What type of glass to use for windows
We are building a house in Southwest PA (Climate Zone 5) with Andersen 400 windows and wondering what glass options we should go for. The front of the house will be facing due west (unideal I know, but that was the only option). On the first floor, only the only room on the west side (besides garage/entryway) is the study. We both work from home so this will be one of our offices. There is no porch/overhang over the windows, so they will be getting hit pretty hard in the summer, but winter should be better. The Kitchen and Great Room are on the east side and there are plenty of windows there, as well as some on the north side and large 1 in the south in the dining room.
First Floor: https://i.imgur.com/lkjh8Lz.png
Second floor has way more windows on the west wide. The southwest bedroom is presently just going to be a guest room, though that may change as we have more kids and they get older, so we aren’t as concerned about comfort yet. Our bigger concern is the upstairs office where one of us will work and the nursery.
Second Floor: https://i.imgur.com/UtQHLWw.png
Andersen has 4 main window options we are looking at. They are:
| Name | Type | U-Factor | SHGC | VT |
|---|---|---|---|---|
| Low E4 | Unknown | 0.3 | 0.31 | 53% |
| SmartSun | LowE-366 | 0.29 | 0.21 | 48% |
| HeatLock | i89 | 0.26 | .31 | 52% |
| SmartSun w/Heatlock | LowE-366 with i89 interior | 0.25 | 0.2 | 47% |
Right now everything is spec’d to be the basic Low E, but we were thinking of upgrading all the west facing to at least SmartSun and possibly SmartSun w/Heatlock to help in winters and having all others be HeatLock. Does this sound like a good idea? Does anybody have a rough idea how much extra these coatings will cost over the basic Low-E one?
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
pahome2023,
FWIW, I went with the lowest available SHGC for all windows even though several face north. Our house design does not have enough overhangs for southern windows and very little for east and west. I went round and round on the supposed winter warming value of a higher gain choice and I do not regret going with the lowest SGHC. My east and west windows are the largest areas and the summer heat load even with a .24 SGHC, is still high. The shoulder seasons are not a whole lot better.
I quite deliberately minimized my southern window area based on experience with two other homes in my area. I also bumped up to triple pane, though being in CZ6B makes that more "reasonable" than for some CZ5 areas. I would still gently push for trying to go triple pane as you will reduce your AC demand. Probably more useful than heat in your area. Depending on available tree shade, the exposure for your west windows may seem quite long as you try to work.
I don't think the Heatlock is a good option. As you can see, the solar gain is almost 50% higher than the Sunlock. The nature of the I89 coating is to bounce inside heat back in, so all that solar gain will be held in the room more effectively. Sounds good in February, not so much in July. I also have seen comments here on GBA that the coating causes the interior glass surface temperature to fall lower than interior glass without. The heat that is bounced back into the room doesn't warm the glass if not passing through it, so the glass cools. Depending on your interior humidity levels, the surface may prove to be a condensation problem during the coldest months.
As to the visible light transmission ratings, you will not notice even a 45% level unless you open the window and compare thru glass to thru air sight lines. Once again, do consider triple pane. It may not make ROI sense, but summers are not getting cooler and you only want to do windows once if you can. Also, be aware that shades don't really do much for rejecting summer heat. Once the sun is inside the window, all you have is a very warm shade and a hot air gap between it and the window.
Keep it simple. What Andersen calls LowE4 is Cardinal's LoE-272 which is ideal for other than west facing windows in your climate/environment, and isn't a bad choice for west windows as long as the glass area isn't overwhelming.
If you are really concerned about overheating in summer, then switch up the west side to L0E-366 and you should stay comfortable winter and summer.
You mention Heat Lock (I89) performance by itself in your post....did someone tell you that you can get that coating without combining it with other coatings in an IGU?