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.
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
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.
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
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
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.
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.
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 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.
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.
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.
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.
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.
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.
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.
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.
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.
About the author
Rebecca Firestone has been working in the Bay Area since 1998 as a technical writer, business content developer, architectural filing lady, marketing director, and sorcerer’s apprentice.