Ratings for Replacement Windows | Compare Performance Numbers Of Top Models


Home : Choosing Replacements : Understanding Window Ratings

Explore Key Ratings for Replacement Windows

Performance ratings for replacement windows are important indicators of how well they will insulate your home and, to a certain extent, how well they will hold up over time. We will first go through the main ratings for replacement windows and explain their importance.


U-factor is a commonly used term that refers to the amount of heat loss of a particular window, either though the glass or frame. U-Factor will a number that ranges from 0.2 to 1.3. The smaller the U-Factor, the smaller the heat loss and therefore the more energy efficient the window.

Air Infiltration

AI measures the amount of air that passes from the exterior of the frame and glass to the interior - AI will be a number that ranges from 0 to 0.3. The lower the air infiltration, the less air passes through and therefore the more energy efficient the window.

Solar Heat Gain Coefficient

SHGC is the amount of heat that passes through the glass and/or frame - SHGC will be a number that ranges from 0 to 1.0. The lower the number, the less heat passes through. SHGC can be tricky though because in hot climates, you generally want a low SHGC but in cold climates you want a higher SHGC to trap the heat inside the home.

Visible Transmittance

VT is the amount of light that comes though the glass - VT will be a number that ranges from 0 to 1. The lower the VT, the less light is allowed to pass through the glass.

How Important Are These Ratings

The first three - U-factor, AI and SHGC - are all important numbers to indicate how well the window performs. In general, you want a window with a U-factor of 0.35 or lower, a AI of .1 or lower and the SHGC will depend on you location and climate. The NFRC provides a great service where you can type in the window and see the performance numbers for different windows.