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.