KNOW HOW

A Window Film Review

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By Jeanne Huber
Special to The Washington Post
Thursday, July 6, 2006

Q My electricity bill is soaring. Would installing window film allow me to run my air conditioner less ?

AWindow film can indeed block a significant amount of the sun's heat, easing the load on your air conditioner. The film is also likely to protect your furnishings from fading, and it can reduce glare, which may make some rooms much more pleasant.

Window film consists mostly of clear polyester plus a tinting agent. Some types are colored with dye or pigment, while others incorporate a whisper-thin layer of metallic or ceramic material that may look colored or clear. A tough acrylic layer coats one side of the polyester. Installers glue the other side to the interior surface of the window. Do-it-yourself films tend to come with water-activated adhesive already on the film, which makes them easier to apply.

Window films absorb sunlight or reflect it, or they do some of each. There are three main types: tinted, reflective and spectrally selective, a category also known as "low-E," for low-emissivity. Emissivity refers to how much heat is allowed through.

Tinted film has dye or pigment particles that physically stop light by getting in its way. The lightwave's energy transfers to the pigment particle, which then warms up. Unfortunately, that causes some of the heat to radiate back into the room, defeating one of the main purposes of adding film. Plus, because the pigment particles can't distinguish between wavelengths of different sizes, they block visible, near-infrared and ultraviolet waves equally. Ideally, you'd want to let in much of the visible light, which helps you see, but stop infrared, which causes heat, and UV, which makes things fade. "Tinting is old technology," says Darrell Smith, executive director of the International Window Film Association, a trade group.

The second big category consists of window films that reflect light, rather than absorb it. These work great on skyscrapers, where architects love the ability to make walls of glass look metallic blue or gold. "But in the residential market, people say, 'I want to save energy, but don't want to have windows look like tinfoil,' " Smith says. Reflective films don't have much appeal on houses except in certain circumstances, such as when people want bronze windows to blend in with brown paint.

Spectrally selective film provides the most benefits in homes. These include the same low-emissivity coatings that allow modern windows to keep people warmer in the winter and cooler in the summer. The only difference is that instead of being applied to the inside surface of one of the glass panes in a dual-pane window or being suspended in the space between the panes, a window film's filter material is bonded to polyester. The filter consists of a super-thin coating of specific metals or ceramic materials that are able to reflect certain wavelengths of light but allow others to pass through unimpeded. This coating is so thin that you can see through it.

The ideal filter for many situations would allow 100 percent of visible light to pass but would reflect 100 percent of the infrared and UV light. The technology is not there yet, but it has come a long way. "You can get almost 80 percent of the visible light through and still stop as much as 50 to 60 percent of the total solar heat from coming through," Smith says. "You don't have to give up as much of your light to achieve the energy efficiency."

But if you want to reduce light, perhaps in a room where afternoon sun makes glare an issue, you can select a spectrally selective film that will do that, too. "With spectral, you can vary which part of the light spectrum you want to control, and how. You can select by the end result you are trying to achieve," Smith says.

When you shop for window film, you'll see ratings for emissivity as well as for shading coefficient, a measure of the level of glare control.

The cost of this light control is lower than you might think. With professional installation, prices start at about $3.50 a square foot, although spectrally selective coatings cost two to three times more. Installers charge more per square foot for small-pane windows because they take almost as much time as big ones. But if you opt to install film yourself, you'll find small panes are much easier to do than large ones. The adhesives are the main difference between do-it-yourself films and those professionally installed; special adhesive-softening products allow both types to be removed without damaging the glass.

You don't need film on all windows, just those where light streams in. The biggest payback comes on windows that you don't cover with draperies or shades because these block light, too -- although only once it's through the glass, allowing some heat into the room.

Window films can't discriminate between winter sun and summer sun. By cutting heat gain during the summer, they also take away free solar heat you might enjoy in the winter. However, just as the coatings reflect the sun's infrared waves back outside, they also reflect inward the infrared rays given off by objects and people inside. This helps keep interior spaces warmer in winter.

If your house already has dual-pane windows, proceed with caution before you add window film. Have a professional installer come to your house and assess whether there is an existing coating that might cause the film to heat up the glass enough to break the outside pane or ruin the seal. Because of concerns about breakage, some window manufacturers void warranties if films are applied. Film manufacturers, however, usually offer their own warranties against glass breakage and seal failure in these cases, provided a professional has helped select an appropriate film.

On the film itself, many manufacturers offer 10-year warranties with professional installation. After a decade or more, films eventually stop working because much of the metal or other filter material is gone. It gradually evaporates through the glass. If you put on polarized sunglasses and looked at the window, you would see ribbons of color, revealing where this has occurred unevenly.

Surprisingly, scratching usually isn't a problem because the acrylic coatings used today are very tough, says Smith, who adds that you can clean a window with super-fine steel wool and not scratch the film. Not that manufacturers recommend it. They do say to clean the films with a soft cloth and non-ammonia window cleaner, because ammonia damages acrylic.


© 2006 The Washington Post Company

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