Windows Should Let Light In And Block The Sun’s Heat

Posted on Monday, June 20th, 2011

Windows vary when it comes to how much visible light passes through into your house. It’s actually a unit of measurement specified by window manufacturers known as Visible Light Transmittance, or VLT for short. VLT is a unit of measurment between 0 and 1. So a VLT of .8 means the window allows 80% of visible light through. Normally, VLT’s below .2 are reflective glass surfaces. Some people might assume the more visible light a window blocks, the more insulated the window is from the sun. It’s a common misconception that a low VLT means high performance in reducing solar heat, but that simply isn’t true.

While the U-Factor of windows is a fairly well known measurement of performance for the insulated quality of a window, VLT only measures the aesthetic quality. For solar heat reduction, there is another measurement of performance known as the Solar Heat Gain Coefficient, or SHGC for short. SHGC is a unit of measurement between 0 and 1. A SHGC of .22 means that a window only transmits 22% of the suns heating energy through a window.

Making Sense Of Window Specifications

Generally, a higher VLT and a lower SHGC would be a good thing. With a high VLT most of visible light is being let through, but the low SHGC mean the window is selectively blocking the sun’s rays that cause heating. That’s a good thing, but there are exceptions. While a high VLT is almost always a good thing, sometimes, its good for a window to have a higher SHGC and let more heat in. That’s because in the Northern hemisphere, north facing windows normally don’t get too much sun. In cold climates with high heating costs, letting the sun’s heat in could help reduce heating costs in winter.

There is a little correlation between VLT and SHGC. Normally, a window with a higher SHGC will have a high VLT as well. That’s because the window doesn’t have to block as much of the sun’s rays and will allow more visible light through as a result. But a low VLT doesn’t imply a high SHGC. Some windows, like windows with brown/grey tints, can create a window with a low VLT and higher SHGC.

To calculate how well a window lets visible light through while blocking solar heat, we can divide VLT by SHGC (VLT/SHGC) to create the Light To Solar Heat Gain Factor, or SHGF. The higher the SHGF, the greater a windows performance will be. For example, Dixie’s Low-E windows block 78% of the solar heat. That’s a .22 SHGC. The windows also allow 80% of the sun’s visible light through. That’s a VLT of .8. Divide .8 by .22 and you’ll find out that Dixie’s windows have a SHGF of 3.63! (Anything above 1.4 is considered good)

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