The U-Factor for replacement windows

Modern windows rely on a wide array of technology to realize a level of energy efficiency as high as five times that of traditional windows. A basic understanding of thermal replacement window ratings will help you choose the window that’s best for you without blindly relying on a sales pitch.

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. 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