Do I want low-SHGC windows on the east and west sides?
I live in sunny New Mexico and am doing research for having my ancient, awful aluminum windows replaced. I’m torn on the question of whether I want low solar heat gain coefficient (SHGC) windows, though. My house faces east-west with only two small, unshaded windows on the south side. Most of the windows are east or west, and these windows cause the house to gain undesirable amounts of solar heat in the summer, especially a large west-facing picture window which makes the living room really hot. Most of the east and west windows are fairly well shaded, but still let in a good bit of direct sunlight in the summer, especially in the afternoon. However, they also let in a lot of desirable solar heat in the winter when the sun is lower and gets under the shading more, especially a large sliding glass door on the east side which really helps to prime the house with heat in the morning.
So I don’t know if I should be spending more for the fanciest LoE coating available for these east and west glazings. On one hand, it seems like it would help with summer overheating, but if it’s going to block an equal amount of desirable winter solar gain, what’s the point? Should I go with a less-serious LoE coating on the east and west windows? What about the south ones? There are only two; does it really matter much?
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Nathaniel,
The point is comfort. In your current situation you aren't comfortable when the sun is shining directly on the west facing picture window so why would you want to spend money on upgrading your windows and still be uncomfortable when the sun is shining directly on the window.
Using a high solar gain LowE coating on west facing windows is almost always a bad idea, even in cold climates, because while the free heat gain is great in winter it is a lot less great during summer and during the transition months.
High solar gain on east facing windows are a bit more ambiguous in part because a lot of people like the warmth of the early morning sun even when the weather is reasonably mild.
Per your thoughts on paying more for the "fanciest LoE coating", I am not sure what you mean.
LowE coatings come in high, moderate, and low solar gain varieties. Unless you are in Canada the low and moderate coatings are much more readily available from most window manufacturers. High solar gain coatings are often considered to be a specialty item and can be difficult to find depending on the window company.
Using a high solar gain coating on east/west facing windows will allow a high percentage of solar heat into the room and the coating will do (as designed) an excellent job of keeping that heat in the room, winter or summer.
Per the south windows, if the window company you are talking with offers high solar gain coatings then go for it - can't hurt.
Thank you for your answer, Greg. That all makes sense to me.
Nathaniel,
Greg has given you good advice. For more information, you may want to see All About Glazing Options.
Martin,
I read your "All About Glazing Options," but it seems to give different recommendations than Greg. He says, "Using a high solar gain LowE coating on west facing windows is almost always a bad idea, even in cold climates." But you say, "If you are in a cold climate, you probably want a double- or triple-glazed high-solar-gain window. If you settle for a double-glazed window, look for these specs: A whole-window SHGC of 0.42 to 0.55. Higher is better than lower."
I'm confused. I live in Rhode Island, and I'm on the verge of ordering Andersen 400 windows with HeatLock and PassiveSun for a master bedroom west wall. It's lakefront, so we're interested in maximizing view, with a large picture window flanked by two casements.
The picture window has U-Factor = 0.23 and SHGC = 0.50; the casements, U-Factor = 0.25 and SHGC = 0.47. VT is also much higher with PassiveSun. If we eliminate the PassiveSun, we get U-Factors of 0.22 (picture) and 0.25 (casement), with SHGC = 0.33 (picture) and 0.31 (casement).
I ran heat gain estimates, and got 190,000 btu/ft^2 per year with the PassiveSun and 120,000 btu/ft^2, with most of the gain coming during the summer months (as expected). Still, I'm inclined to go with the PassiveSun (partly based on your article) because the room gets cold in winter (baseboard hot water heat), and in summer we usually get by with open windows catching a westward breeze from the lake, and we'll have a ceiling fan over the bed. Right now we use A/C maybe 2 weeks per year.
Can you clear up the confusion between the two recommendations and advise on the choice of windows?
I would reconsider using PassiveSun on a west-facing wall, especially if it is a large picture window. I made the mistake of letting my builder talk me out of SmartSun on such a window, and even the standard Low-E4, and it's brutally hot in summer. When I did the rest of the house (Downstate NY), I used SmartSun/Heatlock and am quite pleased. I'm even considering replacing the sash of the picture window with a SmartSun version. BTW, if you're looking at other companies. "PassiveSun" is Andersen branding of Cardinal LoE-180 and "SmartSun" is LoE-366. "HeatLock" is LoE-I89. Careful of the Heatlock, if your climate is particularly cold, condensation can be an issue. The Heatlock actually reduces the temperature of the glass. Triple-pane might be safer.
You are welcome, Nathaniel.
Thanks Martin.
I’m no expert, but I had a similar question when renovating my house recently, and I’ll pass on my experience now that it’s finished and I’ve lived in it for about 9 months. Even though I’m in a northern climate, I followed the advice of people more knowledgeable than me and chose a low SHGC glass for the very large expanse of floor-to-ceiling windows on the west side of my house. I’m grateful every day that I did. Even with the low SHGC glass, I get plenty of free warmth from those windows on sunny days in the winter. And now in the summer, the room is fine, which it clearly would not have been with any more solar gain.
Thanks, TC. This is very helpful.
Another option for east and west facing glass is Suntuitive self-tinting glass:
https://suntuitiveglass.com/specifying-suntuitive/
We have it in all the windows of our west facade, mainly to give us an unencumbered view of our backyard but also to avoid the need for window treatments, regardless of the time of day or season. The glass definitely works as advertised.
We give up some heat gain in the winter because of the low U factor, but it's irrelevant for us since we get ample solar heat gain from our southern windows, which were designed using Passive Solar principles.
You can read about the details of the specs and installation, along with our experience living with the glass here:
https://kimchiandkraut.net/2018/11/12/windows-and-doors-air-sealing-8/
It would require a bit of legwork to find out if it's compatible with your brand of window, but contacting Suntuitive directly should make this relatively painless.
If building again, we'd definitely incorporate the use of the Suntuitive.
>"We give up some heat gain in the winter because of the low U factor..."
The U-factor is almost completely unrelated to solar gain. A low U-factor does not preclude a high SHGC- they're not directly related. Cardinal Glass (and others) have double low-E double pane glass with U-factors close to U0.020 , with an SHGC north of 0.6.
According to the graph in page 2 of Suntuitive's brochure showing the temperature dependency of a representative double-pane Suntuitive glazing, the SHGC is a bit more than 0.35 when it's 20C or cooler, but that only drops to a bit less than 0.30 when it's a fairly toasty 40C, and only hitting 0.20 at a smoking too-hot-to-touch 55C. The cut in visible light transmission is a bit steeper than that, indicating it's better at cutting solar glare than cutting solar gain. The glare cutting benefits are even more evident in their graph of skylight performance on page 3.
https://suntuitiveglass.com/wp-content/uploads/2019/11/Tech-broch.25112019_PL_New.pdf
Dana's right. If you follow the link to our blog and look at the spec sheet for our Suntuitive, I should have referenced the SHGC that varies from 0.18 in its clear state down to 0.07 when the glass is fully tinted.