Every August the calls to this line change. The storm calls fade and the humidity calls start: a closet that smells, a line of dark spots along the bottom of a bedroom wall, a bathroom ceiling that was painted in May and is speckled again, mold on the leather shoes and the backs of dressers. None of it is a leak. It is the air.
Tallahassee's summer dew points sit above 70 degrees for weeks, which means any surface below that temperature sweats. Inside a house that is a cold supply duct, an uninsulated line, a tile floor over a crawl space, the drywall on an exterior corner behind a bed. The mold follows the condensation.
The oversized air conditioner
The most common cause is a system too big for the house. A big unit cools the air fast, reaches the thermostat setting in ten minutes and shuts off, before the coil has run long enough to wring the moisture out. The house is cool and damp at 62 percent humidity, and mold grows at anything much above 60. A right-sized unit runs longer cycles and pulls the humidity down to the low 50s; so does a whole-house dehumidifier added to an existing system. If your house is cold and clammy, that is the diagnosis.
Sweating ducts and fans that vent into the attic
In the attic, flexible ducts carry 55-degree air through 130-degree space. Wherever the insulation sleeve is torn or compressed, the duct sweats, drips onto the insulation and the ceiling drywall, and the ceiling stains and grows mold in a line that follows the duct. Bathroom fans that were never ducted outside dump warm, wet air into the same insulation every morning. Both are fixed by an HVAC contractor or a handyman, and both need fixing before the ceiling is remediated or it will be back.
Closets and exterior corners
Closets on exterior walls have the coldest, stillest air in the house, especially with the door shut and the clothes packed in. The wall sweats, the paper face of the drywall stays damp and the growth starts behind the clothes. The same happens behind a dresser or a headboard against an exterior wall. Pulling furniture a few inches off the wall and leaving closet doors ajar helps; a dehumidifier helps more.
Crawl-space houses
Under the older houses in Midtown, Betton Hills and Indianhead Acres, the summer air coming through the foundation vents is wetter than the air already under the house, and it condenses on the cool joists and the underside of the floor. The musty smell comes up through the floor registers and the family blames the AC. The fix is to seal the vents, lay a vapor barrier over the ground and run a crawl-space dehumidifier; cleaning the joists first if the growth is established.
What to do this week
- Buy a hygrometer and check the humidity in the bedrooms and closets; above 60 percent is the problem.
- Have the AC checked for run time and the ducts for torn insulation.
- Make sure every bathroom fan vents outside and runs for twenty minutes after a shower.
- Move furniture off exterior walls and leave closets open.
- If there is visible growth larger than a few square feet, or a smell you cannot find, book an inspection before the wall is opened.
Useful links
- EPA: a brief guide to mold, moisture and your home – humidity targets and cleanup guidance
Questions on this
What humidity should I keep my house at in Tallahassee?
Between 45 and 55 percent relative humidity. Mold growth picks up above about 60 percent, and most Tallahassee houses without dedicated dehumidification sit there all summer. A cheap hygrometer tells you where you are.
Will a bigger air conditioner stop the mold?
Usually the opposite. Bigger units cool faster and run shorter, and short cycles do not remove moisture. A correctly sized unit, or an existing unit with a dehumidifier added, keeps the humidity down; size is the question for the HVAC contractor, and the answer is often smaller.