I live in sunny New Mexico and am doing research for having my 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 windows on the south side (definitely going LoE with these). 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 just gets scorching hot. The east and west windows are semi-shaded, but still let in a lot of direct sunlight. 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.

So I don't know if I should be spending more for the fanciest LoE coating available for these east and west windows. 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? Would I be better off getting less LoE and putting up adjustable blinds or something?

2 Answers 2


Using LBLs solar gain modeling software (RESFEN) can be instructive. I don't know if they have finally made a less cumbersome version of it yet - last I used it it was still Windows-only and ripe for redoing as a web app. Looks like it's still stuck on Windows PCs only. There are few other tools there on the page linked to as well.

E&W actually have more summertime gain than S (northern hemisphere - swap north for south if on the other side of the equator) due to the solar angles being low during the morning and evening. South windows are more easily made seasonal with shading that keeps the sun off them in summer and on them in winter.

If blinds are on the inside of the window, the heat is still inside the house, with the blind that stopped the sunbeam - exterior blinds or awnings are needed to really keep heat out with a shading product, AFAIK.

I think the most effective way to make use of solar heating for winter is to separate it from your windows, using dedicated collectors. In addition, it's actually become quite difficult to source windows that are good heat collectors (I do understand that what you are really doing is choosing between levels of SHGC - but I wanted some high solar gain but well-insulated windows for my south facade and ended up giving up, as the only ones I could find to buy would have been importing from Canada at 3 times the cost of the low SHGC windows that were all I could get locally - in a mostly-heating climate.) I can build or buy solar collectors with the cost differential I would have had to come up with for the windows I wanted as opposed to the ones I got.

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    OMG, only the government could bungle software distribution so badly. I have to sign up for an account just to download an outdated piece of Windows-only software? Welcome to 20 years ago...
    – iLikeDirt
    Commented Jun 13, 2014 at 19:27
  • 1
    Yep. Must be nice not to remotely meet minimum expectations of 10 years ago for a thing that's supposed to be of service to the public. It simply cries out to be a handy little Java web app, but Nooooo....
    – Ecnerwal
    Commented Jun 13, 2014 at 21:05
  • That's the problem: it's free to the public. No profit in that, so there's no incentive to improve it. They should at least release the source code so the Linux monkeys can improve the usability (lulz).
    – iLikeDirt
    Commented Jun 13, 2014 at 21:29

I managed to get RESFEN working, but it gave me nonsensical results, even when my inputs were validated by experts. I don't recommend that anyone use it. I ended up doing my own Manual J calculation and came to the conclusion that in terms of energy, whether I use low or high solar heat gain windows on the east or west doesn't matter very much, but low-gain windows slightly win out. In the interests of comfort and efficiency, I'm only putting high-gain windows on the south side.

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