Windows are a very important part of our lives - they allow us to see what is going on in the world, they let in sunshine, and they light our houses so we don’t have to use electricity during the day. We need windows.
But they are a mixed blessing. They have very low insulation values so they cost us a lot more than a plain wall would cost to heat and cool. Also, this low R-value means the surface is cold so we can feel cool standing next to them, and they have a lot of moving parts, which means they can leak air.
We want to have windows that have the highest affordable R-value, and that are as airtight as possible.
First, a little terminology - sorry. Window insulation value is listed as “U factor”. U is the same as R, but it is 1/R - or the inverse. When R gets better, it gets bigger, but when U gets better, it gets smaller. U of .5 is an R-2, U of .077 is R-13, and U of .0333 is R-30. Windows have a U factor of about .9 down to about .25, but this a bit confusing, so I will talk about windows in R-value because we are used to that.
Most windows today have an R-2 or R-3 insulative value, while our walls are about R-13 and our attics are about R-38. But since windows are so expensive to change, we must be careful to make a smart investment.
A single pane of glass has an R-1. A double pane has an R-2, and when we double R-value, we cut heat loss in half. So the double pane window has saved 50% of the energy going through the single pane - a very big improvement. If we add Low-E to the glass and fill the space with special gasses, we can get an R-3 and maybe even an R-4 or 5.
New codes require that windows have about an R-3, so this is a good number to consider as a minimum.
Windows have been blamed as a major source of infiltration, and some are. Most windows, however, have minimal leakage, and if they have storm windows, very little. For new windows, the federal government has standards for total window leakage, so even many new windows have some leakage. Double hung windows, like sliding glass doors, have a bit of leakage caused by the sliding mechanism. Casement windows, like your front door, leak much less because they close tightly against a weatherstrip.
Most houses have air leaks that are worth more than air leaks of all of the windows put together that can be sealed for very little money - less cost, more result.
So what do you do with your windows?
If you have double hung windows with a triple track storm window and everything is in good shape, there will be zero payback in changing to a double pane window. That’s right - ZERO. They have the same R-value, and close to the same air leakage. The local Home Performance Programs give no credit for changing windows because they save nothing in most cases.
If you change that window to a Low-E, double pane window, you will save about $4 to $7 a year. If the window costs $400 installed, your payback will be 57-100 years, just in time for your (great?) grandchildren to enjoy. Not a good idea to change windows just to get the Low-E benefit, but if your windows are in bad shape, you should include Low-E on the replacements.
If you don’t have storms but the windows are in good shape, consider a modern triple track storm with Low-E glass in it. These storms can be mounted outside as usual, or inside if you have an historic house. You will get R-3 and good airtightness out of this combination.
There are 2 exceptions to our "no windows" rule. If you have single pane aluminum sliding windows, so many of which were installed in this area in the sixties, consider changing them. And if your windows are in bad shape, consider changing them.
We shudder to think of all the wasted money that has been put into windows. DON’T BE THE NEXT VICTIM!