Oregon State University Extension Service

How to avoid and correct home moisture problems

BEAVERTON, Ore. – Mold season is here, and although excess moisture – both inside and outdoors – can cause a wide variety of problems in the home, knowledge and simple measures can help keep a house mold-free.

When November weather arrives, heated air can trigger moisture problems because it holds more moisture than cool air. When warm air comes in contact with a cold surface, it cools down and excess moisture condenses, usually on the spot – notably single-pane windows or non-insulated walls. The moisture left behind can lead to mold, mildew and high concentrations of bacteria and dust mites.

An important first step to prevent and correct indoor moisture buildup is to use bath and kitchen fans to quickly pull moisture out of the room before it spreads to the rest of the house, according to Jeanne Brandt, family and community health faculty member with the Oregon State University Extension Service in Washington County.

"If a fan that vents to the outside does not exist in the kitchen and bathroom, consider having one installed," she said.

Most indoor moisture is caused by normal respiration of people and pets, but other activities also can be a source of excess moisture, Brandt said, such as baths and showers and hanging wet towels and clothes inside to dry.

Firewood stored indoors, though seemingly dry, can be a problem, as well as uncovered aquariums and large numbers of houseplants (more than five to seven). Indoor humidity levels are best when between 30 and 50 percent relative humidity. You can measure levels with a hygrometer, a low-cost purchase from a hardware or electronics store.

Following are common clues of indoor moisture problems:

  • Musty smells may signal mold, mildew or rot. Household odors that linger may indicate too much moisture in the air.
  • Frost and ice on cold surfaces from condensation can be a sign of excess moisture in the air.
  • A sensation of dampness is common with high humidity.
  • Surface discoloration, staining and texture changes may appear as black or dark streaks or lines. Mold and mildew often appear as a discoloration that can be white, orange, green, brown or black. Conditions often are noticed as a musty odor and are found under carpets, behind cupboards, on framing between sub floors and in crawl spaces and attics.
  • Wood swells when it becomes wet and warps, and it cups and cracks when it dries.
  • Wood rot and decay indicate advanced moisture damage. Fungi penetrate the wood and make it soft and weak.

Outdoor moisture gets into houses by rain, snow or ground water leaking into basements, crawl spaces, roofs and walls. Water also moves in through porous materials, such as vertical movement through a cement block wall. Vapor diffusions allow moisture to permeate even through solid surfaces such as cement, gypsum board or wood.

The following OSU information about mold also is available online:

Source URL: https://extension.oregonstate.edu/news/how-avoid-correct-home-moisture-problems