Understanding Thermal Window Replacement Terms

U Factor Rating

An organization called the National Fenestration Ratings Council (NFRC) has developed a rating system based on the U factor. The U factor is the rating most homeowners look at first and for good reason. This rating measures the heat loss that occurs through the window. Most homeowners choose to install thermal replacement windows because of the heat loss that occurs in the winter with their old windows. Most windows now carry this rating so it's becoming easy to make comparisons. Generally speaking, windows all have a U factor between 1.3 and .2, and the lower the number, the better.
Window Type U Factor Ratings

• Old metal casement window 1.3
• Good quality single-pane window 1.0
• Good single-pane with storm window .6
• Double-pane with low-E glass .4
• Triple-pane with low-E glass .25


Air Leakage:
This rating measures how much outside air will infiltrate through your window installation. Fixed and awning style windows are usually the best. The range is typically between .1 and .3, and the lower the number, the better.
Solar Heat Gain Coefficient: Naturally, this is the exact opposite of the U factor. It measures the windows heat gain during the summer months. It's expressed as a number between 0 and 1 and, again, the lower the number, the better.
Condensation Resistance:
This rating measures the window's ability to resist the formation of condensation. As ventilation and insulation continues to be geared more toward energy-efficiency, condensation, especially on windows, is becoming more and more of an issue in homes—in this case, the higher the number, the better.
Visible Transmittance:
This window rating has more to do with homeowner preference than superior performance. VT measures the amount of light that is allowed to pass through the window. The higher the number, the more daylight you can expect in your home