Huntsville humidity and mold: a month-by-month look

People who move to the Tennessee Valley often notice it first in July: towels don't dry, the bathroom mirror fogs and stays fogged, and the crawl space smells different. That's not your imagination. The figures below come from NOAA's official 1991–2020 climate normals for Huntsville International Airport, the long-term averages forecasters use. They explain why mold problems here cluster in certain months and certain parts of the house.

Dew point, not relative humidity, is the number to watch

Relative humidity swings all day as the temperature rises and falls. Dew point is steadier: it's the temperature at which the air's moisture starts condensing on a surface. Any surface in your home that's colder than the outdoor dew point will get wet if outside air reaches it. That includes air-conditioning ducts, the cold copper refrigerant line, a basement wall, or the shaded underside of a floor.

Dew points are averages of NOAA's hourly normals for each month; precipitation is NOAA's monthly normal. The year totals about 54.3 inches of precipitation, with around 79 days that get a tenth of an inch or more.

What the numbers mean for your house

• June through August is condensation season. In NOAA's hourly normals, every single hour of those three months averages a dew point of 60°F or higher. Warm, wet air is the norm, not the exception.
• Nights are nearly saturated. On a normal mid-July morning around 6 a.m., the temperature is about 73°F and the dew point about 67°F, which works out to relative humidity above 80 percent. Air leaking into a crawl space or attic at that hour is close to its limit.
• Winter is the wet-ground season. December is the wettest month on average and March close behind. That's when water against the foundation, clogged gutters and poor grading show up as damp crawl spaces and basement walls.
• The shoulder months mislead people. May and September can feel pleasant but still carry dew points in the high 50s to around 60, so a house closed up with the AC off can climb in humidity fast.

Indoor targets

EPA recommends keeping indoor relative humidity below 60 percent, ideally between 30 and 50. An inexpensive hygrometer will tell you where you stand. In a Huntsville summer the air conditioner does much of the drying, so a house that sits closed up with the system off can turn damp quickly. Bathroom and kitchen exhaust fans that actually vent outdoors help too, and EPA suggests wiping up condensation on windows and pipes and treating it as a sign that indoor humidity is running high.

When readings stay high

If your hygrometer sits above 60 percent indoors for days, or you see sweating on ducts or windows, look for a source: a damp crawl space, a leak, a dryer venting indoors, or a ductwork leak pulling in humid air. A mold inspection can trace it, and our crawl space page covers the most common culprit under older homes.

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