We’ve learned a lot in the last decade about building new structures to new levels of efficiency. However, windows have not kept up to the level that many other building materials have when it comes to thermal resistance (aka R Factor). Most available windows on the market today are coming in with a U-Factor of around .3. So, we know that we can convert U-Factor to R Factor easily (done so here).
What really matters is that most windows on the market equate to around an R-3 in insulating value. Why is this important? Because the 2012 IECC Energy Code is requiring in Zone 4 that walls must have an R-20 or R-13+5 to meet code. So in this R-20 wall we are going to cut a big hole into this wall and install a window that is only going to perform to an R-3 level. Makes a lot of sense doesn’t it?
I am not saying you should not install windows in your walls, I am saying you should choose the best quality of windows. You should also keep the wall to window ratio as low as you can. Wall to window ratio can fall anywhere from 10-30% to as much as 100%.
As an example how this effects the total wall R-value and the performance of the wall, let’s figure an 8x10’ wall or 80 square foot. In this wall we are going to install a 2x4’ window or 8 square foot of window. This wall now has 10% wall to window ratio. That’s not too bad! Now double it with two windows for a total of 16 square feet or 20%. So 20% of our (R-20, used loosely) is now only an R-3. What is really unfortunate is that this still passes code.
Now if we had only an R-13 cavity insulation in the whole wall, it would not pass. But this does not make a lot of sense in my opinion. There is hope: technology is making most everything better every day. Even window technology is gaining. I recently ran across a new window that can come up to an R-6 to R-20. Yes, I did say R-20.about their product.
So how do windows tie into spray foam insulation? Well, they don’t really directly. But when we look at buildings as a whole, not just piece by piece, we can achieve great results. Adding all the parts to the puzzle create energy efficient, healthy, and most importantly, comfortable buildings. So, leave it to a spray foam insulation contractor to sell you on buying good windows. And no, we don’t do windows!
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