Humidity and Mold at Home

Mold needs water

Mold spores are everywhere indoors and out. They only grow into colonies when they land on a damp surface. The EPA's central message on mold is simple: the key to mold control is moisture control.

Leaky roofs and pipes, condensation on cold surfaces, flooding and high indoor humidity are the usual sources. If you fix the water, you take away what mold needs.

The humidity target

The EPA recommends keeping indoor relative humidity below 60 percent, ideally between 30 and 50 percent. A small digital or dial hygrometer, often sold for a few dollars, shows your level at a glance.

Air conditioners remove moisture as they cool. Oversized units that cycle on and off quickly may cool the air before they remove much water, which can leave a home cool but clammy. Dehumidifiers help in basements and humid climates.

The 24 to 48 hour rule

If something gets wet, such as carpet after a leak, the EPA advises drying it within 24 to 48 hours to prevent mold growth. Run fans and dehumidifiers and remove items that cannot be dried.

For small areas of mold on hard surfaces, EPA guidance describes scrubbing with water and detergent and drying completely. For large areas, more than about 10 square feet, or mold inside the HVAC system, EPA suggests consulting its guidance for professionals.

Mold and the HVAC system

Visible mold growth inside hard surface ducts or on other parts of the system is one of the three situations where the EPA says duct cleaning may make sense. But cleaning alone will not solve it if the moisture problem remains, such as a clogged condensate drain or a leaky duct sweating in a humid attic.

Keep the coil drain line clear and the drain pan dry, and watch for musty smells when the system first starts.

Condensation clues

Water droplets on the inside of windows in winter, sweating cold water pipes and damp spots on basement walls are signs that moisture is meeting cold surfaces. The EPA suggests reducing humidity, increasing air circulation and, where possible, adding insulation to cold surfaces to stop condensation.

Ducts that run through hot, humid attics can sweat on the outside when cold air flows through them. Proper insulation with an intact vapor barrier on those ducts helps prevent dripping and mold growth on nearby materials.

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