Humidity Control & Indoor Air Quality in Corpus Christi, TX

Target: below 60 percent

The EPA recommends keeping indoor relative humidity below 60 percent, ideally between 30 and 50 percent, to limit mold growth (EPA, A Brief Guide to Mold, Moisture and Your Home). In coastal South Texas, outdoor humidity is often far higher, especially overnight and in the morning, so the air conditioner is doing double duty. An inexpensive digital hygrometer tells you where your house stands.

Why a house stays humid with the AC running

Oversized system

A system that is too big cools the house fast and shuts off before the coil gets cold enough, long enough, to wring much water out. Short cycles mean cool, damp air.

Fan set to "on"

With the thermostat fan set to "on" instead of "auto," the blower keeps running after the compressor stops, and water on the cold coil evaporates right back into the house. In a humid climate, "auto" usually dries the house better.

Airflow set too high

Blower speed affects how much moisture the coil removes. Some systems have a dehumidification setting that slows the blower when humidity is high.

Leaky return ducts and attic air

Return leaks in the attic pull in hot, humid air. See ductwork.

Moisture sources

Bathroom fans that do not vent outside (or are not used), clothes dryers venting into the garage or attic, and air leaks around doors and windows all add moisture.

Fixes that work

• Fan on "auto" and a clean filter.
• Correct airflow settings for humid climates, checked during a tune-up.
• Seal return ducts and the air handler cabinet.
• Whole-home dehumidifier tied into the ductwork, which removes moisture without overcooling, useful in spring and fall when it is humid but not hot.
• Variable-speed or two-stage equipment at replacement time, which runs longer at lower speed and removes more moisture.
• Exhaust fans in bathrooms and over the stove that vent outdoors.

Musty smells and mold

A musty smell when the AC starts often comes from the indoor coil, drain pan or ducts. A slow condensate drain leaves standing water in the pan; dirty coils hold moisture and dust. The EPA's advice for mold in homes is to fix the moisture source first, then clean up. Significant mold, more than about 10 square feet, or mold inside ducts, calls for more careful cleanup (EPA, A Brief Guide to Mold, Moisture and Your Home).

Filters and air cleaners

A pleated filter in the MERV 8 to 13 range catches much more fine dust and pollen than a basic fiberglass filter, but a high-MERV filter can restrict airflow in some systems, so check that yours can handle it. The EPA notes that portable air cleaners and upgraded HVAC filters can reduce indoor particles, but they work best alongside controlling sources and ventilating (EPA, Guide to Air Cleaners in the Home).

Sources: EPA, A Brief Guide to Mold, Moisture and Your Home; EPA, Guide to Air Cleaners in the Home.