Solve Winter Home Moisture Problems

Ken Hellevang, Agricultural Engineer

Every breath you exhale flings moisture into the air. Showers, cooking and houseplants all add humidity too.

That humidity combined with cold windows and walls is a recipe for winter moisture problems, says Ken Hellevang, an agricultural engineer with the North Dakota State University Extension Service. Humidity in the warm air of your house will condense on cold surfaces just as humid summer air condenses on the outside of the glass holding your icy beverage.

"A relative humidity of 30 to 40 percent is considered optimum during the winter," Hellevang notes. Reduce humidity further and skin and nasal tissues become excessively dry, furniture can be damaged and heating efficiency is reduced.

If the inside temperature is 70 degrees and the relative humidity is 35 percent, moisture condenses on single-pane windows at 30 degrees, he says. With double-pane windows, condensation will form at temperatures below zero. With three layers of glass, condensation will not form until outdoor temperatures are about 40 degrees below zero, Hellevang says. He notes that condensation will form at higher temperatures if air leakage cools the window surface.

Window condensation can be controlled by warming the window by adding another pane or covering it with plastic.

"Adding plastic will normally reduce air leakage, which frequently contributes to the condensation problem," he notes.

Moisture is added to air in a house from many sources. Each person contributes about 3 pints per day while breathing. Each shower, and air drying towels afterward, contributes about a pint of moisture. Houseplants also contribute moisture to the air.

"In a well-sealed house, this moisture accumulates until there is enough excessive moisture to cause condensation or other problems," Hellevang says.

Some exchange of air with the outside will help maintain relative humidity in the 30 to 40 percent range, he says. The exchange may occur when doors are opened and through normal infiltration into the house.

"If there is not enough air exchange, bathroom fans that exhaust moist air outdoors should be operated during a shower and for about 15 minutes after a shower," Hellevang advises. "Some outdoor air exchange is also appropriate to maintain indoor air quality." Excessive air exchange may make air in the house very dry and result in discomfort and reduced heating efficiency.

Closing drapes over a window typically causes more condensation, Hellevang notes. The drapes insulate the window, cooling the surface, but allow room air with moisture to reach it. Furniture and other objects near a wall can have the same effect.

"That's why closets with outside walls sometimes have mold growth on the wall," Hellevang explains. "There should be warm air circulating behind furniture and onto the outside wall of closets."

Dehumidifiers have limited ability to control winter house moisture problems. Dehumidifiers are designed to remove moisture from air that is very warm and at relative humidities above 50 percent, he notes.