Back when I was starting out as a general contractor, I relied on the salesperson at my lumberyard to determine the window performance level needed. After all, a window is a window, right? Well, as it turns out, no. There are a lot of window selections that should be made based on the location of the project and specific goals of the home. For instance, if you live in a cold climate, you may want to choose a window with more solar heat gain and a lower U-Factor. If you live in a hot climate, less solar gain may be needed.

All this information, and more will be listed on the certification label that’s placed on the window before it leaves the factory. That label is unique to the window configuration and includes performance information specific to its type (casement, double-hung, picture, etc.), materials, glass coatings, and other features. I’ll explain how to put that data to good use but first, a little background.

Visible transmittance measures how much visible light enters through a window. It’s listed as a range between 0 and 1. One is the maximum amount of visible light and 0 would be a window that you’d have a hard time seeing through. Usually, more panes of glass and different glass coatings that help improve window performance will also reduce the amount of daylight moving through the window, which results in a lower visible transmittance number.

Air infiltration measures air leaking through the window assembly itself, not air leaking around the window due to any air sealing deficiencies that might occur during installation. Air that leaks through the different parts and pieces of a window will affect comfort and the operating cost of a home. The number range is from 0.1 to 0.3, with 0.3 being the maximum air leakage allowed.