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Common problems

Humid, sticky air indoors

The house feels clammy even with the AC on, vents or windows sweat, or a hygrometer reads above 60%. Cooling the air and drying it are two jobs, and many systems only do the first one well.

What an audit checks Causes in your climate The fixes

A moisture meter pressed into water-stained basement drywall, an air-sampling pump on a tripod nearby

What it usually means

EPA advises keeping indoor relative humidity below 60%, ideally between 30% and 50%. Above that, mold and dust mites thrive, and the house feels warmer than the thermostat says.
An air conditioner only removes moisture while its coil is cold and the system runs long enough for water to drip off. Short runs, a fan that keeps blowing between cycles and humid air leaking in can all leave a house cool but damp.
An inexpensive digital hygrometer tells you whether you have a humidity problem before you spend money fixing one.

Where it’s most common

Our climate research lists this as a common problem in these metro areas. Each guide covers what homes there need, local audit programs and rebates.

Hot and humid Dallas–Fort Worth · Greater Houston · Miami · Tampa · Orlando · San Antonio · Austin
Mixed New York City · Atlanta · Washington, DC · Philadelphia · Charlotte · Baltimore · St. Louis

Somewhere else? Find your metro guide.

Likely causes, by climate

The same symptom has different causes in Miami, Phoenix and Minneapolis. Find your climate below; the tag marks the climates where this problem is most common.

Hot and humid Florida, Texas and the Gulf Coast Common here

Outdoor air is humid most of the year, so every leak brings moisture in. The usual causes are an oversized AC that cools fast and shuts off before it dries the air, the thermostat fan set to On (which blows water from the wet coil back into the house), return ducts leaking in the attic or crawlspace, and missing kitchen or bath exhaust. Mild, humid spring and fall weeks are often the worst, because the AC barely runs.

Hot and dry the desert Southwest and inland Southern California

Desert air is rarely the problem. If the house feels damp, look at how you use an evaporative cooler: it adds moisture by design and needs a window or vent open to work, and on humid monsoon days it cools less while adding more moisture. Plumbing leaks and a blocked AC drain are the other usual causes.

Mixed the Mid-Atlantic, St. Louis and the Southeast Piedmont Common here

Muggy summers and mild, humid spring and fall weeks bring the same problems as the Gulf Coast, plus basements and crawlspaces that stay damp. A basement often needs a dehumidifier in summer even when the rest of the house is fine.

Cold the Northeast, Great Lakes, Upper Midwest and Colorado Front Range

Upstairs is rarely humid, but basements often are in summer: humid air meets cool foundation walls and condenses, which brings musty smells. In winter, too much indoor humidity shows up as condensation on windows instead.

Marine and mild coastal the Pacific Northwest and coastal California

Summers are mostly dry, so humidity problems show up in winter: long, damp months, little ventilation, and cooking, showers and laundry drying indoors. Window condensation and mold in closets and corners on outside walls are the signs.

Not sure which climate you’re in? Your metro guide says, and the climate zone entry explains the map builders use.

How an audit confirms it

Guessing is how people end up with new equipment that doesn’t fix the problem. These tests measure the cause. Ask for them by name when you book; each guide explains what the results mean and what to do next.

Mold and humidity check Starts with humidity readings and a search for moisture sources: leaks, the crawlspace or basement, drain pans and ventilation.
Manual J load calculation A Manual J calculation includes the moisture (latent) load and shows whether the AC is oversized for the house.
Duct leakage test Finds return-duct leaks that pull humid attic or crawlspace air into the system.
Blower door test Shows how much outdoor air leaks in, and where, while the house is under pressure.

Try first (cheap or free)

Put a digital hygrometer in a few rooms for a week. Aim for below 60%, ideally 30–50%.
Set the thermostat fan to Auto, not On, so the coil can drain between cycles.
Run bathroom fans during and after showers, and use a range hood that vents outdoors.
Check that the AC condensate drain flows freely and the drain pan isn’t holding water.
In a damp basement, run a dehumidifier set to about 50%, draining to a floor drain if you can.

The fix ladder

From the cheapest step to the biggest. Work up only as far as you need to, and test again after.

Hygrometer, fan on Auto, exhaust fans used and vented outdoors Free–$
Seal return ducts and air leaks to the attic and crawlspace $–$$
A portable or ducted whole-house dehumidifier $–$$
Close and condition a vented crawlspace: drainage, a sealed ground vapor barrier and drying $$–$$$
Replace an oversized AC with a right-sized variable-speed system with a dehumidify mode, sized by Manual J $$$–$$$$

Rough cost before rebates: $ is under about $1,000, $$ about $1,000 to $5,000, $$$ about $5,000 to $15,000, $$$$ more. Prices vary a lot by region and house, so get written quotes.

When to call a pro

Mold covers more than about 10 square feet, or the house has had water damage. EPA points to its professional remediation guide for larger jobs.
Water drips from ducts, vents or the air handler, or the drain pan keeps overflowing.
Humidity stays above 60% with the AC running normally. Ask an HVAC contractor to check airflow and settings, and whether the system is oversized.

Common mistakes

Plastic sheeting or vinyl wallpaper on the inside of exterior walls in a humid climate: it traps moisture in the wall when the AC runs.
Buying a bigger AC to fix humidity: an oversized unit short-cycles and dries the air less.

Paying for the fixes

Utility programs often pay toward audits, duct sealing, insulation and heat pumps, and some require an audit first. Your metro guide lists the programs near you, and your state’s page covers state and federal rebates.

Words worth knowing

Relative humidity (RH): How much moisture the air holds compared with the most it could hold at that temperature, shown as a percentage.
Dew point: The temperature at which air becomes saturated and water starts to condense on surfaces, like water beading on a cold glass.
Hygrometer: An inexpensive gauge, often combined with a thermometer, that shows a room's relative humidity so you can keep it in a healthy range.
Dehumidifier: An appliance that removes moisture from indoor air, most often used in basements and humid summers to keep humidity in a healthy range.
Oversizing: Installing heating or cooling equipment with much more capacity than the house needs, usually from rules of thumb, not a load calculation.
Short cycling: When heating or cooling equipment turns on and off frequently in short bursts instead of running in steady, longer cycles.
Variable-speed (inverter) compressor: A compressor that ramps its speed up and down to match the heating or cooling needed, instead of switching fully on and off.
Condensation: Water that forms when humid air touches a surface colder than its dew point, on windows, cold pipes or hidden inside walls and attics.
Mold: Fungi that grow on damp surfaces indoors; they can damage materials and affect health, and moisture is always the root cause.
Hot-humid climate: A climate with long, hot, muggy summers, like the US Gulf Coast and Florida, where controlling humidity matters as much as temperature.

The glossary explains every term on the site.

Common questions

What indoor humidity should I aim for?

EPA advises keeping indoor relative humidity below 60%, ideally between 30% and 50%. A digital hygrometer is the easiest way to check.

Why is my house humid when the AC is running?

Usually because the AC doesn’t run long enough to dry the air, often because it’s oversized; the fan is set to On and puts water from the coil back into the air; or humid air leaks in through return ducts, the attic or a crawlspace.

Should I buy a dehumidifier or a new AC?

Start with the cheaper steps: fan on Auto, exhaust fans, sealed return ducts, and a dehumidifier for a damp basement or crawlspace. Replace the AC only if a Manual J calculation shows it’s the wrong size or it’s due anyway, and then choose a right-sized variable-speed system.

Sources

Where each problem is common comes from our regional climate research (NOAA climate normals, EPA, PNNL, FEMA and others), cited on each metro guide. The guidance on this page follows these sources, checked October 26, 2026:

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