Guide To U-Factor

Our guide to U factor can help you understand more about this energy efficient metric and what it means for your home windows.

Basics Behind U-Factor

U Factor is a measure of the amount of heat loss from a window that ranges anywhere from 0.15 to 1.20. The lower the number, the less heat escapes through the window and therefore the more energy efficient the unit. U factor, often referred to as U-value, is one of the main performance numbers for consumers to look at when buying replacement windows for your home. Other important measures include SHGC, air infiltration and design pressure rating.

Ideal U Factor

Ideally, a window will have a U factor of .28 or below. A mid range vinyl window will have a U-factor of .30. Of course, there are huge variations in U-factor based on the window frame material, glass package and any and all upgrades and adds on that a homeowner opts for. In general, fiberglass and vinyl windows will have the best performance numbers. The best way to drive down U-factor is by upgrading to a better low-e glass or glass package.

Type Of Window
Frame Material
Quality Of Window
Type Of Installation
Number Of Windows
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Sample U-Factors

The following are sample U-factors from popular window manufacturers. Check out our Energy Star program article to see the role that U-factor plays in getting this energy efficient rating.

Alside Excalibur » .35

Simonton Reflections 5500 » .30

Champion Vinyl Windows » .29

Sunrise Restorations » .26