Letting in the Sunlight, Filtering Out the Bad Rays

Few structural features make a house more attractive and livable than windows. In fact, for most of us, the more windows the better. Yet window glass is not a good energy barrier. And it’s paradoxical that, for all we invest in windows, most of us rush to draw the blinds to block out the summer sun. Not only is direct sunlight often too bright and too hot, but also it carries too much ultraviolet radiation and produces glare.

Single-pane window glass blocks less than 25% of the UV rays that damage skin and fade drapes, carpets and furnishings. It reflects less than 8% of the light and heat that strikes it and absorbs only 5%, while allowing roughly 87% to pass through. The year-round numbers for double-glazed windows are only slightly better.

What we really need is a window that’s a little more discriminating--one that lets in as much visible light as we want, while excluding some of sunlight’s less desirable characteristics. Since the advent of window films, we now have a range of products that address this need. However, they’re all not alike, and the usefulness of any one film is directly related to your personal requirements.

Windows may give us the best of both worlds, but not always in the best proportions.

Window films were introduced in 1969 to address problems surrounding the control of sunlight in homes and businesses. Although they were fairly effective, they often played to mixed reviews. The failings were fading film dyes and highly mirrored and easily scratched surfaces. Poor installations also caused blisters, cracks and edge peel.

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Within certain limits, most of the problems of older films have been fixed. Today’s offerings are more attractive (some are virtually invisible) and come in great variety, with excellent performance and better durability. These gains are largely a product of improved technology, nudged along by the market pressures of consumer demand.

With a reasonable compromise of performance and appearance, window films can now block up to 98% of UV radiation and up to 80% of the normal heat gain. They can also provide a degree of privacy, and thicker films can turn regular glass into safety glass by increasing strength up to 300%.

In winter, films can decrease emissivity, allowing us to keep more of the heat we pay for. Moreover, they do it all fairly reasonably. Allowing for the odd exception, window films sell for between $3 and $7.50 per square foot, installed. Where utility companies subsidize installations, through rebates and allowances, costs may be even less. Home centers sell DIY films that range between $1 and $2 per square foot.

If these prices sound a bit steep, they’re really not. Industry studies and Department of Energy models predict a payback in energy savings in three to five years. In fact, site studies show that every 100 square feet of window film can reduce the air-conditioning load of a building by as much as 12,000 BTUs, or one ton of heat. Because the issue is really summer heat gain, the most dramatic savings occur in the Sunbelt states, including Southern California.

Curiously, window films are seldom installed on factory glass, unless you include the sputtered coatings used in making low-E glass. Tinted films are almost exclusively aftermarket products, probably because the selection process relies so heavily on consumer tastes, home sites and climate. Then too, builders are reluctant to darken show homes when buyers prefer bright spaces.