular jurisdiction.

Getting the look of a seamless corner window with double glazing is a challenge, but it's possible. Here's one window detail provided by Swatt|Miers Architects.

Glass Technology

We recently had a question from someone asking what difference the different types of glass such as SolarBan or Cardinal might make. The short version is: refer to the NFRC ratings rather than asking us to evaluate the glass, and bear in mind that some options may cause the glass to appear different. SolarBan 70, which is an option on a lot of Fleetwood window products, offers better performance overall than SolarBan 60 but we had one designer tell us that she didn’t like the look of the 70 – “too shiny”.

For the low-e glass itself, most of the enhanced energy performance is the various coatings on the glass. The good news though is that new products are always coming out with better and more precise performance.

Further Reading

If your thirst for window knowledge still remains unslaked, here’s a good dissertation on windows from the Whole Building Design Guide, a program of the National Institute of Building Sciences.

Don’t say we didn’t warn you. The new Title 24 is tough! In past articles, we harped on the HERS verifications as a way to earn credits towards Title 24 compliance for those hard-to-pass houses. However, there’s another angle that needs attention: issues for additions, alterations, and remodels.

(Above image shows a whole-house remodel and addition by Mark English Architects. Photo: Michael O’Callahan)

When does a remodel need to show Title 24 compliance?

When the changes impact the exterior building envelope, the heating/cooling/water heating systems, or when you’re adding conditioned area or volume. Envelope changes include new walls, replacing windows, and adding or enlarging windows. System changes include upgrading a furnace, changing the type of heating/cooling system, re-doing the ductwork, or upgrading the water heater. There are other circumstances like changing the lighting that may require Title 24 compliance. The focus is on conditioned space only, so garages and sun porches are not counted.

When do you NOT need a Title 24?

When all changes are internal and don’t impact the energy performance of the building. For example, if you move a group of interior walls but leave the furnace alone.

What are the methods of showing compliance?

There are two main methods: prescriptive and performance. The prescriptive method works just like a doctor’s prescription, with mandatory minimums for things like insulation or glazing performance. We do a little of that, but much of the time it’s fairly simple and many architects just do it themselves. The downside is that it’s less flexible – there are minimums, but it’s harder to get credit for exceeding those minimums in some areas in order to make up for shortcomings in others.

Not all remodels will qualify for prescriptive compliance. For example, if glazing is more than 20% of the floor area, or more than 5% of the glazing is on the west, then the project may need to use the performance method. The performance method is what we do, using a CEC-approved software modeling program.

How do I know if my project can qualify for the simpler prescriptive method?

You can use prescriptive compliance if your project matches all of the features listed in Table 151-C of the Title 24 Residential Compliance Manual, which vary by climate zone according to the 16 climate zones of California. (The Bay Area is mostly Zones 3 or 4 with some 2 up north and 12 out towards Sacramento.)

The prescriptive baseline values used for most homes are also known as “Package D”.

What is Package D and where can I find the documentation?

Package D appears to consist mainly of this one table, (Table 151-C) plus several pages of footnotes, which is tucked in the back of the Residential Compliance Manual under Appendix B. Even here not all the information is included; the table just says “MIN” for furnace AFUE for example.

What does the prescriptive method require?

The following is a very simplified summary. For areas like San Francisco, East Bay and the Peninsula, wood-frame walls must be minimum R13, raised floors R19, and ceiling/roof R30. Glazing U factors must be .40 or lower, gas furnaces .78 AFUE or higher, and air conditioners must be SEER 13 or higher.

The prescriptive method places strict limits on the amount of glass that you can add, especially on the west. For additions from 100-1000 square feet, glazing must be less than 20% of the conditioned floor area – no curtain walls, sorry. If you have an addition that is under 100 square feet, that portion can’t have more than 50 square feet of glazing. If the project is an alteration and no area is being added, glazing can’t be more than 20% of the total conditioned floor area. Some climate zones also specify that only 5% of the total glazing area can be west facing.

Medieval castles like this one had very low glazing-to-floor-area ratios.

So, if you want an addition with a glass curtain wall for your Esherick home, you will have to use the performance method.

I wasn’t planning on opening all the exterior walls. And I was going to reuse my old windows, too.

Don’t count on being able to reuse the windows unless they’re of fairly recent vintage. If your existing windows are single glazed, or clear glass instead of low-E, or they’re metal framed instead of wood or vinyl, you can forget it. All windows leak heat, but old ones leak a lot more. Keeping those old windows could easily double the energy budget for the entire home, which would kill your chances for Title 24 compliance.

For additions, alterations, or remodels, where only some walls, windows, or existing systems are upgraded, Title 24 allows several possible approaches or strategies. Again here, we’re talking mainly about the performance method, but these strategies are also available for the prescriptive method as well.

Obviously we can try modeling the project using the old windows, but we’ve ended up having to include a host of other measures to compensate – things like upgrading to a more efficient furnace, HERS-testing the ductwork for air leakage, even adding thermal mass.

What’s a HERS test?

Most of you know this by now, but HERS tests are third-party field inspections for things like duct leakage, and you can earn “compliance credits” for these tests when using the performance method to show Title 24 compliance. Running the software model with one or more of these tests specified can improve the score of the proposed design, sometimes dramatically. They require additional coordination during construction, but are not as inconvenient as having to spend an extra $15,000 on new windows.

For more details, see our recent article on HERS inspections and Title 24 compliance.

What’s the difference between modeling an Addition Alone or doing the remodel as a Whole House?

Additions can be modeled as self-contained if conditioned square footage is being added, the new space is all in one spot, and for modeling purposes it’s best if the addition is at least partially sealed off from the rest of the house. As long as you insulate all those walls, including new interior walls, and use efficient windows,  you can model this additional space as its own self-contained little building. This means you can keep the crappy windows in the rest of the house.

This proposed addition adds square footage and uses new windows, while the rest of the house is left un-altered. This project could use Addition Alone compliance method.

However, if no square footage is actually being added, then you can’t show compliance for only one corner, even if that corner is getting the royal treatment. This can happen if, say, a family room is getting a facelift and new windows, maybe bigger windows than before, but it’s staying the same size as before. At this point you have to either meet the mandatory minimums for the altered portion, including maximum glazing-to-floor-area ratios that may apply to the entire building, or you have to use the performance method, meaning you have to model the entire project within one of those approved software programs mentioned earlier.

It also happens sometimes that the Addition won’t pass by itself. If we have to use the performance method because of glazing area or whatever, we can try running the addition by itself. However, sometimes even the most thoroughgoing modeling efforts will not yield a passing score. Then we have to model the entire building (or condo unit) – and for that, we need to include information on all the existing exterior surfaces: walls, roof, floor, and windows. If the existing conditions are unknown, we have to assume the worst, based on when the house was originally built.

This proposed alteration is not adding new floor area. The main work is occurring towards the rear, including new windows and walls. However, there are also new skylights being added elsewhere in the house, and the front window is replaced with two new windows. This project would require the Whole House compliance method.

How does the Title 24 modeling software show a pass or fail score?

What the software model does is compare energy usage of the proposed design (your remodeling plans) with the energy usage of that same house assuming the mandatory minimums. Model inputs include the home’s compass orientation, wall areas, floor areas, roof areas, glazing areas, actual systems in place, and performance numbers for each. For example, a 1,200 SF home oriented at 90 degrees east might have 270 SF of north facing exterior wall insulated anywhere from R0 to R25. This would be compared to a 1,200 SF east facing home with 270 SF of north facing exterior wall insulated to the minimum, R13. Your design has to beat this baseline, shown in this energy use summary as the “Standard Design”.

This sample Title 24 compliance report shows energy usage breakdowns separately for heating, cooling, and water heating. If the house is not passing, it's easier to see where improvements should be made first.

I really don’t want to open any more walls or replace more windows, because that will put us way over budget! My clients will go ballistic! Why can’t we just add more insulation to the parts that are being opened?

If you have to model the whole building, then all the existing conditions have to be modeled as they are now. This means that if any of the existing walls are un-insulated, that house is going to have a very hard time passing the software model, even if the rebuilt portions are insulated far beyond the minimum.

It is sometimes possible to use blown-in insulation for existing walls without having to open them completely. We have found that even minimal insulation of all walls is far better than leaving any portion of the walls at R0.

So it’s not passing, what do I do?

At that point it’s a matter of incrementally testing in combination various additional measures that you may not have planned on doing. For example, if a house is ahead on heating but behind on cooling, then efficiency measures that aid cooling should be considered first. However, it’s also possible to achieve compliance through improvements to the heating system, even if the cooling is still below the minimum. That’s the advantage to using the performance method, and it’s sometimes the only way that highly glazed designs can pass.

If one measure isn’t available for a project, we can try others instead. Of course if there are too few alternatives – say they can’t afford to replace all the windows or they don’t want to get a newer, more efficient water heater – well, something still has to give. Resorting to elaborate workarounds in an effort to save money can introduce other risks into the project.

Yes, we feel your pain, too, but you still have to replace those windows.

What additional measures should I be prepared to consider?

Based on our own experience of 15 years doing Title 24 compliance for low-rise residential buildings, here are the findings that seem to hold true across projects.

Insulation. For remodels, do not leave any portion of wall un-insulated! This may not mean the entire house, unless we have to model it that way. Insulating to the maximum of what will fit inside the walls should be a given. This can include portions of the interior walls, too. (Radiant barriers are good in hot climate zones, but they don’t make much difference in San Francisco.)

HVAC. Upgrading heating, cooling, or water heating systems. This can be upsetting if the furnace is recent, but not quite recent enough. If your furnace has an AFUE of .90, but the project won’t pass unless that AFUE is .92, we have to deal with the situation as it is and find some way to address it.

For any remodel or addition project in California, additional measures may be required for Title 24 energy compliance. Clockwise from upper left: replacing inefficient windows, upgrading to more efficient heating/cooling/water heating systems, adding extra insulation, earning credits through HERS tests such as this blower door test, and finally, solar shading for homes that have problems with summer solar heat gain.

Windows. Replacing all or most of the existing windows. Obviously this can get expensive, and we try to avoid this. On one project we had to specify every HERS test there was, because they wanted to keep 5 existing windows that were metal-framed with clear glass. This project also had existing window that were wood framed with clear glass, but it was the metal ones that hurt the project the most. Poor window performance is the Achilles heel of compliance.

HERS verifications. We used to discourage the use of these third-party tests because it’s cumbersome to have to coordinate for yet another inspection during construction. And, there’s no guarantee that the test will pass on the first try, although there are ways to prepare for them to help things go smoothly. Now we’ve had to resort to them for about half our Title 24 projects.

Design changes. Our whole raison d’etre is to help architects comply with Title 24 without having to alter the design in a visible way. No shrinking of windows, no adding of south wall overhangs if the original design didn’t call for them. We’ll recommend product substitutions, but we’ve never had to tell someone that they couldn’t have their all-glass panoramic view. Still, I’m sure someday we’ll get a project where adding an overhang or side shading wall makes that 0.01% bit of difference between passing and failing.

Windows can leak heat in cold weather, but they also can admit too much solar heat gain on hot, sunny days, as shown in this illustration. The window on the left would be associated with higher cooling loads.

How accurate is the Title 24 software model? Just because a measure doesn’t help in the model, does that mean it’s really worthless?

Absolutely not! The Title 24 modeling software calculations are actually pretty thorough, although there are some intentional omissions that can, at times, make the building’s real-world performance quite different from what the model would predict. A home that in reality is covered by shade trees and a nearby mountain may show unrealistically high cooling loads in Title 24, because shade trees, adjacent buildings, and landforms are specifically not allowed as factors for compliance. I’m not going to get into the reasoning, but that’s how it is. Title 24 errs on the side of conservatism, so a house that does well in Title 24 should also do well in reality, even if the reverse is not always true.

What it’s good for is comparing the relative impact of one change over another. You can test the sensitivities of using triple glazed vs double glazed windows on just the west or south walls, for example, to see where you can get the most bang for the buck.

I don’t think the Title 24 software models are quite accurate enough to create energy budgets for things like Net Zero Energy homes, or to model temperature flows for passive solar designs. For one thing, appliances like TVs and computers aren’t considered at all, nor are differences in occupant behavior. It could give a rough cut analysis of major opportunities for optimizing the design, but then you’d have to move to something else.

Imagine a home built in the Plains region of the United States that stays warm in the winter without central heating, and cool in the summer without massive air-conditioning. It’s airtight but with an endless supply of fresh air constantly circulating through a filtered, pressure-balanced ventilation system. Every surface is comfortable to the touch, neither too warm nor too cold. Street noise is barely audible through the gasket-sealed, triple-paned windows. 

It sounds futuristic, but so-called Passive Houses have been around for at least 15 years, and it’s yet another strategy for saving energy. Unlike a Net Zero Energy home that might rely on “active” power generation, albeit from renewable sources, a Passive House is just that – passively absorbing heat from its surroundings to release it slowly as it is needed. (In hot climates, Passive Houses are designed to recover and store cooler temperatures.)

Passive House History

The idea of a “passive house” originated in Germany as a result of conversations between two university professors. A Passive House (Passivhaus in German) is a building that requires very little energy for heating and cooling, instead relying on passive energy sources and thermal isolation from its surroundings to achieve temperature stabilization. The notion began around 1988 and is now widely accepted in Germany and Europe. It’s now a standard, with specific and measurable performance requirements that can be field-tested and verified.

Buildings that meet the Passive House standard include public structures such as schools and supermarkets, as well as private residences. In the United States, the Passive House Institute US in Urbana, Illinois is a consulting and research firm working on adaptation and implementation of the Passive House standard within the United States.

This existing home in Ireland has been retrofitted to meet the Passive House standard, and yet it still looks just like every other house on the block. Designer: MOSART Architecture

Here in the Bay Area, Quantum Builders has become a recognized expert in the creation of Passive Houses tailored to our local climate. (Note that there is another builder of that same name in Texas, unrelated). An upcoming project in Tiburon is due to start construction this year, and was designed by award-winning architect Olle Lundberg of Lundberg Design. Bronwyn Barry of Quantum Builders spent considerable time explaining to me how it all works and showing me some of the wall assemblies that Quantum uses in their Passive House projects.

Wall details from above house showing "before" and "after" the Passive House retrofit, from the MOSART web site.

I also spoke with Jonah Stanford of NeedBased Inc., an architect based in New Mexico who’s a Certified Passive House Consultant with a successful track record of completed projects that employ advanced solar design principles in an artful and responsible manner. 

Passive House in a Nutshell

Passive Houses rely on an airtight envelope, lots of insulation, thermal mass, heat-recovery ventilation systems, and a thoroughgoing approach to slowing heat transfer through the walls that leaves no stone unturned. Wall assemblies tend to be thicker – in some retrofits, it’s like wrapping an additional blanket around the existing house – but the most unusual thing about the walls apart from air tightness is the extreme attention paid to eliminating thermal bridging. “A typical home can lose 25% of its heat from thermal bridging,” said Bronwyn.

Thermal bridging can be responsible for 30% of heat loss from a home. Wood frame studs have a lower insulating value than the batt insulation between them, and if the studs are directly in contact with the inside and outside of the wall, they can act to conduct heat out on cold days, resulting in unwanted heat loss.

In a way, it’s not unlike NASA sending up a manned spacecraft and having to account for every last gram of weight, to ensure that there’s enough fuel to get it to its destination. The interior has to be kept warm enough to keep out the cold, which is a chill far more extreme than anything you’d find on the Earth itself. And of course, manned spacecraft have to be airtight, because any air leakage at all would be disastrous.

The idea with a Passive House is to stabilize temperatures by making the thermal mass of the house work for you like a giant hearthstone. You don’t need a conventional furnace at all – even in Northern Europe! Once the house is at the desired temperature, it takes very little energy to keep it there. “Improving a home’s airtightness can result in 25% energy improvement,” said Bronwyn.

Passive Houses have been built in every climate zone. Desert climates of course are more concerned with keeping cool by managing solar heat gain, whereas cold-winter areas are more concerned with staying warm. Tailoring the design for the specific climate and site conditions is of paramount importance.

Clockwise from upper left: Crossway House by Hawkes Architecture in the U.K.; Breezeway House in Salt Lake City, Utah by Brach Design Architecture, the first certified Passive House in the U.S.; zero-emissions research station in Antarctica.

(Note: I’m not sure that they are all “Passive House certified” but they all use the basic Passive House principles. Breezway definitely is certified, and Crossway is “zero carbon” home that has also been accredited by the Passivehaus Institute in Germany. Bronwyn mentioned the Arctic research station as a Passive House, so maybe it’s actually certified as well.)

The Passive House standard doesn’t specifically require the use of non-toxic materials, although the wall assemblies that I saw used materials that were carefully chosen partly for low toxicity – cellulose and rock wool insulation, wood and low-toxicity oriented strand board. Off-gassing isn’t as much of a problem as I had originally thought: the ventilation system has a low but constant rate of air exchange that doesn’t allow stale air to accumulate anywhere in the home. Passive Houses are credited with having excellent indoor air quality; that’s one of their selling points.

Humidity also has to be managed, as with other tight-envelope buildings. Placement of vapor barriers is dependent on climate, similar to other types of construction. The wall assemblies at Quantum Builders showed extensive attention to waterproofing as well as the placement of air and vapor barriers.

Rafter detail showing vapor barrier at interior (blue,) air-barrier at existing siding (red) and bulk moisture barrier at roof sheathing (red.) Arrows show air movement within the roof assembly, allowing air to escape from vents in the roof for passive cooling. Taken from a study for a Passive House retrofit by Quantum Builders.

Like other standards, a Passive House building performance analysis includes the creation of an energy budget. Energy budgets are a key component of many other energy-saving approaches and standards such as passive solar design, GreenPoint Rating, HERS home energy audits, Net Zero Energy homes, or California’s Title 24 energy standard. The Passive House energy budget is thorough and detailed, including occupants, appliances and lighting. Both power consumption and heat generation are considered. 

How do Passive Houses differ from, say, Passive Solar or Net Zero Energy homes?

Passive House is more than a set of principles – and it’s more than a checklist. Passive House is a formalized approach with an associated standard, modeling software, energy budget, and certification/testing process. To be fully certified, each Passive House is verified against actual building performance after the building is completed, as follows:

1. Heating and cooling demand is less than 4.75 kBTU per square foot per year. (A regular house might use 15 times that amount.)
2. An air-tightness rating of less than 0.6 air changes per hour, measured at 50 Pascals. 
3. Energy demand for all uses (called “specific primary energy demand”) including hot water, heating, cooling, auxiliary, and household electricity is less than 38 kBTU per square foot, per year.

The Passive House performance standard.

By comparison, passive solar design isn’t really a “standard” that can be pass or fail. The passive-solar approach looks at building orientation and other principles of solar design, but there’s no specific energy modeling software associated with it (although many software programs can be used to assess most of the solar gains, etc).

Net Zero Energy homes don’t really have a standard or certification other than daily use. They do have a performance goal: use less energy than you produce within a single year, with an annual reckoning every December between you and the utility company. If you’re grid-tied and your energy bill for the year is zero, then yes, you produced more than you consumed, so the house is Net Zero – at least for that year. But there’s plenty of synergy among these approaches. “Passive House gets you super-close to Net Zero Energy”, says Bronwyn. “If you meet the Passive House standard, then Net Zero is easy.” A Passive House requires less energy to begin with, so you’d be able to reduce the size of your renewable-energy systems accordingly.

Title 24 does provide good baseline performance measures, as Bronwyn explained. “The R value needed to meet the Passive House standard varies by climate and is determined per project using the Passive House Planning software. In Minnesota, you might need R30 to R40 walls and R50 roof. Here in California, a Passive House should have around R21 walls, R11 insulated slab, R28 roof, and really great windows.” By comparison, Title 24 mandatory minimums are R30 roof, R13 walls, R19 floor, and Low-E windows. 

The differences lie in the root of each strategy. 

• Passive House is about temperature stabilization as the main focus for reducing the need for actively generated power. 
• Net Zero Energy is about achieving a “net zero” balance between onsite power generation and power consumption, with a strategy that includes active power generation through solar, wind, or other renewable energy sources.
• The GreenPoint Rating system focus is on long-range resource conservation, energy efficiency, community design, and environmental health. So does LEED.
• They all differ from Title 24 in that you can factor in your shade trees for credit, or include these features as part of the energy model.

Tell me about energy budgets in Passive Houses.

Part of the Passive House standard involves setting a specific overall energy budget for the home based on “treated floor area”, or conditioned square footage up to the interior wall area. The Passive House energy budget is imposed purely based on size regardless of activity, and includes all appliances, not just heating and cooling. It’s up to the owners to decide how to use that budget. It’s challenging to meet the standard, but definitely possible. Typically, you have to be very careful when selecting appliances. The Energy Star rating only sets a minimum efficiency; within that, appliances can vary widely in how much power they actually use.

The Passive House software tool, called the Passive House Planning Package, is an elaborate Excel spreadsheet that helps to create a detailed energy model of the home. Although use of this tool isn’t strictly required, it seems to cover every possible angle and takes all the Passive House principles into consideration.

What if you want to light a couple of candles over dinner? Will this throw the house off?

A good ventilation system can take that into account. What most owners do is they open a window! But yes, you do have to be aware of every heat-generating activity that you do. However, Passive Houses can accommodate a wide variety of activities and lifestyles. You don’t have to be afraid of exercising in a Passive House or of hosting large groups of people. In Germany there are entire kindergartens and office buildings that are Passive House certified, even an indoor pool!

What happens if you leave for a long weekend and forget to take out the trash?

The first question everyone asks when they hear about a hermetically sealed, airtight house is “What happens if you fart indoors?” Even though the question itself is crass, the concerns about stale air are reasonable enough, given all we’ve heard about airless offices and flu-laden airliner jets.

“Passive House ventilation systems usually have a ‘flush’ feature nowadays,” said Bronwyn. “If you need to clear the air, you can activate this cycle and then the system resumes normal operation.” As with the above question, you can also open the windows for 10 minutes, air the place out, then shut them again without seriously disturbing the temperature balance inside the home. 

Passive Houses have better air quality than so-called normal buildings. Bronwyn and I spent time discussing the chronic health issues so many urbanites face, from asthma to migraines. She quoted me longitudinal studies from a school in Germany that showed reduced absenteeism, improved occupant health, increased attention span, and reduced CO2 levels. “In a Passive House, the indoor air is constantly being filtered and circulated, while stale air is constantly being expelled.”

Can you use carpeting inside a Passive House? Are there certain conventions for indoor furnishings and materials that need to be re-examined?

Yes, you can, and any dust it generates will be less of a problem because airtight houses are less drafty. There are no stray air currents to kick up dust into the air. There is no reason why you couldn’t use all the same interior design techniques that you would in any other home.

What’s it really like inside a Passive House?

I asked this question of Jonah Stanford. “It’s like being on the moon,” he answered. “It’s amazing, really. The house acts totally different in some ways from what we’ve been conditioned to expect. If you stand near a window on a cold day, you won’t feel a thing. Normally you would feel a thermal draw from the window in cold weather.”

A cross section from a window designed using Passive House principles. It features argon gas-filled triple glazing, thermal breaks, insulation inside the frame, full gasket seals at three places inside the frame, and a waterproofing system on the outside to trap and guide rainwater out and downward.

The evenness and stability of the temperature inside a Passive House eliminates hot and cold zones that we may be used to. “I went to a 2 story office lobby that used Passive House principles and we measured the temperature at the floor, the wall, and the roof. It was all exactly 21.5 degrees Centigrade. Phenomenal.”

Do occupants feel separated from the outdoors?

Not at all. Passive Houses are quieter, but they actually have more fresh air. Occupants can open windows when it’s nice outside as much as they want, just as they would in a conventional house. The only differences is they don’t HAVE to.

Does the Passive House standard REQUIRE that you purchase special building materials all the way from Germany? Can’t you do it using local materials?

Yes you can. Quantum has chosen to work with German manufacturers because they’ve got more experience building to the Passive House standard. Importing assemblies from Germany to the Bay Area actually uses less embodied energy than, say, trucking them from Minnesota. Apparently to be really “carbon compliant” everything trucked by surface has to come from a distance of under 300 miles. 

It’s not the materials, it’s the details that have to be reworked. All thermal bridging must be eliminated, which requires special measures. In addition, airtightness, vapor protection, and waterproofing all need to be addressed.

Insulated roof ridge detail before and after, showing how R-values were improved from a mere 2.2 up to R-50. Taken from study for a Passive House retrofit in Larkspur, by Quantum Builders.

There are no books on typical detailing for Passive Houses – yet. Builders on the East Coast can often use details from German books on Passive Houses, but these details are optimized for a cooler climate and rely more on masonry than on wood frame construction which is most commonly used here in California.

Aren’t the floors cold? Is every surface supposed to be the same temperature?

There is slab insulation under the home to keep the floors from leaking heat out into the ground. A Passive House has a lot of thermal mass partly to keep every surface temperature constant. Thermal imaging via a software package called THERM is a useful supplementary tool. Bronwyn showed me two thermal images, similar to the example image shown below, comparing the effect of placing slab insulation either above or below the slab. 

Imaging example showing a thermal model of a floor to wall assembly, from the software package THERM. Image courtesy of Quantum Builders.

Although both floor surfaces were warm where the floor met the air, the warmth went deeper when the slab was exposed and could warm itself. With the slab underneath, it sucked cold up from the ground and stayed that way.

How NOT to design a foundation. This shows how thermal bridging can effectively drain all the heat out of your home.

What does the ventilation system need to do?

For a Passive House, you need a good mechanical heat-recovery ventilation system with balance between air intake and exhaust, delivering 0.6 air changes per hour. The house should have an even pressure balance between inside and outside air. Air filtration components may be selected based on the location and occupant needs, but are always present. Special attention is paid to the location of openings for air intake, which may vary by climate as well as site. For example, intake in very cold climates may require some form of pre-heating via earth tubes.

“HEPA filters aren’t always necessary,” said Bronwyn. “Passive Houses have lower airborne particulates already, because there are no indoor convection currents (drafts) to stir up dust. The fact is, so-called ‘normal’ indoor air quality is poor to begin with.” 

What additional costs are associated with building a Passive House as opposed to a “regular” one?

The Wikipedia article on Passive Houses contains the statement that overall, Passive Houses cost on average 14% more to build and are more expensive in Northern latitudes above 60 degrees. Other sites claim 10% overall or 7% in Germany. I didn’t get a figure from Quantum, although it’s clear that the additional insulation and thicker walls do add somewhat to the cost. 

“It’s really important to have a fully committed client,” said Bronwyn. “Otherwise they may not want to go all the way.” I observed that most people don’t view their houses as legacy homes to be handed down to their children. It seems that most people stay in their homes about 5 years or so and then move on. They’re not as willing to invest in improvements that you can’t see.

Do Passive Houses have a thermostat?

Yes. Typically this would be set to 68 degrees, and is adjustable to suit occupant preference.

Can you have multiple heating zones for sedentary vs vigorous activity?

If you want to have a small office that’s nice and warm, while the rest of the house is at a cooler temperature, you can use a small portable space heater. This can be accounted for in the home’s energy budget during the early planning stages.

How well do Passive Houses do in extreme climates?

There are Passive Houses built in all seven climate zones in the US. There’s even one in Antarctica, a research station. A desert Passive House will be geared more towards cooling, but the actual wall assembly is similar to what you would use in Minnesota, in both cases well-insulated and protected against thermal bridging, because in either case you want to minimize thermal transfer through the walls.

This desert home by Kendle Design is not Passive House certified, but it uses the same principles of solar design that would also be employed to build to the Passive House standard: massive earth walls and solar shading. It might be challenging to make a glass wall this large using the airtight, triple-paned construction details shown on other Passive House windows, but who knows?

Within the U.S., the most challenging climates seem to be the cold regions around the Canadian border, and the extreme heat and humidity in places like Florida and the Gulf. Sometimes the use of geothermal or earth warming tubes buried in the soil can act as heat exchangers to pre-heat or pre-cool outside air before it goes through the ventilator.

Let’s talk about special building techniques for Passive Houses.

The Passive House standard is performance-related rather than material-specific. Quantum Builder’s South African case study is 100% predesigned and prefabricated from custom-produced wall and roof assemblies. In the Ukraine, according to Jonah Stanford, there are Passive Houses built with monolithic wood walls, although I wasn’t able to find any immediate specifics online. Regardless of material, airtightness, moisture management, and the elimination of thermal bridging are important considerations when designing specific wall assemblies. 

What’s inside this wall assembly here in your office?

From inside to outside: 

1. Drywall
2. Furred-out mechanical chase
3. Oriented strand board layer for air-barrier & structural sheathing
4. Cellulose between the structural framing
5. Insulated fiberboard impregnated with wax
6. Rain-screen furring 
7. Exterior siding

One of the wall assemblies on display at Quantum Builders' offices.

What about the windows in a Passive House?

The one component that isn’t easily obtainable here are the windows. Windows that meet the Passive House standard are hard to come by. They must be airtight, triple glazed, with insulated frames, with a very low U value – .14 or even as low as .11. All moving parts must be precisely fitted, like airlocks in a spaceship.

“R values of typical window are poor. A typical vinyl window is around R2, and the best Marvin windows are around R3.2. The R value of a Passive House window needs to be more around R7 to R9,” said Bronwyn. Considering that the minimum wall insulation in CA is now R13, the windows present the primary avenue of heat loss in a home, and it pays to make them as thermally efficient as possible. Installing the windows presents an opportunity for further insulation. In some cases the window frame can actually be layered behind additional insulation extending from the walls of the house.

The thermal performance of a window is influenced by the performance of the frame, the glass, and the spacer. In addition, the installation method can affect the performance of the entire wall. Each of these components within a window should be as thermally efficient as possible.

Bronwyn had special words about vinyl windows. Although they’re encouraged in Title 24 as being efficient, they have a reputation for off-gassing. And they’re still not airtight enough. The windows used by Quantum are made from wood, sometimes with aluminum or fiberglass cladding. The fiberglass clad windows are enhanced with a special insulating foam: Neopor- a super-insulating carbon impregnated type of EPS, on the outside of the window.  Any gas that escapes can’t penetrate the air barrier to get inside the house.

If you import special materials and such, do these products meet local building codes and standards?

I was especially interested to know if the imported windows were NFRC rated. Bronwyn informed me that their window manufacturers were in the process of getting their products rated, which can take up to a year. 

Why would you choose to go with the imports rather than building it locally, then?

Passive House materials must be built to the most exacting standards possible. Air-tightness must be controlled at every joining, every assembly, every switch box. Rather than try to manually assemble everything onsite, it can be both faster and more quality-enhancing to produce components such as wall systems in a factory that is already set up to achieve these standards. Having vendors and suppliers you can really rely upon is vitally important. Right now, most of these factories are in Germany because the Passive House standard was originated there, by building scientists, with strong support from the German government.

Quantum Builders already had strong ties with Germany, and has chosen to work with factories that achieve precision and who are committed to using high-quality, non-toxic products. These producers offer custom details as well as a wide range of standard products to satisfy design-oriented architects.

Jonah Stanford confirms that Passive House standard does not mandate a particular type of material, only a specified performance threshold. “The German assemblies that Quantum uses are actually quite reasonable in terms of cost. We’ve also price-compared both the German imported assemblies versus site-built or prefab assemblies made locally, and it came out about 20% less than importing – basically the cost of shipping.” 

When building manually, you have to pay a lot of attention to thoroughly sealing all electrical and plumbing penetrations, to keep the vapor-lock tightness. “You have to be obsessed with it, and even so, the seals might not last as long as the building,” says Stanford.

Stanford is working on his own assembly, a double stud framed wall. What distinguishes this wall from a “normal” wall is the layering. “The interior wall is load-bearing. Then, I use oriented strand board – NOT particleboard, followed by another layer of studs that are not vertically bearing.” This idea was, he says, inspired by the Ukranian wood frame Passive Houses, which are literally built from the inside out. 

There can be conflicts with local codes or incentives. In NM the incentives are generous, but require adherence to ASHRAE Standard 62 which requires outside venting for appliances like dryers. In a Passive House, however, the heat from that dryer should really be kept inside the house, at least in the wintertime.

What special skills are needed to design systems for, and actually build, a Passive House? 

Bronwyn had a couple of thoughts on this. The first was to have an integrated team from the start. “Architect, owner, builder, energy analyst, structural engineer, mechanical – they all have to review the early drawings together,” she emphasized. The second was to produce good construction drawings and details. “If the details are clear, any builder should be able to build to them – as long as they understand the why.”

Can an existing home be remodeled to meet the Passive House standard?

Yes, although some details such as under-floor insulation, strongly encouraged in Passive House construction, can be difficult to retrofit in existing slabs. Bronwyn showed me some details for Quantum’s remodel project in Larkspur. 

Although the added thickness does increase the home’s footprint very slightly, this in and of itself is not a problem unless the home is on an urban lot right up to the property line. In that case, the retrofit might have to concede a little space on the interior.

For the roof, you might have to actually raise the roof in some cases, in order to create additional room to fit the necessary amount of insulation. In jurisdictions where they might be picky about adding 6 inches to the building height, you might have to build down or lower an interior ceiling.

Isn’t rigid foam toxic, though?

During the roof discussion, Bronwyn and I got into a side discussion of different insulation types and R values, which is a measure of the resistance to heat transfer (higher is better). Typical batt insulation has an R value of around 3.7 per inch, meaning you can fit up to around R13 into a typical 2×4 framed wall. However, some types of rigid foam insulation can do better. Polyisocyanurate, for example, has been claimed as being R8 or even R11 per inch. I’d been wondering about the toxicity of this – “polyisocyanurate” just SOUNDS toxic!

Bronwyn pointed out that in the case of the roof assembly, the foam is on the outside of the air barrier, and the polyiso isn’t the worst thing out there. Formaldehyde from conventionally made engineered lumber products is a LOT worse, lasts a lot longer after installation, and it’s ubiquitous in buildings already.

What happens as part of the Passive House design and certification process?

The steps to Passive House certification are as follows:

1. During the schematic design phase, use the software to determine which wall assemblies (R-value) will meet the Passive House standard for the particular project based on climate. From there, you can proceed to create a detailed energy model of the project including surface areas, ventilation, windows, shading, even prevailing wind speed. It is essential to have input from your design and build team during this refinement. When design is complete, the drawings and the project modeling file are sent to the Passive House Institute US for pre-certification prior to construction. This takes around 4-6 weeks and costs around $800.
2. During construction, a third-party inspector comes out to verify that the house is actually built to the drawings.
3. After construction is completed, a third-party inspector conducts an official blower door test, to verify that the home is airtight. This test can be done by a HERS rater, as long as that person knows how to test specifically to the Passive House standard. This includes verifying a neutral air pressure balance inside and outside the home.
4. The final step in certification is to re-verify the home against the as-built drawings. This takes another 4-6 weeks and costs an additional $300 depending on complexity and size of the project.

As Bronwyn noted earlier, special attention should be paid to construction detail drawings. Construction Documents are one phase that sometimes gets short-changed, because clients mistakenly believe that it will “save them money” – and then those details get worked out in the field by the builder. With a Passive House, you can’t do this because those detail drawings will be required for verification during construction.

What do architects need to know in order to design a Passive House? Would they work with a special builder or consultant to do the modeling?

Jonah Stanford mentions that you have to design to the Passive House standard from the beginning of the project, which would seem obvious but it’s worth pointing out that if you start out designing a standard home (or standard remodel) and you’ve already gotten as far as construction drawings, and THEN you decide to meet the Passive House standard, you will have to re-do all the wall assemblies. Expensive. “You can’t change horses in the middle of a stream,” I said, to which Stanford responded, “No, it’s more like switching from horseback riding to driving a herd of pigs through the water.”

How did you choose Lundberg Design for a passive house project? 

There was a long and careful selection process, where we interviewed several architects.

This straw-bale house, also built by Quantum Builders, isn't a certified Passive House, but it uses passive solar design principles, and it's quite a nifty shape.

What’s the response from Planning to your project?

They’re fully behind it – as long as it still complies with the building code.

Green living is sometimes viewed as a sacrificial process whereby one by one, all our pleasures and comforts must be set aside in the name of saving the planet: walking instead of driving, sweeping instead of vacuuming, home cooking instead of take-out, turning the thermostat down in the winter while our hands and feet turn into blocks of ice, low-flow showerheads designed by bald men that take forever to rinse the shampoo out of a long-haired-girl’s mane, limiting one’s diet to only locally available seasonal produce (which could be nothing but cabbages if you live in Chicago), calling three hardware stores to find one that carries low-VOC paint, giving up meat because it takes too much grain to feed a cow, trudging everywhere with a backpack filled with stuff that otherwise we could just keep in the car. In essence, the increased physical hardship comes from asking our own bodies to start doing more of the work. And what’s our reward? A nice warm feeling of altruistic glow, and maybe a slimmer figure.

Efficiency is often seen as achievable only at the cost of comfort – some of us East Coasters remember shivering through the 1970s oil crisis as our dads re-defined 58 degrees during the day as “normal” and turned the thermostat down at night till the pipes froze, and our mothers finally complained. Well, so what? What’s the big deal? We all have to give up something. Well, the problem is that this “fix” didn’t really fix anything. Reducing consumption is not the same thing as having an efficient building, and neither approach presents qualitative factors like comfort or contentment as worthy of consideration.

It’s time we started measuring the benefits, started quantifying the value of comfort in both human and performance terms. After all, a bottom line applies both to corporate and to individual operations, even if the qualitative benefits of “improved morale” and “comfort” aren’t seen as “adding value”. People with severe allergies and asthma can already tell you exactly how much more productive they are in a mold- and chemical- free environment when they’re not gasping their life away, but what about the rest of us who can breathe diesel fumes all day with nothing worse than a little nausea?

Cats have long known how to enhance their own thermal comfort through reclaimed heat from TVs and other appliances.

Current Focus Is On Efficiency Alone

California’s energy code requires efficient buildings in order to reduce fuel consumption, reduce peak demand, and – yes it’s a stated goal – reduce fossil fuel consumption as well. Title 24 energy compliance trade-offs in residential designs are usually a matter of running the proposed design through a software model with different options, with an assumed average air temperature that is not configurable, to simulate the building’s performance throughout the seasons for a specified climate zone.

Title 24 seeks to make homes more efficient so that they leak less heat in the winter. Infrared photography shows that this home is losing heat through the windows and the attic.

If the proposed design does not meet compliance standards, one can weigh various building improvements against the costs and time needed to achieve each measure. Is it cheaper to replace all the windows or to upgrade the furnace and add some field verifications? Does it make sense to add a radiant barrier to the roof, or would it be better to spend that money on better wall insulation?

On paper, it’s the same either way. In practice, however, which option you choose can have a big impact on the comfort of the occupants – who may be your own design clients. Every time they sit in their patio, or in their kitchen, or in a reading nook looking out the window, it’s an opportunity for them to remember their architect with fondness. However, if every time they sit near their picture window they catch a chill, they’ll remember that, too – especially if they’re me. I’ve spent far too much of my life in buildings that were too cold for me, and most of it was bad design – needless discomfort. (In office buildings, the chill was exacerbated by inappropriate professional dress codes that required suits, pumps and nylons when a wool hat, a big bathrobe and slippers would’ve saved me countless weeks of colds and sinus trouble.)

Contemporary lifestyles presume that people can comfortably wear light clothing or go barefoot indoors, even in winter. However if the walls or the floor are cold, this home will feel much colder even when the air temperature is the same. That wall of glass just visible on the left could make it chilly just to sit on the sofa, if it were cold enough outside.

The thing is, indoor air temperature is easier to measure, and that’s what Title 24 takes into account. However, the indoor air temperature alone may not be enough to ensure comfort. Other factors, such as humidity, air movement, temperature of surfaces in direct physical contact with the user, the occupant’s level of activity, and radiant heat transfer from windows can make a huge difference.

So how the heck do you measure something as individual and subjective as “comfort”? How are these “findings” actually validated in practice? And how can the proven findings from this type of research be useful for residential designs to go beyond energy compliance? For this article, I visited the offices of Loisos + Ubbelohde in Alameda, California.

Advanced Modeling and Thermal Controls – Is It Really Any Better?

When most people hear words like “daylighting” or “integrated facade systems” they think of the elaborate sensors and controls that are increasingly employed for commercial high-rise buildings to reduce heating and cooling loads and ensure optimal light levels, mostly without human intervention. Of course, the public then hears of supposedly cutting-edge “sustainable” buildings that are no more efficient or comfortable than the old, wasteful kind – at least from the point of view of the occupants themselves.

An "old-school" skyscraper on the left by Louis Sullivan is a precursor to the modern curtain wall, but at least the windows still open. The all-glass facade of the Prudential building in Boston - famous for occasionally shedding windowpanes onto nearby sidewalks - incurs high heating and cooling loads.

The usual complaints are the lack of manual overrides and the general inaccuracy or insensitivity of these control systems to what is actually happening at different points in the building. People can’t open the windows for air or control for glare on their own, and they don’t like it. I personally spent years in offices where we taped the vents closed when they blew frigid air down our necks in the summertime – and then the management would come around at night and open them up – then the next day the war would continue, along with incessant bouts of colds and flu.

(Apparently these systems are a lot smarter now, and they actually DO know when the sun is shining in each window of a 15,000-window high-rise. At every minute, for every single day of the year. Just like Stonehenge! But that’s a topic for another article. Meantime, back to comfort.)

The New York Times Building by Renzo Piano incorporates a state-of-the-art multi-layered skin, as well as advanced daylighting controls that know exactly where and when the sun shines into each window, all year round. Loisos + Ubbelohde worked on the daylighting.

Who’s Using Comfort Research Now?

The firm of Loisos + Ubbelohde takes these daylighting criticisms seriously enough to address user comfort – and user behavior – using fairly sophisticated measures developed in conjunction with ASHRAE and the Daylighting folks at the Lawrence Berkeley National Laboratories. Apparently it *is* possible to measure something as variable and subjective as “comfort” with enough precision to make design decisions based on it.

My first questions to the folks at Loisos + Ubbelohde were:

• How do you know that these comfort predictions are valid?
• How do you explain your findings to your clients, who may not have the scientific background to understand all the reasoning behind it?
• How can you clearly and convincingly demonstrate to your client the VALUE of occupant thermal comfort?

Typical office workers huddling together for warmth.

San Francisco Office Building Example

They say a picture is worth a thousand words. So I’ll start with three pictures from a recent analysis done by Loisos + Ubbelohde for an office building in San Francisco. These three color-coded images show how much usable floor space you lose by having clear glass windows in the wintertime that draw radiant heat out of the room, and also from anyone who’s sitting too close to the glass. Basically, if you sit too close to a cold surface, you’ll feel colder – even if you’re not actually touching it, and even if the actual air temperature is the same. A consistently cold surface can, through radiant heat transfer alone, literally suck the heat out of your body.

Each image shows the same office space with a desk, with three different curtain-wall options. The color-coding indicates the Mean Radiant Temperature (MRT) which is a combination of air and radiant temperatures. The yellowish zones are comfort zones with an MRT of 71-72 degrees – neither too hot, nor too cold, for the majority of individuals. Each dot represents one square foot of vertical space as measured on the wall behind the desk, which moves closer to or farther from the window.

Thermal comfort zone on a San Francisco winter's day next to a clear plate glass window requires almost a six-foot setback.

In the first image, you see the “baseline” – clear glass. Right next to the window (indicated in cross section on the right), are almost 6 linear feet of unusable wall space. The blue and green color indicates “too cold” and “much too cold”. In this particular building, that’s a total of 26,000 SF going to waste!

A medium performing curtain wall reduces the arctic zone by half. Now only three feet of wall space is wasted.

In the second image, the desk has moved three feet to the right; and by the last image, showing the best curtain-wall option, total space victory is achieved.

With a better-performing curtain wall, the desk can go flush against the window, with no wasted floor space.

The most interesting thing about this graphic was that it wasn’t self-evident to present the data in this manner – but, once created, these illustrations are immediately convincing, even for non-technical people, of the value of thermal comfort. The study examined a number of other factors, including the feasibility using a perimeter heating system in the winter – but the impact of radiant temperature on thermal comfort remains the same.

Why Not Just Freeze Your Workforce in the Winter?

I have to ask this question, because it’s been the de facto answer for all the years that I was actually an office worker. What’s wrong with treating office workers like the commodities that they are, and letting them suck it up and deal? Why coddle them with expensive thermally insulated windows when cheaper glass will do? Well, aside from the fact that it sounds really bad to say it this way, it’s a poor idea to ignore conditions that promote ill-health. Even mild illnesses can cause absenteeism, or worse. People who can’t afford to stay home will continue to come to work – and less efficiently, too – while sick, thus spreading illness, reducing their own productivity, and prolonging their own recovery time.

Desirable office wear for poorly conditioned buildings can present the wrong image at the annual board meeting.

A 2003 report on sustainable building for one particular facility cites productivity increases of 5 percent and absentee decreases of 40 percent after moving into a renovated facility. [1] Other site-specific studies also cite increased productivity, reduced absenteeism, improved recruitment, and reduced turnover resulting from improved workplace environments. One might add that this would be particularly important for public agencies that don’t always pay competitive salaries, and who can no longer offset lower salaries by guarantees of job security.

How Do you “Measure” Comfort, Anyway?

I’m going to shamelessly plagiarize a summary from an article titled “Window Comfort and Energy Codes” [2] because I can’t say it any better than the writer himself – Jim Larsen of Cardinal Glass. He’s citing ASHRAE Standard 55, Thermal Environmental Conditions for Human Occupancy:

“Comfort can be evaluated with a statistical index called predicted percent dissatisfied (PPD). The calculation of PPD requires a knowledge of room conditions (air temperature, air velocity, humidity, and mean radiant temperature), and the occupant conditions (clothing level and metabolic rate). When comparing two conditions, a lower PPD is desirable as this reduces the risk of occupant discomfort.

Some common examples where cold weather PPD will be improved (lower):

• Increase thermostat setting;
• Adding layers of clothing; and
• Increase level of physical activity.

During hot weather the converse of these will improve comfort as well as increasing air movement and/or reducing humidity.”

(Note: ASHRAE Standard 55-2004, Thermal Environmental Conditions for Human Occupancy, is not a free download. Here’s one place that you can purchase it.)

How Hot Is Really Too Hot?

So far we’ve discussed thermal comfort and warmth, but of course in some climates cooling is a much greater concern, and even modern urbanites are questioning whether we really need it to be 65 degrees inside in the summertime, when that means a 40-degree differential with the outdoors. After all, humans have survived for millions of years without air conditioning, and complaining about the weather at least gave people something safe to talk about.

It turns out that yes, humidity and air movement have a lot to do with how overheated we feel, and a study I can’t locate at the moment was cited showing that subjects in a blind trial were able to perceive 92 degrees as comfortable – as long as there was sufficient air movement.

Stone lattices were a pre-industrial solution for desert climates that filtered incoming light while allowing air movement - although, making it dark enough to be comfortable might be too dark for most types of "productive" office work.

Maybe We’re Just Spoiled Here In America

This is another topic unto itself, but yes, cultural expectations can affect perceptions of “comfort”. However, those same cultural expectations that might induce one to tolerate greater extremes might also be more forgiving of seasonal and even diurnal fluctuations in productivity – AKA the afternoon siesta. Could siestas save the planet? Well… it’s probably an easier sell than telling everyone to just tough it out.

[1] Kats, G., Alevantis, L, Berman, A. et al. “The Costs and Financial Benefits of Green Buildings. A Report to California’s Sustainable Building Task Force”, October 2003, cited in “Occupant Thermal Comfort and Curtain Wall Selection” by Susan Ubbelohde, in the Journal of Building Enclosure Design, Summer 2006, pp 32-34.

[2] “Window Comfort & Energy Codes”, by Jim Larsen,  in the Journal of Building Enclosure Design, Summer 2006, pp 37-38.