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.
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.
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."
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.
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
condensation requires maintaining the surface
temperature of the window above the dew point, he said.
reduce the amount of heat that gets transfered 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.
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.
are two things to consider when choosing windows, Herron
from single-glazed windows to multiple-glazed windows or
insulating glass units reduces the potential for
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.