University of Wisconsin - Extension

Condensation, and even ice, on windows is a common problem in Wisconsin homes during this time of year.

"Understanding the causes of moisture problems is the first step in solving it," says John Merrill, University of Wisconsin-Extension housing specialist. "Condensation and ice form on windows because the window surface is relatively cool. So, like the cold drink glass in the summer, some of the moisture in the air near the window condenses on the glass."

Ice on storm windows usually means that air is leaking into the cavity between the prime window and the storm window. The storm window glass often is cold enough that, even with low relative humidity in the house, condensation and ice form.

The air usually leaks into the space between the windows through gaps between window sashes and the frames. Merrill says 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 as much moisture as possible to escape.

The long-term solution is to improve the weatherstripping on the primary windows. There are various types of replacement weatherstripping available that are suitable for surfaces that will slide or compress. Another possibility for double hung windows is to replace the tracks that the windows run in. This can help the windows work better, as well as make them tighter.

This time of year it is probably best to consider a temporary solution. There are two options. The first is to use rope caulk to plug all cracks. Rope caulk, as the name implies, comes in a roll and handles like putty or modeling clay. You can reform the caulk into a bead the size of the opening and press it into the crack. In the spring, it can be removed without damaging the paint or varnish. It can be applied in cold temperature conditions but needs a dry surface.

The second option is to cover the entire window with a sheet of clear plastic. Kits with the plastic, double-stick tape and instructions are available at hardware stores and home centers in various sizes. It is important to seal the plastic to the window so air can't leak around it. The instructions that come with the kit usually recommend wrapping the window frame and sealing it to the outside edge of the frame or casing. The tape that comes with most of these kits can be removed in the spring without damaging the finish. However, if you leave the tape in place for several years, it may be difficult to remove.

If the condensation occurs on the primary 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 F, condensation should not occur on double-glazed windows until outside temperatures drop below zero, if relative humidity is kept below 40%. If you have additional glazing, including Low-e coatings or tight fitting storm windows, it takeas even colder temperatures before condensation occurs.

There are a couple of exceptions to this rule. First, if you cover the windows with blinds or drapes at night you will reduce heat loss through the windows, but you also cool the windows enough for condensation to occur. Also, if you set back your thermostat at night, you may notice condensation on windows in the morning. This should disappear as the temperature rises.

If you have relatively new double-glazed windows and you are still having persistent condensation problems, your relative humidity is probably above 40%. You can check the relative humidity with a humidity gauge. However, beware that humidity gauges are often unreliable. It is a good idea to have a couple of different types of gauges or borrow a second one for a while to confirm your reading.

You can drop the relative humidity by using kitchen or bath exhaust fans for an hour or two while you have a window in another part of the house open an inch or two. Be sure that the fans vent to the outside.The cold outside air is very dry and will quickly drop the overall relative humidity. The amount of heat lost is usually modest with this type of controlled ventilation.

For more information about window moisture problems, contact your local UW-Extension office for a copy of Merrill's housing publication, "Moisture Problems in the Home" (NCR 312). 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.