The short answer
The EPA says to keep indoor humidity below 60 percent, ideally between 30 and 50 percent. A simple humidity meter (hygrometer) tells you where you stand. Too humid: fix drains and moisture sources, make sure the AC is not oversized, and consider a dehumidifier. Too dry in winter: a humidifier can help, used with care.
The right level
The EPA’s guidance on mold and moisture says to keep indoor humidity below 60 percent, ideally between 30 and 50 percent. Relative humidity is how much moisture the air holds compared with the most it could hold at that temperature.
Measure it with a hygrometer; many digital thermometers and smart thermostats show it too. Check a few rooms, including the basement, and in different seasons.
Signs your home is too humid or too dry
| Too humid | Too dry |
|---|---|
| Air feels clammy or sticky even when cool | Frequent static shocks |
| Musty smells, especially in basements and closets | Dry, itchy skin and dry nose or throat |
| Condensation on cold pipes, windows or AC ducts in summer | Gaps opening in wood floors and trim; cracking furniture |
| Mold spots, peeling paint or damp stains | Instruments and wood items going out of shape |
In winter, heavy condensation on the inside of windows usually means the air is too humid for the cold weather, not too dry.
How your air conditioner removes moisture
An air conditioner dehumidifies as it cools. Warm, moist air passes over the cold evaporator coil, water condenses on it like on a cold glass, and the water runs out through the condensate drain.
- The drain must flow. DOE notes that clogged drain channels prevent a unit from reducing humidity, and the EPA advises keeping AC drip pans clean and drain lines unobstructed and flowing properly.
- An oversized AC dehumidifies poorly. It cools the air so fast that it shuts off before much water has collected – the short cycling pattern that leaves a home cold and clammy. Correct sizing is covered in our load calculation guide.
- Fan on Auto. Running the blower nonstop can return moisture from the wet coil to the house – see thermostat settings.
- Variable-speed equipment runs longer at low output, which tends to remove more moisture – see two-stage and variable-speed.
Dehumidifiers
When the AC cannot keep up – in basements, in humid climates in spring and fall when it is not hot enough for cooling, or in tight, well-sealed homes – a dehumidifier helps.
- Portable units suit a single room or basement. Empty the bucket or connect a hose to a floor drain, and clean the filter.
- Whole-house units tie into the ductwork and drain on their own. They are installed by an HVAC company.
Set the target within the EPA range rather than as low as possible.
Humidifiers
Cold winter air holds little moisture, so heated homes can get very dry. A humidifier adds moisture: portable units for a room, or a whole-house unit mounted on the furnace or air handler.
- Keep it clean and follow the maker’s instructions; standing water can grow mold and bacteria.
- Do not overdo it. Condensation on windows is a sign to turn it down.
- Whole-house units need their water panel or pad replaced and their drain checked, often as part of a fall tune-up.
Ventilation and moisture sources
A lot of indoor moisture comes from everyday life: showers, cooking, drying laundry, plants and a damp basement or crawlspace.
- Run bathroom and kitchen exhaust fans, and make sure they vent outdoors, not into the attic.
- Vent clothes dryers outside.
- Fix leaks, and direct gutters and downspouts away from the foundation.
- In tight homes, mechanical ventilation brings in fresh air in a controlled way. An ERV or HRV exchanges stale indoor air for outdoor air while recovering heat – and, in the case of an ERV, some moisture.
A note on mold
Mold needs moisture. The EPA’s core advice is to control moisture: fix leaks, dry wet areas promptly and keep humidity in the range above. Cleaning mold without fixing the moisture source means it comes back.
If you find substantial mold inside the heating and cooling system or ducts, see our duct cleaning guide and have the moisture cause found before anything is cleaned. Large areas of mold, or mold after flooding, are a job for professionals.
Sources
- EPA: A Brief Guide to Mold, Moisture, and Your Home (checked October 2026)
- DOE Energy Saver: Maintaining Your Air Conditioner (checked October 2026)
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Questions
What should indoor humidity be?
The EPA says to keep indoor humidity below 60 percent, ideally between 30 and 50 percent.
Why is my house humid with the AC running?
Common causes are an oversized AC that short-cycles, a fan left on On instead of Auto, a clogged condensate drain, and moisture sources such as unvented bathrooms or a damp basement. A dehumidifier can help where the AC cannot keep up.
Does an air conditioner remove humidity?
Yes. Moisture condenses on the cold indoor coil and drains away. It only works well when the system is sized correctly, runs long enough and has a clear drain.
Why are my windows wet inside in winter?
Usually the indoor air is too humid for the cold weather. Turn down any humidifier, run exhaust fans and improve ventilation.
Related guides
Updated October 2026 by the Heating and Cooling editorial team. General information for homeowners, not professional advice – rules and equipment differ. When gas, carbon monoxide, electricity or anyone’s safety is involved, call 911 or your utility. How we write our guides · HVAC glossary