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NDSU Extension Service, Published December 20 2011

Flooding and wet soil may worsen winter home moisture problems

FARGO - Moisture that still remains in building materials and moisture vapor transfer into basements are likely to aggravate home moisture problems this winter.

"As outdoor temperatures cool, home moisture problems increase because moisture condenses on cool surfaces," notes Ken Hellevang, an agricultural engineer with the North Dakota State University Extension Service. "It's common to see damage to homes from window condensation and mold growth on walls beginning at this time of the year. The mold growth also can be a health hazard because mold spores in the air in large numbers can trigger allergies and other respiratory problems."

Hellevang predicts that those problems may be worse than usual this year because of high subsoil moisture levels in some areas and moisture-laden building materials.

New homes typically have more moisture problems because new lumber may have a moisture content of up to 19 percent. Those materials will dry to about 6 percent in the dry environment of the home during the winter, releasing moisture into the home. Homes that got wet through basement seepage or flooding, for example, will experience similar drying during the winter.

"Wood and building materials that got wet will continue to release moisture into the home even though they dried during the summer and fall because they only dry to 10 to 13 percent, which is in equilibrium with air conditions," Hellevang notes. "Also, moisture vapor will move into the house through concrete basement walls and the floor if the soil around the basement is wet."

The best remedy for home moisture problems is to try to maintain the relative humidity at 30 to 40 percent during the winter, Hellevang says. Lower humidity levels dry out nasal tissue, skin and furniture. Higher humidity levels increase the chances of moisture damage to the building and creating wet surfaces prone to mold growth.

Moisture condenses on windows when the window surface is cold or the air relative humidity is too high for what the home can handle.

"You can limit window condensation by warming the window surface by adding storm windows, another pane of glass or by covering windows with plastic," Hellevang says.

Covering the window with plastic normally will reduce air leakage, which also helps keep the window warmer. Air leakage cools the window surface, increasing the potential for condensation.

Closing drapes over a window typically causes more condensation on the window, Hellevang notes. Drapes insulate the window, cooling the surface, but allow moisture-laden room air to reach the window. 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 says. "Warm air needs to circulate behind furniture and onto the outside walls of closets to keep the surfaces warm."

Condensation also can be limited by reducing the air's relative humidity. The best way to reduce indoor humidity during the winter is by providing more ventilation, exchanging inside air with outside air.

In addition to moisture from the structure, many normal activities, such as taking showers, cooking and growing plants, release moisture into the air. That moisture needs to be removed to maintain the relative humidity in the 30 to 40 percent range. Air exchange removes moisture because the colder outside air is drier than air inside. Thus, outside air at 30 F and 90 percent relative humidity has a relative humidity of only 20 percent when pulled inside and warmed to 70 F. Therefore, the air is very dry, so it can remove moisture from the house.

Air exchange also improves indoor air quality by removing fumes and pollutants, Hellevang says.

Air exchange may occur naturally when doors are opened and through normal infiltration. Operate bathroom fans during showers and for about 15 minutes or longer at other times if humidity is a problem, Hellevang advises. Opening doors and windows may be necessary to achieve the required air exchange to reduce the moisture level in the house.

Both an inlet and an exhaust opening are required to exchange the air. Air-to- air heat exchangers provide for air exchange while reducing heat loss.

Information on air-to-air heat exchangers is available in NDSU Extension Service publication AE-1393, "Air-to-Air Heat Exchangers for Healthier Energy-efficient Homes."

Monitor the humidity level using a meter that can be purchased at hardware and discount stores. Hellevang recommends verifying the accuracy of the meter by placing it in a sealed container, such as a gallon plastic bag, along with a cup containing ¼ cup of salt and ½ cup of water. After 12 hours, the relative humidity should be at about 75 percent.

Hellevang notes that dehumidifiers may have limited ability to control winter condensation problems. Dehumidifiers are designed to remove moisture from air that is warm and at high relative humidities. They may not reduce the humidity level enough to prevent condensation.

More information on home moisture is available in NDSU Extension Service publication AE-1204, "Keep Your Home Healthy," and on NDSU's home moisture website at http://www.ag.ndsu.edu/homemoisture.