What is the “MPG” of your windows?

What is the MPG of your windows? Sounds like a silly question right? After all, a window can’t actually be measured for miles per gallon.  That said, a window CAN (and is) be measured and rated for its performance and efficiency, very similar to the MPG on your car.

There is a long list of nerdy, complex ratings for a replacement window, both for thermal and structural performance. The big ones discussed by most experts however, would be U-value and Air Infiltration. Most homeowners learn enough about windows when shopping around to compare the U-value (essentially measures how well the window insulates), but Air Infiltration is not talked about quite as much. This is particularly ironic given that a top three complaint about your old windows will be “they are too drafty”.

So why isn’t this rating talked about more frequently? Frankly, because most windows stink in this area. It would be like a car maker advertising that their new model gets 8 MPG. Not gonna happen. That leaves the ball in your court as a consumer, to ask for and verify these ratings so that you can make an informed buying decision.

For a reference point, a window with an Air Infiltration rating of .20-.30 cfm(lower is better) is about like a gas guzzling truck or suv. Maybe 8-12 MPG. On the opposite end of the spectrum, you can find windows in the range of .05-.01 cfm, and those would be more like 40-50+ MPG hybrid or electric car. Of course there is everything in between as well.

Lastly, one of the best things about considering these ratings– air infiltration in particular– is that unlike mileage, these ratings can be great indicators of a window’s quality in addition to it’s performance and efficiency. A small, junky car with a small engine can still get pretty good gas mileage, but for a window to be very airtight, it needs to be engineered and built to exacting tolerances.


Brandon Erdmann is the owner of HomeSealed Exteriors in Milwaukee, WI.  He’s a window installation expert, a supporter of this site and an all around great guy.  If you’re in the Milwaukee area and you’re thinking about new windows you should give Brandon a ring.  You’ll be glad you did.