Charleston Humidity and Mold: Why It Grows and How to Stop It

Charleston humidity and mold

Mold grows where things stay damp, and Charleston's air keeps things damp. About 140 days a year at the airport average a dew point of 65°F or more. The EPA says to keep indoor humidity below 60 percent, ideally 30 to 50, and to dry anything wet within 24 to 48 hours.

Updated October 28, 2026 · Sources listed at the end

The numbers

We counted every day since 1973 in NOAA's daily record for Charleston International Airport. Over 2016-2024, the airport averaged 140 days a year with a daily mean dew point of 65°F or higher, and 96 at 70°F or higher. July and August average about 73.2°F and 73.2°F. In 2024 it was 144 days.

Every day of 2024 at Charleston International Airport, colored by the day's average dew point. 144 days averaged 65°F or higher, the muggy air that keeps a crawl space damp.under 45°F45 to 5455 to 5960 to 6465 to 6970 to 7475+NOAA NCEI Global Summary of the Day, station 72208013880.

Days per year with an average dew point of 65°F or higher

Source: NOAA NCEI Global Summary of the Day, Charleston International Airport (72208013880), 1973 to 2024.

That count hasn't climbed much over the decades; Charleston has always been this humid. What changes is the house. Air conditioning, tighter windows and ducts run through crawl spaces and attics create cool surfaces where that moist air condenses.

Where mold shows up in Lowcountry houses

Crawl spaces under raised houses: on joists, subfloor and ducts, from humid air coming through the vents. Encapsulation.
Closets on exterior walls and behind furniture, where air doesn't move.
Around the air handler, from a clogged condensate line or a sweating duct.
Bathrooms without a working exhaust fan.
Behind walls after a slow leak nobody saw.

Keeping humidity down

Get a cheap hygrometer and watch it. The EPA's target is below 60 percent, ideally 30 to 50.
Run the AC in summer, and keep the fan on "auto," not "on," so moisture drains off the coil.
Use bath and kitchen exhaust fans that vent outside.
Fix leaks right away and dry anything wet within 24 to 48 hours.
Keep gutters clean and water draining away from the foundation.
In a closed-up house or a sealed crawl space, a dehumidifier helps. Crawl space dehumidifiers.

Health

The CDC says people with asthma or who are allergic to mold may have severe reactions, and that immune-compromised people and people with chronic lung disease may get infections in their lungs from mold. If someone at home is in one of those groups, treat a mold problem as urgent and talk to a doctor.

Questions Charleston homeowners ask

Why is everything in my house getting moldy?

Usually indoor humidity above 60 percent for long stretches. Check with a hygrometer. Common causes in Charleston are an oversized or short-cycling AC that doesn't dehumidify, a crawl space open to summer air, and bathrooms without exhaust fans.

Will a vapor barrier stop mold in my house?

It helps if the crawl space floor is bare dirt, which gives off moisture all year. In a vented crawl space it doesn't stop humid outdoor air from coming in through the vents; that takes sealing and humidity control.

Is it normal for a Charleston house to smell musty?

No. A musty smell means mold or mildew is growing somewhere, often in the crawl space, a closet or the air handler. Find it before it spreads.