Window Rating Terminology

Windows are rated by the National Fenestration Rating Council. The rating takes into account the following five criteria:

•U Factor – A measure of the resistance to heat transfer on a scale of 0 to 1.2. The lower the number, the greater the resistance. To qualify for tax credits the window must have a rating of 0.30 or less.

•SHGC – Stands for Solar Heat Gain Coefficient. This rating measures how well a window blocks solar heat gain on a scale of 0 to 1.0. The lower the number the more efficient at blocking the suns heat from a home’s interior the window is. A rating of 0.80 means 80% of the solar energy hitting the window gets through. To qualify for tax credits the window must have a rating of 0.30 or less.

•Visible Light Transmittance – A measure of the amount of sunlight that can pass through a window on a scale of 0 to 1. The higher the number the more light will reach the interior of your home. Tinted windows have relatively low visible light transmittance.

•Air Leakage – A measure of a windows air infiltration in cubic feet per minute per inch of crack or seam.

•Condensation Resistance – A measure of the windows ability to resist the formation of condensation on its interior surface. This is very important in evaluating the durability of a window relative to others. It is rated on a scale of 0-100, with a higher number indicating a better resistance to condensation.

These ratings are listed on a sticker located on the windows themselves or often available in manufacturers literature or on their websites.
For more information:
www.nfrs.org

www.efficientwindows.org