Low-SHGC in Ski Home southern wall of windows? Did I make a mistake?
Just ordered windows for my full-time home in Vermont’s Green Mountains. The huge 12 ft. picture units will result in a 13% ratio of south-facing glass to conditioned floor space. (Did it for the gorgeous view.)
I ordered Andersen Smartsun coatings, with a low 0.24 SHGC, but was that the right move? I could have ordered 0.54 SHGC Passivesun—or skipped the coatings, saved $500, and gone with 0.36 SHGC. All options are Low-E4 with HeatLock.
Reading on this site, my first thought was that I messed up BIG time. The low SHGC will mean forgoing winter heating potential here in ski country, right? And despite a 0.23 U-factor, that much glass will seriously increase my heating load. (Probably need some cellular shades or something for nighttime?)
On the other hand, a 13% glass-to-floor ratio is high, so maybe I’ll still get some solar heating on sunny winter days? And to that point, most winter days on our mountain aren’t all that sunny, with a clear day being quite rare, even if there’s sunshine down in the valley. I also have wood floors with no significant thermal mass.
With no A/C or overhangs, my bigger concern was summer cooling. With so much glass, I was afraid of turning the place into a solar oven. (That might happen anyway. Might need to invest in some mini split A/C units.)
I might still be able to change the order to a different SHGC option, but should I? Please help, and thanks!
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Powderski, the only way to know for sure is to do energy modeling. But as general advice, I'd say that on the coldest days of winter you'll regret not going for higher SHGC. The rest of the year you'll be glad you went with low SHGC, especially on warm or hot shoulder season days where you also get a low sun angle. If you don't want to have someone do an energy model, I would keep your order as it is, and invest in air-sealing other parts of your house (and insulation, if appropriate) which will make it easier to heat in winter and cool in summer.
Hi Mike, Do you think .5 is a decent solar gain for south side? I see many PH high SHGC windows are more like .6-.65.
And is there any reason to upgrade to low SHGC on windows that won't get sun anyway (like the N?)
Thanks, Michael! I was concerned about this, but you're helping set my mind at ease. :)
qofmiwok, I don't recall needing or wanting SHGCs that high for high-performance homes here in Maine, where shoulder seasons (and more glazing than ideal for performance) make overheating an issue. I use energy modeling to find the glazing details that work for each house. Every project is different so that's really the only way to make a fully informed decision. That said, I often find that SHGCs of 0.4-0.5 are high enough.
You can use higher SHGC on the north. The reasons might be to save money, decrease U-factor or raise the VT--visible transmittance. I would not use high-SHGC on the east or west because those facades see sun at a low angle and can significantly contribute to overheating.
I'm in the exact boat including the ski town. The consensus lately seems to be the solar gains are minimal (in terms of contribution to heating) and not worth worrying about. But personally I want some high solar gain windows for emotional reasons. I (and the dog) just like the feel of warmth from sun, especially in the morning. So I was planning that on the south, which does have big overhangs. But I'm leaning toward Zola R7 windows and just learned they don't have high solar gain. Their standard is .5 and their low .29. I'm starting to compare what all the coefficients are by the different manufacturers. Maybe .5 is a decent amount for the south. My "best" view is West so that wall is almost all windows (although my ceiling is only 10' so they aren't as tall as yours). I probably should do the .29 there.
I'm not too worried about overheating because my friends built in my town 3 years ago, similar amount of glass facing west, way lousier windows than mine, and their house is always freezing even in summer. So I am *hoping* to get some heat through my windows. But I'm not sure if your mountain climate is the same as ours here in the west. Here in the west it's 50 degrees at night even in summer, and there's no humidity. So it's actually pretty hard to get too warm unless you're really dumb with your window walls.
Having lived in Vermont, I'd go with maximum solar gain in your location. But like qofmiwok we're designing for Colorado, 7300', S facing lot, LOTS of sunshine. We have ~400 sq ft of S facing windows and the estimated heat gain from the Sustainable by Design calculator (https://susdesign.com/windowheatgain/index.php) in winter is staggering. If the estimates are anywhere near correct, we'll be fine in summer (w/ eaves) but opening windows in winter to cool things down.
I have triple pane windows with Saint Gobain .54 SHGC glazing and I love them. Nothing like the feel of warm natural sunlight on a cold winter's day. When I got up this morning it was 22 degrees outside and 66 degrees in my south facing living room (heat pump was off). By 10am it was 25 degrees outside and 69 degrees in the living room. I also like that the glazing in these windows have high VT. I am not a fan of windows that look green or blue. I like windows to look like clear glass.
I have blackout shades in the south facing bedrooms and they do a good job of keeping the rooms from overheating. However, we use one of them for an office and with the shades up, it got up to 73 degrees in there today and it felt good.
If I recall correctly, IRC prescriptive method require a SHGC of .3 or less, so you may need to use the performance method if you want to go high SHGC.
Good points about VT. I like to have 0.4 or higher, and for all of the windows to have similar VT even if the other performance factors vary.
The IRC requires 0.3 or less for a U-factor in climate zones 5-7 but SHGCs are not specified for those zones.
https://codes.iccsafe.org/content/IRC2018/chapter-11-re-energy-efficiency#IRC2018_Pt04_Ch11_SecN1102
Michael,
Yes, the SHGV limit is only for zones 4 and below, so not relevant to the OP.
Thanks, everyone, for all the input! The picture units I've ordered have 0.54 VT. That's relatively good, right?
And regarding SHGC, this morning we woke to 0° F, so this was definitely one of those days where in future I might regret choosing low-SHGC. Hopefully the summer will make up for it.
We certainly had condensation at the base of the current windows this morning. I'm guessing the new windows with HeatLock will probably have the same issue on these cold nights.
PowderSki,
.54 VT is very good, I think you will be very happy with that.
As far as overheating is concerned, it is more of a shoulder season than summer season issue because of the height of the sun in the sky. The summer becomes an issue if you have a lot of western facing windows as the late afternoon sun is low enough to beam its rays directly at those windows.
I have not had any interior winter condensation issues and I don’t think you will with your new windows. However, other times of the year we will get condensation on the exterior of the windows which is a good thing as it shows that they are limiting the amount of heat escaping from the house and allowing the exterior pane to get cold enough to drop below the dew point.
I'm firmly in the camp that says always go as low SHGC as you can. Every window, every wall. Let your HVAC system handle the HVAC.
Sometimes my mind wanders a little and I wonder if I should push the SHGC up a little to get higher VT. Then I come back to my senses.
Right now I'm in Maine, basking in warmth coming through my big south facing window array (11' x11') SHGC of .49. I've never felt it overheats the space. I have a small portable thermal mass cat who is melted into the floor, though.
You went with Anderson, so you did not make a mistake. I have a large Anderson picture window in my garage. It is about 10 feet long, I installed it for the view, a beautiful double pane window. When they installed it, it was so windy, the installer from Home Depot nearly got killed when the window blew out and fell down the hill. Well, they ordered another window, and this time, prepared better. What do I think about the window, it is awesome. I also have Anderson sliding doors, simply awesome. In the garage, birds will sometimes fly in and hit the window because the window is so inviting. That is how good the window is. Don't worry about the heat, if the windows are double pane, not much of a difference over time. If you are skiing in Vermont don't worry about windows. Start collecting more skis. My brother has the Nordica Enforcer at 93 under foot. I did not like the ski that much at Alpine Meadows, CA, and would go with 100 under foot. The details matter, must like in windows. But the bottom line, any good modern window is such an improvement, you won't feel any difference. Get out there and ski and forget about the slight difference in R values.
Right now I'm in Maine, basking in warmth coming through my big south facing window array (11' x11') SHGC of .49. I've never felt it overheats the space. I have a small portable thermal mass cat who is melted into the floor, though.
I have some large south facing windows with low-ish SHGC (.29 ballpark). They still warm the place up considerably-even when it is -20F. We do have some over-heating though in the fall. I fixed that by building some exterior shutters that slider over the windows. Problem solved.