What is a U-factor, and what number should I look for?

U-factor measures how well a window keeps heat from passing through, and lower is better. For a cold climate like Chicagoland’s, look for an ENERGY STAR rated window, which generally means a U-factor at or below about 0.30. Treat that as a plain guideline, not a hard rule for every home. We explain the ratings in clear terms during your consultation so you are comparing real performance, not marketing.