Understanding Energy Star Labels on Replacement Windows for Your Home

If you are looking at replacing any or all of the windows in your home, reading the Energy Star label will provide a wealth of information about the windows you're purchasing - if you can understand the information that's being provided.

The Energy Star label provides the government performance standards for the three separate climate zones of the United States: Northern, Central and Southern. Choosing a window which is suited for the climate zone you live in is critical to maintaining heating and/or cooling efficiency with your new windows. This guide will help you to understand the Energy Star label so that you can know exactly what you're looking for when it's time to buy new windows for your home.

U-factor
The U-factor listed on the Energy Star label indicates the rate of heat loss through the window. The lower the U-factor, the more insulation protection it provides.

SHGC
SHGC stands for Solar Heat Gain Coefficient. This measures how well a window blocks heat from sunlight. The SHGC is the fraction of the heat from the sun that enters through a window. SHGC is expressed as a number between 0 and 1. The lower a window's SHGC, the less solar heat it transmits. Therefore, someone living in the Northern climate zone would likely prefer a window with a higher rated SHGC value.

VT
VT stands for Visual Transmission, or the amount of light that is allowed to get through the window. VT is also rated on a scale from 0 to 1, with 1 representing the maximum amount of light that it allowed to pass through the window.

AL
AL stands for Air Leakage and is a measure of how much air is allowed to slip past the weather stripping on the window. The lower the leakage the better the window, however, window manufacturers say that air leakage isn't as important a concern as the U-factor or SHGC.