Humidity, Mold and Air Conditioning in Florida

Why Gulf Coast air is so wet

Warm water on three sides and frequent afternoon thunderstorms keep the air around Sarasota humid for much of the year. The National Weather Service office for the Tampa Bay Area tracks dew points and heat index values that reflect that moisture through the summer rainy season (National Weather Service Tampa Bay Area). Every time a door opens, or air leaks in through gaps, some of that moisture comes inside.

The target

The U.S. EPA recommends keeping indoor relative humidity below 60 percent, ideally between 30 and 50 percent (U.S. EPA, A Brief Guide to Mold, Moisture and Your Home). Mold needs moisture to grow; dust mites thrive above roughly that level too. A small digital hygrometer shows where your house sits and is worth having.

How an AC dries the air

The indoor coil runs cold. Moisture in the air condenses on it and drips into a pan, then flows out the condensate drain. Drying is most effective after the coil has been cold for several minutes, so long, steady cycles remove more moisture than short ones.

Common reasons a house stays humid

1. The thermostat fan is set to "on." After cooling stops, the fan blows across the wet coil and evaporates water back into the house. "Auto" usually fixes it.
2. The system is oversized. It reaches the set temperature quickly and stops before drying the air.
3. Mild weather. In spring and fall the AC runs less, so it dries less, even though the air is still humid.
4. Air leaks. Gaps in the house and leaky return ducts pull humid air in.
5. Dirty coil or low charge. Both reduce how much water condenses.

Mold warning signs

• A musty smell in closets, bathrooms or when the AC starts.
• Spots on ceilings near vents, on window frames or on stored leather and shoes.
• Condensation on vents, windows or ducts.

The EPA advises fixing the water problem first, cleaning mold off hard surfaces, and drying wet materials within 24 to 48 hours to prevent growth (U.S. EPA, A Brief Guide to Mold, Moisture and Your Home).

Seasonal homes

Many Sarasota-area owners leave for part of the year. A home closed up with the AC off through a Florida summer can grow mold quickly. Common approaches: set the AC to a moderate temperature rather than off, use a thermostat that can control humidity, or install a dehumidifier. Ask someone to check on the house after storms. Florida Power & Light publishes energy tips for setting a thermostat while away (Florida Power & Light, energy-saving tips).

When a dehumidifier makes sense

If the AC is properly sized, clean and charged, the fan is on "auto," and humidity still runs above 60 percent, a whole-home dehumidifier is the next step. See humidity control and what shapes humidity work.

Related pages: humidity control & dehumidifiers, indoor air quality & filtration, ac maintenance & tune-ups, and what shapes an estimate.

Sources: U.S. EPA, A Brief Guide to Mold, Moisture and Your Home; National Weather Service Tampa Bay Area; Florida Power & Light, energy-saving tips.