Q&A: Condensation on Windows

A.Energy and sustainable design consultant Andy Shapiro responds: Two factors affect the humidity level in a house: how fast water is being introduced and how fast it is leaving. A very tight house doesn’t need a lot of moisture input to result in high humidity and condensation on the windows, while the same amount of water introduced into a very leaky house won’t raise the humidity much.

You can use a blower door to check the house air leakage rate, though your wet windows may already be telling you that it is relatively tight. You can also check the humidity in the house with a Radio Shack temperature-humidity indicator. The windows should tolerate 40% relative humidity without condensing in cold weather. If they don’t, then they are part of the problem — they’re not well enough insulated at the edges. If the home-owner isn’t willing to upgrade the windows or add storms, then you’ll have to lower humidity levels further.

Showering contributes a lot of moisture to a home. A bath exhaust fan should take care of it, but often the fan is missing, undersized, or little used. Drying clothes indoors also releases a lot of water. (Dryers should always be ducted outside.) Every drop of water that goes to houseplants ends up as moisture in the air. Drying firewood in the basement can add quite a lot of water. Cooking, particularly if the occupants don’t use a range hood that is vented to the outside, can generate a lot of moisture.

If the house is new, the construction materials contain literally tons of water that will evaporate over the first winter. Therefore, condensation problems that show up the first winter may not show up again. Poor basement drainage on a wet site can also be a major source of water. (Your nose will tell you if there is water in the basement — you can usually smell the damp or the mold.) Consistently bringing wet or snowy cars into an attached garage that is not adequately sealed from the house can bring in a lot of water.

To solve condensation problems, first reduce the sources of moisture and then ventilate to get the humidity down to acceptable levels. I recommend powered ventilation for all houses. An inexpensive ventilation approach is to install a quiet, efficient bathroom exhaust fan, like the Panasonic FV-08VQ. The fan can either run continuously or be wired to a control like the Airetrak, which runs the fan at a constant adjustable speed and has a push button for 20 minutes of high speed (see “Simple Whole-House Ventilation,” 8/95).