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.