Frost on Windows

This time of year in Colorado, when nighttime temperatures get down to 20 below zero, we get calls from people worried about frost on their windows.  The beautiful crystalline pattern of the frost is not appreciated when it thaws and drips onto the window sill, creating a problem for the homeowner.  This moisture can soak into a wood sill, degrading it and causing discoloration.  Eventually, if it keeps happening, there is the potential for mold growth.

Why does this frost occur?

Its all about dew point, the temperature for a given relative humidity at which airborne water vapor condenses on surfaces.  So, when the surface temperature of the interior pane of the window is below the dew point, condensation on the glass surface will occur.  The dew point depends on both temperature and relative humidity.

What can you do to avoid the frost?

1 - Use high-performance glass.
In snow country, we recommend using a high-performance glazing product like a triple-pane window.  The better the insulation value of the window, the higher the temperature of the interior glazing surface, and the less likely it will be to frost. 

2 - Use insulating shades appropriately.
Many of us are using insulating shades to increase the R-value of the window assembly, a good idea, but consider this:  Even though your insulating shades help keep the heat in the room, they also help keep the cold on the glass.  Unfortunately, since there is not a true air seal between the insulating shade and the glass, the shade allows moisture-laden air into that space.  There it will condense onto the glass surface, causing frost on those cold winter nights.  In order to keep from having frost on the window and subsequently water droplets on the sill, Mike Dollahan of Architectural Windows and Doors recommends keeping the insulating shades open at least 4" at the bottom.  This allows warm air from the interior to heat the glass surface so that it does not drop below the dew point.

3 - Watch your home's interior relative humidity.
In Colorado, an interior relative humidity of 25% to 35% is normal.  But if you are humidifying your home, or if you have a pool or hot tub inside, relative humidity levels will go up.  As they go up, the chance for frost on the windows goes up, too.  (you may want to buy a hygrometer to keep tabs on those humidity levels!)

4 - Use fiberglass or vinyl windows.  The frames of these windows are typically better insulated than wood-framed windows, and they won't discolor or become saturated when the frost thaws and drips on them.   Fiberglass windows are considered to be the best choice for overall performance and longevity.  The process of making fiberglass is not too impactful; and its made from a very abundant resource, ...sand!