Sun-control film for your windows

I would like to get the complete story on solar film which can be applied to the inside of glass. Does the film help to reflect heat as well as the sun? What are the advantages of the different tints? Robert H. Beckman Cape Coral, Florida

A sun-control film will reduce both heating and air-conditioning costs, lessen the fading of inside furnishings, make glass shatterproof, increase daylight privacy, and cut glare.

Numerous commercial and industrial buildings around the country are turning to solar film. So are many homeowners who put the film on large glass areas especially, such as sliding-glass doors, Florida rooms, and the like.

For the past 20 years the 3M Company, Minneapolis, Minnesota, has been making a solar film which it called Scotchtint. But other companies turn out sun-control film as well.

It is not inexpensive; and obviously, quality enters into the equation. A very-low-cost product may not have the same heat- and cold-reduction properties as a more expensive product. You will have to be your own judge as to what you buy.

You ask about the advantages of the different tints. Simply, the color that is most like a mirror reflects away the most sunlight and heat.

If a window is open, everything that the sun is producing comes through the opening. But if you put a piece of clear glass in the space -- in other words, close the window -- 77 percent of the sun's output comes in. The glass reflects away about 7 percent and absorbs the remainder, or 16 percent.

The minute you go to a tinted glass, such as bronze or gray, the glass will absorb, not 16 percent, but 50 percent, according to Daniel E. Callanan, a marketer and installer of 3M Scotchtint. As a result, his company avoids putting a colored film on colored glass, such as a bronze film on bronze glass. Simply, if the glass absorbs too much heat, you might have a problem, he says.

"That is why most of the film we install is mirrorlike," reports Mr. Callanan. "All of the advancements during the last 20 years have been in that film," he adds.

Before buying a sun-control film for glass, make sure it is scratch-resistant. You wouldn't want to mar the surface when washing the windows, for instance.

Two years ago 3-M introduced a window-insulation film which is blue-gray in color. In addition to keeping out 75 percent of the sun's heat, it also will bounce back into the room 40 percent of the heat that would be going out through the glass. The noninsulation-type film keeps only 20 percent of the heat from going out.

How about cost? If you want to save money, you can go to a store, such as Sears-Roebuck or perhaps even a discount-type store, and buy a do-it-yourself kit for "insulating" a pair of sliding-glass doors for about $50, I'm told. If the glass is monolithic -- one large sheet of single-glazed glass per panel -- the payback can come in a year or two. But if you opt to take this route, make sure you can do the job without leaving air bubbles under the film, etc.

In other words, if you can't wallpaper a room, then you could have trouble trying to apply a sun-control film on glass.

A professional job might cost twice as much.

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