Condensation forms on a surface when the temperature of the surface is below the dew point of the air. During the winter, when the coldest surface in a room is often the window, it’s fairly common to see water droplets or ice on window glass — especially in a room with elevated indoor humidity.
Condensation is more likely to form when indoor relative humidity is high. That’s why it’s more common to see condensation on a bathroom window than a bedroom window.
Condensation is more likely to form on cold surfaces than warm surfaces. That’s why it’s more common to see condensation on a single-glazed window than a double-glazed window.
Older Americans who grew up in the days of single-glazed windows remember the joy of waking up on a cold winter morning to see beautiful frost patterns — swirls, vines, and lacy leaves — on their bedroom windows. And who can forget the frost-covered windows in the Hollywood version of Dr. Zhivago?
On today’s low-e windows, however, such frost patterns are extremely rare.
In theory, a window with a high R-value (that is, a low U-factor) should do a good job of resisting condensation. However, even a low-U window can have a condensation problem. That’s because a window’s U-factor is an area-weighted average of the U-factors of the different window components. Window condensation is a thermal bridging problem, not an average U-factor problem.
The coldest part of most modern windows is the bottom half-inch of glazing. (Although the width of the space between the panes of modern insulated glazing units has been optimized to minimize the effect of convective looping in the gas between the panes, convective looping still occurs. Naturally, the gas at the bottom of the gap — whether air or…