High-Performance Windows and Glazing

About this course.

Windows are usually the weakest thermal point in the building envelope because glass and window frames conduct and radiate heat far more readily than an insulated wall, so a window lets much more heat escape in winter and pour in during summer, for the same area, than the wall around it. On top of that, windows admit solar radiation directly — sunlight passes through the glass and heats the room — and they are common places for air leakage and for condensation. As a result, the windows in a typical home can be responsible for a large share of its unwanted heat loss and heat gain, which is exactly why improving them matters so much.

To see why, compare a window with the wall it sits in. A modern insulated wall is a thick sandwich designed specifically to slow the flow of heat: layers of cladding, a cavity, insulation and lining all resist heat moving through. A single sheet of glass, by contrast, is thin and conducts heat readily; heat flows through it, and the inner surface of ordinary glass also radiates heat away to (or toward) the room. The frame that holds the glass can be a weak point too, especially if it is made of a highly conductive material. So even a good-quality ordinary window is, thermally, a hole in the insulated envelope — not a hole for air, but a hole for heat. This is the first and most important idea: for the same area, an ordinary window loses and gains many times more heat than the insulated wall around it.