We have a passive solar home, so we used neither of those in our south windows. We spec'd windows with LoE 179 instead. All other windows have 272 glass.

Get the u and SHG values of the windows you are considering, not the glass they are made of. It is the rating of the window itself that is important. Typically, a high u means a low SHG. The reverse is also true.

If the windows are right on true south or close to that, and you have the proper overhangs to shade them in summer, then you should really worry about the u value in the summer and the solar heat gain (SHG) value in the winter.

If solar heating is the most important, concentrate on the SHG value. If summer cooling is the most important, concentrate on the u value. The lower the better. U is the reciprocal of R and vice versa. If you are more comfortable dealing with R values, just do a little math and use them instead.

I suspect you will find that windows made with LoE 366 are not cost effective, unless they get a lot of sun in the summer. Generally, these are west windows or unshaded south windows. If you have a lot of west or unshaded south windows, you need all the help you can get.

Interesting thank you!

Ok the Jeld Wen we are interested in are

272 - U =.29 and SHG = .34
366- U =.29 and SHG = .22

Same with sliders and awnings you could see a difference

272 (awnings) U = .29 SHG = .27
366 U = .29 SHG = .18

Weird the U is the same on 272 vs 366. Does that seem strange? I got these numbers right from the Jeld Wen website.

Thank you for the info, creek side, I had no idea about this SHG rating.

Also, the builder spec'd out the windows with Argon. Do we need that?

Yes on the argon. If the U is the same, make your determination on how much sun will hit your windows in summer versus winter. SHG in the winter is good. In the summer, it is bad. Make your decision base on how shaded the window(s) will be in summer.

I think the 266 might be better then, as we are in the Midwest where we have about 3 months of summer lol

Thanks again!

It sounds like the 266 is better for you. But it all really depends on orientation, climate, shade etc. There is no one answer. Even mixing windows on a house might be a good idea.

The Midwest is a large area - could be Minn - could be St Louis. You might have a/c or you might not and are willing to spend some money so that you can continue to live without a/c.

If you are predominately cold - then obviously a little more solar heat gain is a good idea. U value becomes the most important. If you have some heat - then low solar heat gain is important if you are unshaded and the windows get sun in the summer. I live in mixed and my North windows never get direct sunlight - SHG doesn't really matter there. Now my West windows - it would make sense to pay for less SHG. And generally in South windows, a higher SHG is a good idea to maximize winter heat gain.

So the answer depends on too many things to say one is better than the other. But in the midwest, you probably want relatively more SHG.

Not a simple subject and you would hope that your builder would have some guidance in your particular situation. If not your builder, then some energy consultant. It seems like an energy consultant (like Energy Star) should almost be standard on new construction since a lot of builders don't seem to care.