What is Visible Light Transmittance (VLT)?
The visible Light transmittance (VLT) is an optical property that indicates the amount of visible light transmitted through the glass. VLT is expressed as a number between 0 and 1. The higher the VLT, the more daylight is transmitted. A high VLT is desirable to maximize daylight. What is Air Leakage (AL)? The air leakage rating (AL) is a measure of how much air leaks through the crack between the window sash and frame. The rating reflects the leakage from a window exposed to a 25-mile-per-hour wind, and is measured in cubic feet per minute per linear foot of sash crack. Heat loss and gain occur by infiltration through cracks in the window assembly. The lower the AL, the less air will pass through cracks in the window assembly. An air leakage rating (AL) of 0.30 cfm/sq ft or less is recommended. Back to top What is an NFCR rating? The National Fenestration Rating Council (NFRC) is a non-profit collaborative of manufacturers, builders, designers, government officials, utilities and consu
