University of Wisconsin - Extension

MADISON, Wis.--Fog or frost on your windows? Mold on your walls? During fall, moisture stored in your home's wood and trim during summer is released into the air as temperatures cool. It is a common problem in cold climates. The beginning of the heating season is a good time to prevent cold weather moisture problems that can damage your home or pose health risks.

Understanding the causes of the problem is the first step in solving it. Condensation and ice form on windows because the window surface is below the dew point for the air near the window, so some of the moisture in the air condenses on the glass. The higher the relative humidity of the air near the window, the higher the temperature of the dew point.

For example, if the thermostat is set at 70 degrees, the temperatures on the surfaces of your windows will be much lower, how much lower depends on the outside temperatures and the insulating value of the window. When the outside temperatures are at the freezing point, temperatures on the inside surface of a double glazed window could easily be in the mid fifties. As the temperature of the air near these cold surfaces drops, its relative humidity climbs to the point where condensation can occur. To eliminate condensation, homeowners either need to raise the temperature of the window surface or reduce the relative humidity in the room.

Ice on storm windows is not caused by moisture leaking in from outside as is commonly thought. Air leaking in from the outside would not contain enough moisture to cause condensation problems. The problem is caused by warm moist air leaking into the cavity between the prime window and the storm window from the inside. The leaking air has enough moisture to cause the condensation, but not enough heat to warm the surface. If the temperature is cold enough, the condensation turns to ice. Making the storm windows tighter will not reduce an icing problem, nor will closing the weep holes at the bottom of the window. These should be kept open to allow condensation to escape.

The air usually leaks into the space between the windows through gaps between window sashes and the frames. Sliding windows are historically among the most leaky, however, they have improved recently. The aluminum track at the base of sliding windows can be part of the problem. The aluminum transfers the temperature from outside to the inside track and causes condensation and potential icing problems.

In some cases, air may leak through openings in the window frames. It first finds its way behind the window casing and then through the frame. This is most common in retrofit situations where the original window frames had penetrations for a window weight system. Even if the original window weight wells are filled with fiberglass insulation, this does not stop airflow.

A temporary solution may be to carefully caulk all the joints where the sashes meet the frame with rope caulk, which is removable in the spring. Alternately, you could install a plastic film over the windows, making sure to wrap it around the frame. In the spring, remove the sashes and check the weather stripping. The manufacturer should be able to supply you with replacement weather stripping, if needed.

If the condensation occurs on the prime window and the window is double-glazed, that usually means the relative humidity is too high for the temperature conditions. With a thermostat set at 70 degrees, condensation should not occur on double-glazed windows until outside temperatures drop well below zero, if relative humidity is kept below 40 percent. If you have additional glazing, including Low-e (energy efficiency) coatings or tight fitting storm windows, it takes even colder temperatures before condensation occurs.

Several conditions can increase condensation problems. If you close drapes over windows, this can cause the glass temperature and the temperature of the air between the drape and the glass to become cooler, making condensation more likely. If the thermostat setting is dropped substantially at night, this will cause an increase in relative humidity and may increase condensation. This condensation should vanish once the temperature rises in the morning.

The best way to deal with condensation on high-quality, double-glazed windows is to reduce indoor relative humidity to no more the 40 percent. You can do this by using kitchen or bath exhaust fans until condensation disappears. The fans will be more effective if you open a window elsewhere in the house to provide replacement air. Be sure that the fans you use exhaust to the outside, instead of going to the attic or just recirculating air within the house.

For more information about solving moisture problems, contact your local UW-Extension office for a copy of my housing publications, "Winter Home Moisture Problems " (NCR 312) and "Controlling Molds and Mildew" (B3768). You can also obtain copies from Cooperative Extension Publications, (877) 947-7827. See Home & Family, Home Care at: cecommerce.uwex.edu. Single copies may be printed from this web site.

In an earlier column on sprayed-in-place foam insulation, I stated that a vapor barrier was usually not required with such foam. I have since learned that the Wisconsin Uniform Dwelling Code does require a vapor barrier. The reason being that the foam does not cover the framing members and thus does not provide the required continuous vapor retarder. Another reason for the vapor retarder requirement is that some foam products are open celled and may shrink, leaving voids that could allow air leakage and potentially cause moisture problems.

If you have a housing question, you may send it to John Merrill, 1300 Linden Dr., Madison, WI 53705. Or, visit the UW-Extension housing web site at http://www.uwex.edu/house .