Window Building Science or just plain window BS? You decide. A lot of home energy auditors feel that windows are over-sold, meaning that the typical window salesman exaggerates the benefits of new windows. Let’s run some numbers and you can decide, because in truth, like most things in building science, it all depends on the specifics.

For our example, we are going to use a 36” x 60” window, or 15 square feet. We’re also going to assume that our example home has an 80 AFUE natural gas furnace, meaning roughly that it runs at 80% efficiency. The home also has an air conditioning unit for cooling with an efficiency of 13 SEER, the federal minimum standard. The 80 AFUE furnace is probably about average and the 13 SEER A/C is probably a little better than most installed units. (Those homeowners in rural areas who have electric heat are going to have heating savings substantially higher than what we calculate below.) Further, we’re going to pretend this house is in St. Louis, Missouri (so it has 4900 HDD and 1527 CDD). So now we’re ready to go.

Let’s say we have an old single glazed wood frame window and we purchase a brand new ENERGY STAR replacement window for it. Uninstalled we could expect to pay about $109 for the new window. Remember, that’s uninstalled. Given the assumptions we made above, we can expect to save about $9.92 in heating costs and about $16.36 in cooling costs annually. In other words, that may not be a bad investment, especially if the windows are leaky and the installation cost wasn’t too exorbitant. By “leaky” here I mean air leakage since if they leaked water they’d probably have been replaced long ago – or should have been.

But most of us don’t have old single glazing type windows alone. They probably had storm windows added to them a long time ago, and that presents a completely different scenario. Replacing a single glazed window that has a storm window with a new ENERGY STAR replacement window (same price), we might expect to save only $3.14 in heating costs and $5.18 in cooling costs annually. It’s going to take some time then to pay off that new window. Now, if this is the meager savings I get with a single glazed window with a storm, you can pretty much assume that if you have an old double-glazed window which a window salesman is trying to get you to replace, that your savings wouldn’t be any better.

Ok, ok – let’s say we did have an old single glazed window without a storm. It would cost us about $46.00 to purchase a storm window for it (uninstalled again). Once installed we could expect it to save us $6.78 in heating costs and $11.18 in cooling costs annually. Wow! But, true, there is the hassle of putting them up and taking them down each year if you are so inclined to do so.

So that’s window building science in a nutshell. Given the savings above, it’s probably better to spend the window money for energy efficiency upgrades elsewhere in the home. But not always. There are some situations where new windows is a very good investment.

And finally, for those of you engineering types who just can’t put your pencils down, I’m using $1 for the cost of a natural gas therm and $0.086 for the cost of kWh (read this and weep, Californians). The U-values used for the window examples above are:

.9 – Single glazed wood frame

.3 – ENERGY STAR qualified window

.49 – Single glazed with storm window

That should be all you need to check my calculations.