Nashville humidity and mold, month by month

Plenty of Nashville homes grow mold without ever having a leak. The cause is the air itself. For a good part of the year the outdoor air in Middle Tennessee holds enough water vapor that anything cool inside the house becomes a condensing surface. Knowing when that happens, and where, makes it much easier to stop.

Rain is steady, humidity is the bigger factor

The National Weather Service's Nashville climate page gives a 1991–2020 normal of 50.51 inches of precipitation a year at the airport, spread fairly evenly. May is the wettest month at 5.02 inches and October the driest at 3.36. On about 79 days a year at least a tenth of an inch falls. That steady rain keeps soil around foundations damp, but it's the summer humidity that puts water on surfaces inside the house.

The dew point tells the story

Dew point is the temperature at which air's moisture turns to liquid. Any surface cooler than it gets wet. Here are the monthly averages of NOAA's 1991–2020 hourly normal dew points for Nashville International Airport, which we computed from the published hourly file:

Two things stand out. Every single hour of June, July and August has a normal dew point of 60°F or higher, with the lowest hourly value at 61°F in June. Counting the whole year, 2,740 normal hours sit at a dew point of 60°F or more, and all of them fall between May and September. In practice that means an air conditioning duct, a cold-water pipe, a basement wall, or the floor framing over a vented crawl space can sit below the dew point for most of the summer.

Where that shows up in a house

• Crawl spaces. Vents are meant to let outside air dry the space, but in humid weather they do the opposite. The Department of Energy's Building America research found that in humid climates outside air is often wetter than crawl space air, causing structural, mold and pest problems, and that closed, conditioned crawl spaces cut humidity by more than 20 percent in the homes it studied.
• Basements. Below-grade walls stay cool all summer, so humid air touching them condenses. Finished basements hide that dampness behind drywall and carpet.
• Ducts and air handlers. Uninsulated supply ducts in attics and crawl spaces sweat, and a clogged condensate drain can leak quietly for months.
• Closets on outside walls. Poor air circulation behind stored clothes keeps those walls cooler and damper than the rest of the room.

Keeping it in check

EPA recommends keeping indoor relative humidity below 60 percent, and ideally between 30 and 50. A cheap digital hygrometer in the basement, a bedroom and the crawl space will show you where you stand. From there:

1. Run bath fans during and after showers, and vent the dryer outdoors.
2. Run a dehumidifier in a basement that stays above 60 percent, and empty or drain it reliably.
3. Keep gutters clear and downspouts discharging away from the foundation, so the soil next to the house isn't saturated after every storm.
4. Have the air conditioner's condensate line and drain pan checked each spring.
5. Look at the crawl space: standing water, wet insulation hanging from the floor or white growth on joists all point to a moisture problem that a contractor should assess.

When humidity has already caused growth

Lowering humidity stops new growth but doesn't remove what's there. Small patches on hard surfaces can be scrubbed with detergent and water and dried, per EPA. Larger areas, growth inside walls or anything over about 10 square feet is better handled by a pro; see mold remediation. If the dampness followed a specific leak or a flood rather than plain humidity, start with the risk checker. Sources: NWS Nashville, Nashville Climate Records; NOAA NCEI, 1991–2020 hourly normals for Nashville International Airport; U.S. DOE, Unvented, Conditioned Crawlspaces; EPA, A Brief Guide to Mold, Moisture and Your Home.

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