U-factor is the rate heat escapes through the whole window assembly, glass, frame, spacer and all. It’s measured in Btu per hour per square foot per degree Fahrenheit, and the scale runs the opposite way to what most people expect. Lower is better. A 0.25 window loses heat more slowly than a 0.30 one.
The number comes from the National Fenestration Rating Council, and it’s printed on the sticker of every rated window, so you can compare two quotes directly instead of taking a salesperson’s word for it.
One caution. U-factor describes the unit, not the hole it goes into. A window rated 0.24 that’s shimmed crooked with a gap behind the trim will underperform a 0.28 window set square and sealed properly. The rating is the ceiling on performance, and the installation decides how much of it you actually get.
Any ENERGY STAR certified window carries an NFRC label with its U-factor on it, so ask for the number on each option you are quoted. Our page on St. Louis window replacement lists the styles and glass packages we install.
What Is a Good U-Factor for Windows in St. Louis?
St. Louis sits in ENERGY STAR’s North-Central climate zone, where a certified window has to hit a U-factor of 0.25 or lower and an SHGC of 0.40 or lower. So 0.27 or 0.28 is a decent window, just not a certified one here. If certification matters to you, 0.25 is the line to clear.
