FAIRBANKS — The “Ask a Builder” series is dedicated to answering some of the many questions Fairbanks residents have about building, energy and the many other parts of home life.

Q: How can I keep mold out of my house?

A: Maybe you’ve seen it in the bathroom, around the window frame or along the corner of an outside wall. Mold is a hazardous — though not uncommon — reality in Fairbanks. How can you keep it out of your house?

Mold requires several ingredients to grow: moisture, above-freezing temperatures, oxygen and nutrients (drywall or wood, for example). Mold spores exist in abundance outdoors and can enter your home relatively easily through open windows and doors, clothing, shoes or pet fur. 

If spores land on a surface with available nutrients and moisture, they can grow into a colony. In cold climate homes, the main sources of moisture leading to mold growth include condensation forming on indoor surfaces, plumbing leaks and weather-related water intrusion.

Because humans need warm, moist air be comfortable, we can’t eliminate those factors from our homes. That’s why mold prevention is largely focused on managing condensation. 

Condensation occurs when humid air encounters a cool surface, such as windows in an exterior wall. When moist air cools to the dew point, the water vapor in that air forms liquid water on cold surfaces nearby. Hence: bathroom windows fogging up after a shower, or water droplets forming on the kitchen window while cooking on a cold winter day.

Inside an exterior wall or roof during winter, where temperatures may be below freezing, indoor air leakage also will condense to form hidden frost much the same way frost occurs on the siding around an exterior dryer exhaust.

Preventing mold growth

Controlling indoor humidity levels is a big step toward preventing mold growth. For tight houses, ventilation is absolutely essential. 

At minimum, this means using exhaust fans in bathrooms and kitchens. 

Ideally, a heat recovery ventilator will provide continuous ventilation throughout the house. Humidity-related issues also will vary depending on the insulation level of your home. In a home with better insulated exterior walls, doors and windows, these surfaces stay warmer. Consequently, they will be able to withstand higher levels of indoor humidity and colder outdoor temperatures before condensing events occur.

Although indoor humidity levels between 40 percent and 60 percent are optimal for human health, the reality is in an extreme cold climate like Fairbanks, levels higher than 30 percent can lead to condensation on windows, exterior walls behind furniture and in closets. Humidity levels lower than 30 percent can be tolerated by humans, however a greater percentage of occupants may experience the physical discomforts associated with drier air.

Indoor humidity levels between 20 percent and 30 percent are much safer in terms of reducing the condensation risk during the winter, especially during cold snaps. To measure the humidity level in your home, you can buy a hygrometer at a hardware store or online for between $20 and $60.

Even homes with low overall humidity may have damp microclimates where mold can grow. Inspect areas such as crawl spaces periodically. A crawl space can produce large quantities of water vapor if damp soils aren’t covered with a well-sealed vapor retarder. 

Be on the lookout for water leaks, air leaks in ducts, condensation on pipes or discolorations on wood surfaces — particularly around the rim joist areas in crawlspaces and basements. Be sure to address any issues promptly. 

If you find standing water as a result of a leak, you have 24 to 48 hours to dry the area before mold spores can settle and grow. Clean up the water as soon as possible and use a dehumidifier or fan as needed to help dry out the area.

Creating an air space between the wall and furniture, clothing or other objects promotes air circulation and can eliminate damp areas on walls. Drying firewood indoors can also contribute to moisture loads. A plugged or disconnected dryer vent can introduce large amounts of water vapor into the air and can go unnoticed. Inspect all exterior vents periodically to ensure they are in good working order.

Eliminate any standing water in the home. You can prevent standing water in showers and sinks by keeping drains clear and clean. Keeping a pot or kettle full of water going on the stove should be avoided.

Finally, if you do discover mold growth, clean it up as soon as possible to stop the mold from spreading and to prevent further occupant exposure.

Additional resources

The Cooperative Extension Service at UAF has links to several publications on mold and moisture problems: http://bit.ly/2fcY2vM

The Canada Mortgage and Housing Corporation has compiled a list of resources on mold in homes: http://bit.ly/2fjnUT6

CCHRC has a variety of articles at cchrc.org/publications.

Ask a Builder articles promote home awareness for the Cold Climate Housing Research Center (CCHRC). If you have a question or to sign up for classes, contact us at info@cchrc.org or 457-3454.