I know I’ll be getting this question at least once as cooler weather sets in, so here’s an explanation of window condensation, courtesy of Tom Herron of the National Fenestration Rating Council.
Condensation appears as a light coating of water, frost or ice. Unless the condensation is between the window panes, humidity inside the home is the cause, Herron said.
“Humid air holds water vapor until it contacts a surface whose temperature is less than or equal to the dew point,” he said. “When this happens, the water vapor turns to liquid.”
Because the interior surface of your windows is typically the coldest part of your home, condensation forms there first. Once the air becomes less humid or the glass warms, the condensation vanishes.
Condensation is “a naturally occurring phenomenon,” Herron said, but it can be destructive because excessive moisture can damage curtains, walls, carpets and wooden window frames. In some cases it leads to mold, creating health risks.
Minimizing condensation requires maintaining the surface temperature of the window above the dew point, he said.
Manufacturers reduce the amount of heat that gets transferred through a window, called the thermal transmittance or U-factor. The higher the U-factor, the higher the potential for condensation to form on the glass.
Reducing the potential for condensation requires each of a window’s three thermal zones to be efficient. Heat from inside the house will conduct its way through the parts of the window that are least efficient, causing those parts to have lower indoor surface temperatures.
Here are two things to consider when choosing windows, Herron said:
▪ Upgrading from single-glazed windows to multiple-glazed windows or insulating glass units reduces the potential for condensation.
▪ Going from single-glazed to dual-glazed or insulating glass units reduces the potential for condensation on the edge of a glazing surface. Choosing high-performance glass further reduces the chances for condensation.