Quick answer
If the house feels humid while the AC runs, the usual causes are an oversized system that cools fast and shuts off before drying the air, the fan set to on instead of auto, leaky ducts pulling in damp attic air, or mild, muggy weather when the AC barely runs. Fixes range from a fan setting to a whole-house dehumidifier.
Why a cool house can still be damp
An air conditioner removes moisture only while its coil is cold and running. In the worst weeks of summer, dew points here sit in the upper 70s, and average summer humidity is around 76 percent, so the AC has a lot of water to pull out of the air. If the system cools the house quickly and shuts off, it never runs long enough to finish the job, and the air stays damp.
Indoor humidity above about 60 percent feels clammy and lets mold and dust mites thrive; somewhere around 45 to 50 percent feels comfortable in summer. A simple hygrometer from a hardware store tells you where your house sits.
The usual causes
Fixes, from free to bigger
- Set the fan to autoFree, and often a noticeable difference within a day.
- Change the filter and clear returnsGood airflow lets the coil do its job.
- Seal leaky ducts and returnsStops damp attic air from being pulled in.
- Right-size or slow down the systemTwo-stage and variable-speed equipment runs longer at low speed.
- Add a whole-house dehumidifierHolds humidity steady whatever the temperature.
Start with the first two, which cost nothing. If the house is still damp after a week, have the ducts checked and the system's run times and airflow measured before buying anything.
When a dehumidifier makes sense
A ducted whole-house dehumidifier sits in the attic, a crawlspace or a closet, pulls air from the house, dries it and sends it back through the ducts. It runs on its own humidistat, so it works on the mild, muggy days in spring and fall when the AC barely runs. It suits houses with oversized systems, tight new construction and homes that stay damp after the basics are fixed.
Portable dehumidifiers help one room at a time, but they need emptying and add heat to the room. One ducted unit covers the whole house and drains on its own.
If you are replacing the system anyway
A replacement is the time to fix humidity at the source. Size the new system from a load calculation rather than the old label, and look at two-stage or variable-speed equipment that can run long, low cycles. A slightly smaller system that runs longer often feels better here than a big one that cools fast and stops.
Ask how the installer will set the blower speed and the fan settings for humidity, and whether the thermostat has a dehumidify feature. Those settings matter as much as the equipment.
Sources
Figures in this guide come from these sources. Programs and prices change, so check current terms.
About this guide. Ward Teague writes the HVAC guides for Byron Vegas. Each guide starts from questions people ask when they call, and the figures in it come from the sources listed at the end of the article. More about the writer.