Reducing residential energy use can be achieved by selecting energy-efficient heating and air conditioning equipment and by properly insulating a home against heat loss in the winter and heat gain in the summer. While the importance of sufficient insulation in walls and roofs is well recognized, the importance of well-insulated windows cannot be over-emphasized.

Compared to solid walls and roofs even the best windows are poor insulators. They are often equally poor at blocking unwanted solar energy that as a function of heat entering through existing glass, according to the California Energy Commission, accounts for as much as 40 percent of a home’s cooling requirements. Never the less, the desire for visible light and views motivates homeowners to want more not less glass. In terms of energy conservation how many windows and how much glass are appropriate for both new and renovated residential construction?

Optimum wall and ceiling insulation performance measured in familiar R-values (resistance to heat flow) depending on location may be from R-13 and up for walls and up to R-30 for ceilings. Considering that the typical vinyl-framed, low-E coated, gas-filled insulating glass window has an R value of R-3 to R-4 even the most energy efficient windows and glass may seem to pose a serious energy conservation problem.

But in terms of insulation there is a point of diminishing returns. In saving energy it is much more important to increase resistance to heat loss from R-0 to R-10 than from R-10 to R-infinity. Given that reality, energy efficient windows are not the net energy losers a superficial evaluation of the comparative R-values of windows and walls would suggest.

But insulating against heat loss is not the same as blocking unwanted solar heat that can be a problem even in winter in homes with significant amounts of south and west facing glass. For existing glass, conventional tinted and reflective window films successfully block a significant amount of solar heat. However these same films block a significant amount of light.

Most conventional window films transmit less than 34 percent of visible light, a good 36 percent less than the 70 percent necessary to be undetected by the naked eye. The result is interiors are correspondingly darkened, often requiring the use of increased illumination. This may lead to higher electricity consumption that may increase inside temperatures requiring more air-conditioning. Increased utility costs defeat the major benefit of the film—cost savings.

Clear spectrally-selective applied window film offers the best ratio of visible light transmission to heat rejection. Spectrally-selective refers to the ability of the film to select or let in desirable daylight, while blocking out undesirable heat. Given rising electricity cost, the rate of payback for spectrally selective film is always improving—averaging less than four years.

V-Kool, Inc., Houston, TX, is a sales and marketing distribution company of spectrally selective applied films for commercial, residential and specialized vehicular applications. For information contact V-Kool, Inc., at 800-217-7046 and at www.v-kool-usa.com.