Window Ratings – For Your Information

Window ratings can help us to decide the window that's best for our needs. But you should always ask the company if the ratings on their windows are for the whole window and not just a part of the product. A window rating can be a bit confusing, so here's a look at some ratings to be found on windows and what they mean...

Air Leakage (AL): This rating term is the amount of air that still passes through a window between its sash and frame when the window is shut. It is measured in cubic feet of air per minute per square foot. So the lower the number, the better the window is sealed. An AL rating of no more than 0.30 is recommended.

Condensation Resistance (CR): This rating number, between 1 and 100 this time, is an indication of how well a product can prevent condensation from appearing on the interior surface. The higher the CR rating number; the better the window will be at resisting condensation.

Solar Heat Gain Coefficient (SHGC): This number, again between 0 and 1, is all about how much heat from the sun goes through the window. The heat directly transmitted, and the heat absorbed and then re-radiated is both included in the rating. The lower the number means that less heat can pass through. For hot climates, a SHGC of 0.40 or under is what's widely recommended. In colder climates you will want the number to be higher of course, as otherwise there will be a reduction in the amount of passive heating that your home can accrue from available sunshine. So it's important to weigh up the needs for a higher or lower SHGC rating if your budget is neither dominated by heating costs in the cold season, nor cooling costs in the hot weather.

U-factor (or U-Value): This is probably the most well known of the ratings and is a measure of how well a window can stop heat energy from passing through it. For cold climates you should look for a window with a U-Factor of 0.35 or less. In hot climates, a low U-factor can help with keeping the heat out, but the Solar Heat Gain Coefficient (SHGC) is more relevant.

Visible Transmittance (VT): This rating deals with how much daylight passes through the glass. It will be a number between 0 and 1 with the higher VT number meaning more light is allowed through. Note that this evaluation deals with visible light form the sun and not infrared light. The latter is dealt with by the SHGC rating.

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Author Notes:

Anthony Lee contributes and publishes news editorial to http://www.replacement-windows-n-shutters.com. Find everything you need to know about replacement windows, shutters, tinting, basement, fiberglass windows and more.