Mildew and mold

The Old House Web

The high humidity of an East Coast summer, combined with winter weather conditions, can make homeowners fall victim to household mildew.

"A large snowfall prevents the ground from freezing deeply, or a mild winter can fail to freeze the ground, allowing for easy water penetration," says Phyllis Adams, associate professor of agricultural and extension education at Penn State's College of Agricultural Sciences. "Any rain or melting snow, followed by a humid summer gives you perfect conditions for mold and mildew growth."


Adams says the causes of most mildew problems are not difficult to understand or to solve. "Air can absorb moisture. It's like a sponge," she says. "Warm air can hold more water vapor than cold air. When the temperature of the air drops, its ability to hold moisture is lessened until the air reaches the dew point. Droplets of water will form on any surface that is at or colder than the dew point of the surrounding air."

Warm, moist air within a house is attracted to all cold surfaces, including windows, mirrors, the corners of basements, exterior corners of a room and inside closets. The buildup of condensation creates the perfect environment for mold spores to thrive.

Adams says mold spores grow especially well on a dirty surface. Cleaning regularly will make it more difficult for mold and mildew to grow.

According to Alletta Schadler, director of Lebanon County's Cooperative Extension office, mildew and mold spores are present in homes all the time, growing when temperature and moisture reach high enough levels.

Mold can spread from basement surfaces and along cold walls upward through the floor, often onto rugs, behind furniture placed against a wall, onto bookshelves, or on cardboard boxes placed against a wall.

"The first reflex when people smell mold is to throw open the windows, which is exactly the wrong thing to do. It actually makes the mildew worse," Schadler says.

In summer, mildew most often plagues older homes that do not have central air-conditioning, which can act as a dehumidifier by circulating air.

Although mildew flourishes in high humidity, several easy-to-follow preventative tips will keep surfaces, furniture, clothing and other possessions from looking like moss-covered logs. Schadler lists the following measures:

  • Keep doors and windows shut during periods of high humidity. Open them at night, when the temperature outdoors is the coolest, and then close the house up in the daytime.
  • Circulate air as much as possible with portable fans. Make sure air in the basement is being circulated as well by using a small fan.
  • Buy a dehumidifier for the house. Schadler warns that most dehumidifiers have water pans that hold far less than the machine can extract from the air. To prevent constant trips to empty the water pan, Schadler suggests rigging a hose from the appliance into a drain.
  • If dampness in the house is a problem, consider buying a smaller dehumidifier for the home's upper floors.
  • Make sure the outside landscaping is designed so rainwater will drain away from the house. Poor drainage and poor air circulation around shrubs next to the house will encourage mold and mildew growth. Winds can bring mold spores right in through an open window.

Unfortunately, mold and mildew are not just summer afflictions. In winter, moisture can collect within a home as a byproduct of burning fossil fuels (such as gas stoves, kerosene heaters or gas fireplaces), cooking, using clothes dryers, or taking long showers without using an exhaust fan.

Winter mildew can be a problem in newer, energy-efficient homes. Often built with heavy insulation and plastic moisture barriers, newer homes can retain enough moisture to begin condensation -- again creating a perfect environment for mold.

Adams recommends these preventative measures to control indoor moisture:

  • Limit the number of plants in your home.
  • When cooking, use lids on pots and pans.
  • Use the kitchen exhaust fan when cooking, and make sure the fan vents directly outdoors. Many range hoods filter only food particles generated during cooking, returning steam back into the kitchen.
  • Use the exhaust fan when showering. "Most people, especially older individuals, don't use the exhaust fan because it makes them feel colder," Adams says.
  • Determine how the clothes dryer is vented. Adams says dryers often are vented into the attic, where hot, moisture-laden air meets cold surfaces. "Where this situation exists, I've seen water droplets dripping off the rafters," Adams says.
  • Make sure air is able to circulate in each and every room. Keep closet doors open.
  • Diligently clean the house and its surfaces at least once, preferably twice a year.
  • Make sure compost piles are well away from the house. "These produce huge quantities of mold spores than can enter the indoor environment when the wind blows," Adams says.

Once mildew appears, Schadler recommends quick action. All affected surfaces should be washed with a chlorine-based or fungicidal cleaner. Do not mix chlorine bleach with detergent cleaning solutions, Schadler warns.

Because mold and mildew spores are ever-present, Schadler says even the most obsessive housekeeper can never completely rid a home of the threat of mold.

"The best way to prevent mold is knowing what conditions make it grow," Schadler says.

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