Before 1973, nearly 5% of the national energy consumption was attributed to windows--that is, to the heating, cooling, and lighting required to compensate for the effect of windows. Advances in window technology have substantially reduced those losses and have the potential to make windows net sources rather than sinks of energy, especially in cold climates.¹³ Unlike insulated walls, which at their best prevent the outward flow of heat, optimal windows can accept solar gain and hence provide net heating.

Great advances have come from research on coatings for windows. Although many kinds of coatings have been developed, such as solar control coatings seen on high-rise office buildings, which reflect across the whole spectral range to reduce glare or overheating from the sun, the most effective coatings for reducing energy consumption are those with low emissivity (e). Such coatings can greatly reduce the radiative heat losses, which account for two-thirds of heat transfer through a double-glazed window. The coatings have a high reflectance, hence low e, in the thermal infrared (IR) and a high transmittance (T) in the visible. Additionally, some coatings are designed either to admit solar near IR (NIR) to help heat a building in a cold climate or reflect the NIR back in a warm climate. Since their introduction in 1981, windows with low-e coatings have captured 35% of the sales and generated gas savings that are equivalent in energy to one-half the output of oil in Prudhoe Bay. But the windows will long outlast that dwindling northern reserve.

To engineer these optical properties of a coating, researchers can either select a material with the right intrinsic properties or combine several materials to achieve the desired performance. One class of high-T, low-e materials consists of doped oxides of tin or indium, which are wide bandgap semiconductors. Adjusting the dopant level can tune the wavelength cutoff between transmittance and reflectance.

Another class of materials comprises very thin films of noble metals, especially silver. Although thick films of silver are highly reflective, the reflectance of very thin films (10­20 nm) can be suppressed by thin-film interference effects. Adding dielectric layers to the front and back of the metal layer thus reduces the reflectance of the thin film for a limited range of wavelengths. These coatings can be made highly transparent to visible radiation, but remain reflective in the NIR.

The figure above shows the transmittance (solid curves) and reflectance (dotted curves) of three actual window coatings. The single-layer conducting oxide coating (gray) has a high T throughout the IR, and the other two, both thin metal coatings, have reduced T in the NIR. The double metal layer (green) cuts out more of the NIR and has a sharper transition between transmittance and reflectance than does the single metal layer (red).

Low-e coatings insulate best when combined with features such as multiple panes with low-conductance-gas fills that minimize convection, and with insulated window frames that minimize conductive heat transfer. Researchers are now trying to design windows whose properties can be switched either actively or passively as ambient conditions change. For example, electrochromic windows can be electrically switched to give greater or less transmittance. The key to these more complex devices is better understanding of materials properties. Another thrust is to make coatings that are easier and cheaper to manufacture. (The cover of this issue shows a chamber used to deposit experimental low-e coatings on windows for research.)

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© 2001 American Institute of Physics