Signs Your Home Is Too Humid, and What to Do About Each

The signs

Common signs and the guidance behind them.

Sign	What it means	Source
Hygrometer above 60%	Above EPA's upper limit	EPA
Condensation on windows, pipes, walls	A sign of high humidity	EPA
Mold or mildew spots	Moisture has gone uncontrolled	EPA
Musty smell	Often mold or mildew out of sight	General observation
Water-damaged materials	Need drying within 24 to 48 hours	EPA
Clammy air on mild days	AC not running, so not dehumidifying	PNNL

Step 1: measure it

EPA's mold guide suggests using a meter to check humidity. Put a hygrometer in the main living area and one in the basement for a few days. If the basement reads 75 to 90%, ENERGY STAR calls that very damp; 90 to 100% is wet. Those bands are what you need to size a dehumidifier with our sizing calculator.

Step 2: find the sources

• Showers and baths. EPA advises running the bathroom fan or opening a window when showering.
• Cooking and dishwashing. EPA advises exhaust fans or open windows.
• Clothes dryers, stoves and kerosene heaters. EPA says to vent them outside where possible.
• Leaks. EPA says to fix leaky plumbing and other water problems.
• Ground moisture. Basements and crawlspaces draw moisture from soil; see dehumidifiers for crawlspaces.
• Humidifiers. EPA says not to humidify above 50%. Check that a humidifier is not the cause.
• Humid outdoor air. In swing seasons the AC may not run, and PNNL says it removes no moisture when it is not cooling.

Step 3: fix it

Ventilate and fix leaks

These cost little and stop the problem at the source. Dry anything that got wet within 24 to 48 hours, as EPA advises, and insulate cold water pipes to stop condensation.

Run a dehumidifier

EPA advises using air conditioners and dehumidifiers when needed. ENERGY STAR suggests 25 to 40 pints a day for a very damp space under 2,000 square feet and 30 to 50 for a wet one, and says it is better to oversize than undersize. Set it to 30 to 50%, ENERGY STAR's optimum, and drain it with a hose or pump so it never stops on a full bucket. See the best dehumidifiers.

Use the AC in summer

Air conditioning dries the air while it cools, though PNNL says it may not keep up in very humid conditions. Upstairs in summer, the AC plus closed windows often brings humidity into range; basements usually still need a dehumidifier. See dehumidifier vs air conditioner.

If you already see mold

EPA says that in most cases, if visible mold is present, sampling is unnecessary. Areas under about 10 square feet can usually be cleaned by a homeowner; larger areas call for EPA's more extensive guidance. EPA says bleach is not recommended as a routine cleanup method and should never be mixed with cleaners containing ammonia. Fix the moisture, or the mold will come back. See the humidity level that prevents mold.

Too humid vs too dry

Some homes swing between the two: too humid in summer, too dry in winter. EPA's 30 to 50% range applies year-round. A dehumidifier in summer and a humidifier in winter, both on humidistats, keep you in range. See signs your home is too dry and the ideal indoor humidity level.

While you fix the moisture, a HEPA purifier can capture airborne spores; see the best air purifiers for mold.