U-Factor Rating (U):

The lower the U-factor (value) the more energy efficient the window will be. The U-Factor calculates the total window's energy efficiency (glass and frame). A difference of .01 in the U-factor changes the energy efficiency of the product by almost 4%. The U-factor is the most important energy number when choosing a window. Consumers can go to www.nfrc.org to look up any window's U-factor.



Solar Heat Gain (SHGC):

A lower SHGC means more radiant heat is blocked, but that is not always better. Solar Heat Gain has to do with how much radiant heat from the sun passes through the glass. The higher the number the more radiant heat can pass through. This is beneficial when the weather is cold and extra heat is beneficial, but detrimental when cooling your house becomes a priority. 



Air Leakage Rating (AL):
The lower the Air Leakage rating, the tighter the window. A window with a great U-factor and SHGC but a poor Air Leakage rating can seriously hurt the window's overall performance.



Visible Transmittance (VT):
Visible Transmittance measures how much visible light comes through the glass. The lower the number, the less light is transmitted. Often differences in VT are difficult to notice, and a good experiment is to hold a sample of the glass up to daylight and look through it. 



Condensation Resistance (CR):
CR measures how well a window resists the formation of condensation on the inside surface. CR is expressed as a number between 1 and 100. The rating value is based on interior surface temperatures at 30%, 50%, and 70% indoor relative humidity for a given outside air temperature of 0 degrees Fahrenheit under 15 mph wind conditions. The higher the number, the better a product is able to resist condensation. CR is meant to compare products and their potential for condensation formation.