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By James Piper, P.E.
Windows & Exterior Walls Article Use Policy
A variety of measurements are used to rate the properties of solar films and coatings.
The visual transmittance (VT) value indicates how much visible light passes through the film or coating. With a maximum value of 1.0, the higher the VT value, the more visible light that is passed. A typical uncoated, dual-glazed window will have a VT of approximately 0.81. The same window with a low-e coating will have a VT of approximately 0.78.
Adding a window film or coating can reduce this to as low as 0.33. At that value, the glass will have a dark appearance. For most applications, a good minimum target value is 0.65.
A second measurement is the solar heat gain coefficient (SHGC). This metric ranges between zero and 1 and rates the solar heat gain through the film or coating. The lower the number, the more effective it is at blocking solar heat gain.
A third useful measurement is the light-to-solar-gain (LSG) ratio. The LSG is a measure of the visible light admitted through the film or coating relative to the solar heat gain. A typical dual-glazed window with no coatings has an LSG of 1.06. Adding a low-e coating changes it to about 1.16. The higher the LSG number, the better the film or coating blocks heat gain while admitting daylight.
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