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.