BackPage Primer from Environmental Building News
November 1, 2008
Reducing Heat Flow Through Windows
Windows are the weak link, thermally speaking, in most building envelopes. Modern windows are much better than old single-pane windows and store-fronts, but they still represent a compromise—we accept their mediocre thermal performance because we want the daylight, views, and ventilation they offer. It’s true that south-facing windows in a good passive-solar design can heat a space in winter, but even these have to be managed carefully to avoid losing more heat at night than they provide during the day or introducing too much solar gain when it isn’t wanted.
Windows and glazing systems allow unwanted heat flow in several ways. They tend to have cracks and gaps around their edges that allow air to leak in or out. They allow heat transfer via conduction across the glass and frames. And they allow heat to radiate through, both as visible light and as invisible, infrared radiation. This solar gain is the biggest source of cooling loads in buildings with large areas of glass.
In the 1950s and 1960s, designers began using tinted glass to reduce solar gain, but that approach reduces visible light transmittance, affects the appearance of the glass, and impairs views. The introduction of low-emissivity (low-e) coatings on glass in the 1980s was a big breakthrough because these “spectrally selective” coatings are more transparent to visible light than they are to invisible radiation, so they cut down on heat flow without affecting visibility as much. Low-e coatings work in two ways: they reflect certain wavelengths of radiation back towards the source and they prevent heat absorbed by glazing from re-radiating on the other side of the window. Newer generations of low-e coatings have gotten even better at distinguishing light from heat, and they can be specified to optimize their performance in relation to solar gain, visible light transmittance, and heat loss from indoors.
These coatings are especially effective when used in insulated glazing units (IGUs) with a low-conductivity gas such as argon or krypton instead of air between two, three, or more layers of glazing. Those with more than two panes often use one or more polyester films suspended between two panes of glass to provide the additional layers. Double-pane, low-e, argon-filled windows are commonly used in the U.S. today. These achieve a U-factor of about 0.3 (R-3), and reduce solar heat gain by 30% to 50%. Windows with U-factors as low as 0.2 (R-5) and solar heat gain reductions of 70% are readily available.
As the glazing systems themselves improve, heat gained or lost from conduction through the edges and frames and from air leakage become relatively more important. Special “warm-edge spacers” (in place of conventional, highly conductive, aluminum spacers) can help to reduce heat loss and prevent condensation around the edges of the windows. Insulated window frames significantly reduce heat flow compared with standard frames. And good gaskets, sealants, and weatherstripping are important in reducing air leakage, which is often a huge problem in older buildings.
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