During the winter, windows have the potential to be both an enemy and an ally. They let light into your home, but they can also bring cold air, frost, and condensation. However, with a few minor adjustments, homeowners can get more friend than foe out of their windows.
Over the course of a subzero night, windows, (especially those older, single-glazed, metal-framed ones), will often become very frosty. This frosting isn't exclusive to older windows; windows all over will be exhibiting varying degrees of the same effect. Frosty windows are a result of condensing moisture in the home. Vapor droplets in the air that come in contact with the cold surfaces of the window will, if the surfaces are cold enough, cool down into water droplets that form on the cold surfaces. If this happens all night long, there can be considerable accumulation of water. In some cases, the water drops freeze shortly after forming on the window, causing ice to build up. To help correct this issue, attention should be paid to the amount of moisture in your home and the interior temperature of the window glass and frame.
Moisture cannot be eliminated from your home entirely, but it can be reduced. This can be achieved by: Turning your furnace humidifier down or offEnsuring your clothes dryer is venting properlyUsing kitchen and bathroom exhaust fans when cooking and showeringOpening a window periodically when things feel "stuffy" We recognize that it's only possible to do so much, and if your windows are cold enough the sweat will still form - which is why we also encourage homeowners to warm up the surface temperature of the window glass and frame. If the window is old, it may be drafty. This will be apparent in cold air whistling through around the edges. Replacing or improving weatherstripping, a relatively easy DIY project, can often solve a draft problem. If the room has only one pane of glass between it and the outside, install a storm window. This will warm up the interior pane. With a newer double-glazed window, unless it's very cheap or very poorly installed, the glass and frame temperature should be pretty reasonable.
The trick now is to assess the heat source in the room. In a perfect world, the heat for the room is delivered at floor level right below the window. The idea is that the warm air, either from a furnace register or a radiator, washes up the window, keeping the glass and frame nice and warm, reducing or eliminating condensation. A related problem in many homes is the window treatments. California shutters look great and can block out light, but when closed they also block warm air from reaching the window, leading to condensation in cold spells. The solution is to open up the louvers, or open the shutters. Many blinds and drapes have the same effect. When closed, find a way to prop them out at the bottom so that the warm air can go up between them and the window.
If you've tried everything, and still you have a window or two that sweat uncontrollably, hardware store plastic sheeting will work nicely. This remains the cheapest solution for your windows. A better, but more expensive solution? Replacing those old windows with new multi-glazed coated windows.