Updated November 1, 2026 · Reviewed for accuracy by a licensed Uniform HVAC technician before publication
The EPA recommends keeping indoor humidity below 60%, ideally 30–50%. An air conditioner removes moisture as part of cooling, but only when it runs long enough with the right airflow. Several common conditions stop it doing that well.
Why a home stays humid
| Cause | What happens |
|---|---|
| Oversized system | It cools the air fast and shuts off before removing much moisture. |
| Fan set to On | Moisture on the coil re-evaporates into the house between cycles. |
| High airflow | Air moves across the coil too fast to drop much moisture. |
| Duct leaks | Return leaks pull hot, humid attic or garage air into the system. |
| Infiltration | Gaps, open windows and exhaust fans bring outdoor humidity in. |
| Low cooling load | Mild, humid days and empty homes don't run the AC enough. |
Options, in order
Measure
A humidity reading over time tells you if there's a real problem.
Fix settings
Fan on Auto; humidity-aware thermostats where suitable.
Fix airflow and ducts
Correct blower speed and seal return leaks.
Right-size at replacement
A load calculation, and variable-speed equipment if humidity is a priority.
Add dehumidification
A whole-home dehumidifier when the AC alone can't do it. See dehumidifier vs. AC.
We don't make medical or mold-eradication claims. Controlling humidity reduces conditions that favor mold; existing mold should be assessed by a qualified specialist.