According to the U.S. Department of Energy, around 30 percent of the energy used to heat and cool our homes is lost through inefficient windows, even though windows often make up less than 10 percent of the surface area of a house.
Swapping out old, single-glazed windows with high-performance dual- or triple-pane units is costly. Moreover, it is often not allowed in historic buildings. Architectural historians insist on maintaining the appearance and character of windows in classic old buildings, irrespective of their performance.
A less costly possibility, rather than replacing inefficient windows, is to install an insulating window film. Glazing films have been around for more than 50 years, with high-performance films only in the past decade. They have evolved along with the great gains in window technologies. Coatings and films have become standard in high-quality windows. The large choice of offerings can yield greater or lesser heat gain, visual clarity, ultraviolet blocking, and insulating values, to name a few of the available options.
The cost of replacing residential windows can easily be $125 per square foot or more. Window films such as 3M’s Thinsulate Climate Control film offers good performance for a fraction of the cost. This type of low emissivity, or “low-E” film, improves the insulation value of a typical single-pane window to approximate a double-pane unit (up to about 92 percent). The emissivity rating can be as low as 7 percent, meaning that 93 percent of a room’s heat is reflected back into the space and doesn’t escape outside. Low-E coatings reduce heat loss in winter and heat gain in summer. As a result, the initial installed cost of $5-$15 per square foot (depending on film type and size of the job), is a two- to five-year return on investment (ROI). The comparable ROI for replacing old windows with high-performance units is 30-50 years.
Some of the recently introduced low-E films have high visible light transmission (visible transmittance of 70 percent), meaning they have basically no impact on views — no darkening and no distortion. Furthermore, these coatings block 99 percent of ultraviolet rays to fully protect furniture, rugs, upholstery, and artwork from fading. Window film application does not require a professional installer. Professional installation, however, is highly recommended for quality purposes and to have the 20- to 30-year warrantee apply. Installation by professionals takes only minutes per window. Full curing of the film takes a few days to a month.
For reasons of energy savings, modest cost, comfort, and fade protection, low-E window films are important to know about.