Window Insulation Films Tips and Tricks
Check the specs of window insulation films before buying.
There are a variety of different specifications that you will see listed on window insulation films. Although they have different names, they measure more or less the same thing, the amount of the various components of sunlight that they block. To get the maximum benefit from window insulation films, you need to pick the film that best fits your needs.
Check that you are going to get the right amount of Visible Transmittance.
Low Visible Transmittance (VT) values (0.33 to 0.55) mean that very little visible light is passed through the film and can give the glass a dark appearance. From the outside, the film will be more reflective (i.e., look more like a mirror).
Make sure that the solar heat transmission is low.
The U.S. Department of Energy defines the Solar Heat Gain Coefficient (SHGC as a number between zero and one, with a lower number expressing less solar heat transmission or a lower solar heat gain. The SHGC value needs to be low to be effective—the lower the better.
Make sure that you get more light with less solar gain.
The Light-to-Solar-Gain (LSG) ratio, which is the VT divided by the SHGC, measures the ability of window insulation films to provide light relative to the solar heat gain. A high LSG means that the film allows more light in with less solar gain. Extremely high-performance windows have an LSG of around 2. It is difficult to find window insulation films that have an LSG than of more than 1.2.
Check the amount of solar energy that will be selected.
Total Solar Energy Rejected (TSER) indicates the amount of solar energy that is rejected by a reflective film. A TSER of 50% would mean that the film rejects half of the solar energy. But keep in mind that the TSER does not tell you how much of each component of the sunlight is being rejected, just the total amount of solar energy removed. The relationship between SHGC and TSER is very simple: TSER = (1 – SHGC). So if you have a TSER of 30%, the SHGC = .70. The TSER tells you how what percentage of solar energy is rejected (higher being better) while the SHGC tells you how much solar energy is passing through the film (lower being better).
A high TSER is not always the measure of a high performing solar film. The TSER can always be increased by making the film darker and more reflective. Remember that TSER looks at all of the components of sunlight, so to make appropriate comparisons, you need to look at films with the same or similar VT values. Imagine two films that have a TSER of 50%, and that one has a VT of .8 while the other has a VT of .2. Even though these films have the same TSER, the .8 film is a much higher performing film because it blocks out more infrared radiation than visible light.
Check the Luminous Efficacy.
The Solar Coefficient (SC) is the ratio of the amount of solar energy passing through a film to the solar energy which passes through a standard piece of glass. Luminous Efficacy (LE) tells you how much a film shades the interior in the presence of direct sunlight. Window insulation films with a high LE rejects infrared radiation and lets visible light through.
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