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Window Ratings: The Key to Understanding Window Efficiency

Key topics for facility professionals. Keywords for this topic: Windows

Compiled by FacilitiesNet Staff

Whether replacing windows in an existing facility or evaluating new windows, the challenge remains the same: Finding the most energy efficient window for the job. Selecting efficient windows — whether replacement or new — requires understanding four key window ratings.


One window rating to start with is the U-factor of a window. The U-factor is a major consideration for any building where a heating system is required to make building occupants comfortable, says Ray McGowan, manager of technical services for the National Fenestration Rating Council (NFRC), the organization responsible for giving windows their energy and optical performance ratings.

The U-factor represents a measure of heat loss through the entire window system, including the frame. The lower the number, the higher the insulating value of the window.

Insulating ability is affected by both the glass and the frame. Using non-metal frames such as wood, fiberglass or vinyl can improve the U-factor compared to a window with an aluminum frame. Thermal breaks will improve the U-factor of metal windows.

A single pane of glass is given a base U-factor of 1.0. U-factor ratings of a whole dual-pane gas-infill insulated window system can range from 0.59 for the thermally unbroken aluminum frames to 0.26 for fiberglass window frames.

U-factor is the inverse of another widely used measure, R-value. With R-values, a higher number is better.

Solar Heat Gain Coefficient and Visible Transmittance:

Regardless of outside temperature, heat can be gained through windows by direct or indirect solar radiation. The ability to control this heat gain through windows is measured in terms of the solar heat gain coefficient (SHGC) or shading coefficient (SC) of the window. The SHGC rating on windows is represented by a number between 0 and 1. The lower a window’s solar heat-gain coefficient, the less solar heat it transmits.

Visible transmittance (VT) indicates the amount of visible light transmitted through the glass (also referred to as visible light transmittance — VLT). Glass with a high VT provides more daylight, offering a better opportunity to reduce electric lighting and cooling loads. Like SHGC, VT is measured as a number between 0 and 1. A good window system will have a low SHGC and a high VT.

Any glass will have some insulating value and will restrict some visible light. A typical clear glass single pane window can have an SHGC of 0.82 and a VT of 0.88, according to the Efficient Windows Collaborative. Add a bronze or gray tint to the window system and it will reduce SHGC to 0.65 and VT to 0.56. A VT that low is cutting half of the light coming into a space and will appear quite dark, says John Carmody, director for the Center for Sustainable Building Research at the University of Minnesota. Ideally, VT numbers should therefore be in the range of 0.60 to 0.80.

Air Leakage:

Heat loss and gain also occur by air leakage through cracks in the window assembly. This effect is measured in terms of the amount of air (cubic feet or cubic meters per minute) that passes through a unit area of window (square foot or square meter) under given pressure conditions. In reality, infiltration varies slightly with wind-driven and temperature-driven pressure changes. Air leakage also contributes to summer cooling loads by raising the interior humidity level.


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