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Window Film Can Reduce Excess Heat, Light

April 19, 1998|POPULAR MECHANICS | FOR AP SPECIAL FEATURES

Window film is an excellent strategy for reducing the heat and ultraviolet level from the sun that enters your home. It's important to know how window film performance is measured.

Manufacturers measure performance in a variety of ways; the most significant indicator for the homeowner is the film's shading coefficient, which is expressed as a decimal number that has a reciprocal value. A shading coefficient of 0.51, for example, would allow 49% of the sun's energy to pass through the window.

Industry experts say that for a reasonable energy savings and a significant improvement in comfort and other benefits, a shading coefficient of at least 0.45 is needed. Any number below 0.45 is better, any above will be less effective. This does not mean that a 0.51 won't make a difference. It may, in fact, be just right for some applications. The most popular choices, however, are in the 0.35 to 0.40 range.

Why would anyone opt for a less efficient film? Film selection should be goal-oriented and may mean a compromise. If your primary goal is not heat rejection, as when a group of windows receives only moderate sun, when ultraviolet protection is paramount, or if you're concerned about strengthening the glass, efficiency can give ground to other considerations.

Though performance can be achieved with any of the three technologies, site limitations and aesthetic preferences may come to bear. If your windows are double glazed, you won't be able to use a highly absorbent interior film, whether that's a dyed film or a combination film. In that case, a metalized film is usually in order. Should that metallized film be highly mirrored, lightly mirrored or nearly clear?

These are subjective judgments with practical implications. A highly mirrored film is not everyone's favorite, but mirrored surfaces provide daytime privacy because they always reflect toward the light source. If daytime privacy is a goal, then why spend the extra money on sputtering technology?

On the other hand, if you'd like only a slight mirror effect and a bronze cast, you may need to spend the extra money for sputtered metal film. If you'd like a nearly clear film, then your only choice is a sputtered metal film. If you don't care to spring for the high-end films, then a combination film of a different color may work.

As you can see, external factors and individual tastes have a lot to do with the choice. At its most basic level, every selection is a marriage of performance and appearance, with some trade-offs implied.

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