December 4, 2018

This blog was originally posted in Healthy Indoors magazine. Subscribe for free to print or digital editions here.

Mold can post serious health risks – especially for people with allergies and asthma. The key to controlling mold growth is reducing moisture, whether from leaks, high humidity, or variations in temperature in your home.

Below are 9 tips to help you keep your home drier and, thus, mold-free.

1. Dry up the drips

Even a small water leak can develop into a major mold problem, so check pipes under sinks, and behind your washing machine and refrigerator (if it has a water line). Repair leaks right away.

2. Don’t be sloppy when watering your indoor plants

I can't tell you how many homes I've seen with moisture stains on the floor under or near an indoor plant. Be careful not to spill or overwater when you are watering a plant and put a dish under the pot in case the pot leaks. It's also not a great idea to put a plant pot on a carpet or rug.

3. Watch the hot water tank

Put a battery-operated floor-water alarm near your hot water tank, so you'll be alerted if the tank starts to leak. If you have a central alarm system, consider having a floor-water alarm tied into the system. If you have a gas-fired hot water heater, plan to replace it right before the warranty expires. Electric hot water heaters tend to last longer than their warranty, but still, keep an eye on its warranty date.

4. Control the relative humidity below-grade

Mold growth doesn't always require standing water. Some molds can begin to grow when the relative humidity (RH) is over 80%. As air cools, its RH rises. Below-grade (below ground level) spaces like basements and crawl spaces are naturally cool and damp, so the RH must be adequately controlled.

Use a thermo-hygrometer to measure the RH. The RH should be kept at or below 50% in unfinished basement spaces and in crawl spaces, and below 60% in finished basement spaces.

During the spring and summer months, between mid-April and mid-October in the northeast, add dehumidification as needed, even if your finished basement has air conditioning. Be sure that your dehumidifier is adequate for the space, and attach the machine to a condensate pump, so it can drain into a sink or to the exterior. That way, you won’t have to empty the reservoir (when the reservoir is full, the dehumidifier will shut off).

In the winter, you do not need to dehumidify an unfinished basement. However, a finished basement must be kept consistently warm, whether in use or not, with the thermostat set at a minimum of 57° F.

5. Don’t over-humidify above grade

In the winter, we don't open windows and doors that much, so moisture can build up in a house in above-grade (at or above ground level) rooms. If you have an exhaust fan over your cook stove that vents to the exterior, use the fan whenever you cook or bake, and try to cook and bake at the same time.

After showering or bathing, operate the exhaust fan in the bathroom for at least twenty minutes. In addition, leave the door open and operate an oscillating fan in the bathroom to help dry surfaces.

If you have a central humidification system, keep the RH in habitable rooms under 40% (lower in extreme cold). The same holds true if you use a portable humidifier in any particular room. Always measure the RH with a thermo-hygrometer.

6. Keep your exterior closet(s) warm

A closet with one or more walls facing the exterior can develop conditions of elevated RH. This can lead to mold growth on those walls, as well as on goods stored on the cool closet floor. It's best not to overstuff a closet with personal goods; keep your things off the floor and away from cool walls.

Don't install carpet or put rugs on the closet floor, as carpeting captures biodegradable dust. Remove the dust from the floor and baseboard trim.

To warm the air and help control the RH in an exterior closet, you can heat the closet with a heater specially made for the purpose (or just the light). Or, you can keep the door open to increase the flow of warm air into the space. You can also install a louvered rather than solid closet door.

7. Don’t keep some rooms cold while you heat other rooms

In the winter, it's tempting to heat only the rooms that you use and leave other rooms cool. In homes with wood-burning stoves, I've often found that some rooms near the stove are toasty, while others are uncomfortably cold. This may help you save on your heating bills, but unfortunately, elevated RH conditions can develop in the rooms that aren't adequately heated, even if they are above grade. And then you know what may happen next… mold growth.

8. Isolate your crawl space

A crawl space should be isolated from the exterior and be dehumidified. Otherwise, humid air can enter the space and lead to condensation and mold growth on cool surfaces. If a crawl space has a dirt floor, the dirt should be covered with a mesh-enforced vapor barrier, or even with appropriate cementitious material, to limit the evaporation of moisture from the soil.

9. Don’t introduce unnecessary amounts of moisture into your attached garage

In the summer, dehumidify your garage during particularly humid weather; just keep the RH under 70%. In the winter, wipe snow off your car, including kicking snow off the tires, before driving the vehicle into the garage. If snow still falls onto the garage floor, sweep it to the exterior before it melts, or sweep the water out of the garage after the snow has melted.

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Jeffrey C. May, Guest Poster

Jeffrey C. May is a mold expert and author of three books on mold and other indoor air quality (IAQ) issues. After 17 years as an ASHI inspector, May switched gears to become an IAQ professional and founded May Indoor Air Investigations LLC in Cambridge, Mass. He has investigated IAQ problems in thousands of structures, and he has a lot to say about mold.