Air leakage rating
The air leakage rating shows how much air passes through a closed window or door, measured in cubic feet per minute per square foot. Lower is tighter.
Even a closed window can let air through the seals and joints. The air leakage rating measures that flow under a standard test. A lower number means a tighter product, less draft and less strain on your air conditioning.
The figure sits on the NFRC label with the U-factor and SHGC. Do not mix it up with a leak from rain. Water is a separate test. Good weatherstripping, a rigid frame and correct flashing all help a unit stay tight after install. For product comparisons, see our guide on planning a window and door project.
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