Indoor Humidity, the Right Level and How to Hold It

What Relative Humidity Means

Relative humidity is how much water vapor the air holds compared with the most it could hold at that temperature. Warm air can hold more water than cold air. That is why the same moisture can read 40 percent in a warm living room and 80 percent in a cool basement, and why a cold window can drip even when the room feels fine.

The temperature where air can hold no more water is called the dew point. When air touches a surface colder than its dew point, water condenses on that surface. Cold pipes sweat, windows fog and the back of a closet on an outside wall stays damp. Those damp surfaces are where mold starts.

The EPA suggests keeping indoor humidity below 60 percent, and ideally between 30 and 50 percent. Mold growth on building materials becomes likely when a surface stays damp for a day or two, and the air near cool surfaces is often wetter than the room reading suggests. Dust mites also thrive when humidity sits above about 50 percent for long stretches.

Too dry is a problem too. Below about 30 percent, many people notice dry skin, static shocks and irritated sinuses, and wood floors and furniture can shrink and crack.

Measure Before You Buy Anything

A digital hygrometer costs about $10 to $20 and tells you the humidity and temperature in a room. Guessing by feel is unreliable, so measure for at least a week before you buy a dehumidifier or change your habits.

• Put one on each floor, including the basement if you have one.
• Keep it away from kitchens, bathrooms, vents and direct sun.
• Check it in the morning and evening, and after showers and cooking.
• Note the highest reading and how long it takes to come back down.

Cheap meters can be off by several points. You can check yours with the salt test. Put a spoonful of table salt in a bottle cap, add a few drops of water to make a damp paste, and seal it with the hygrometer in a zip top bag for about 8 to 12 hours. A correct meter reads about 75 percent. If yours reads 70, add 5 points to every reading.

A room that jumps to 70 percent after a shower and drops back to 50 within an hour is normal. A room that sits at 65 percent all day is not, and it needs a fix.

Cut Moisture at the Source

A family of four can add many pints of water to indoor air every day through showers, cooking, breathing, plants and laundry. Before you remove moisture with a machine, stop adding so much.

• Bathrooms. Run the exhaust fan during every shower and for about 20 minutes after. A timer switch makes this automatic. Hold a tissue to the grille. If the fan cannot hold it, the fan or its duct needs attention.
• Kitchen. Use a range hood that vents outside, not one that just blows air back into the room, and turn it on when you boil water.
• Laundry. Make sure the dryer vent runs outside and is not disconnected behind the machine. Avoid drying loads of clothes on racks indoors in humid months.
• Plants and aquariums. A large collection adds real moisture. Cover tanks.
• Firewood. Store it outside, not in the basement.

Also look outside. Gutters that dump at the foundation, soil that slopes toward the house and sprinklers that soak walls all push water into crawlspaces and basements, where it rises into the rest of the home. A dehumidifier fighting a drainage problem will run forever and never win.

Choosing and Using a Dehumidifier

If humidity stays above 60 percent after you cut the sources, a dehumidifier is the next step. Basements, crawlspace adjacent rooms and closed up summer homes are the usual candidates.

Capacity is rated in pints per day. The Department of Energy changed the test in 2019, so newer models carry lower pint numbers than older models of the same real size. Compare models rated under the same test. When in doubt, size up, because a larger unit running less often works better than a small one running nonstop.

• Set it to about 45 to 50 percent.
• Use a continuous drain hose to a floor drain or sump if you can, so it never stops with a full bucket.
• Close windows and doors to the space it serves.
• Clean the filter monthly.
• Many units stop working well below about 65 degrees Fahrenheit, so check the rating for a cool basement.

Air Conditioning and Humidity

Your air conditioner removes moisture as it cools. Water condenses on the cold coil and runs down the condensate drain. That only works well when the system runs in long cycles.

An oversized AC cools the room fast and shuts off before the coil removes much water. The house feels cold and clammy at the same time. If your AC is new and the house still feels damp, ask the installer whether the system was sized with a load calculation, and whether the fan speed or a dehumidify mode can be adjusted.

• Set the thermostat fan to Auto, not On. With the fan running nonstop, water on the coil can blow back into the house between cycles.
• Change filters on schedule so airflow stays right.
• Keep the condensate drain clear so collected water leaves the house.
• Do not shut the AC off completely in humid weather when you leave. Set it a few degrees higher instead.

In a closed up house in a humid climate, humidity can climb fast with the AC off. A few days in the 70s of percent can be enough to start mold on furniture, closets and shoes.

Winter, Condensation and Humidifiers

Cold weather flips the problem. Outside air holds little water, and heated indoor air feels dry. But the inside of your windows and outside walls gets cold, and humid indoor air condenses there.

Water on the inside of your windows in winter is a sign the indoor humidity is too high for the outdoor temperature. A common rule of thumb is to lower your target as it gets colder, to about 30 to 40 percent in mild cold and lower in deep cold. Wipe windows dry and run exhaust fans to bring it down.

If the air is truly too dry, a humidifier can help, but use it with care.

• Keep the reading at or below about 40 percent.
• Empty, rinse and dry the tank daily, and clean it as the maker directs.
• Use distilled water in ultrasonic and mist models so they do not spray minerals as fine white dust.
• Do not aim mist at walls, windows or furniture.

A humidifier that keeps a bedroom at 55 percent in January can grow mold on the window frames and the back of the closet within a season.

When Humidity Will Not Come Down

If you have cut the sources, run a correctly sized dehumidifier and the reading still sits above 60 percent, something else is putting water into the house. Common hidden causes include these.

• A slow plumbing leak inside a wall, under a slab or under a cabinet
• A crawlspace with bare soil and no vapor barrier
• A basement wall that wicks groundwater
• A clogged AC drain or a pan that overflows into ductwork
• A disconnected bath fan or dryer duct that dumps moist air into the attic or a wall
• A roof or window leak that wets insulation

A moisture meter helps you track it down. Test drywall and wood near the high reading. Readings that stay high in one spot point to a source nearby. Your water meter can also reveal a hidden supply leak. Turn off all water use and watch whether the meter still moves.

When the source stays hidden, a plumber, an HVAC tech or an independent moisture inspector can find it with tools like thermal cameras. Find the water first. No machine can dry a house faster than a leak can wet it.

Step by Step

1. Place Hygrometers

   Set one hygrometer on each floor, away from kitchens, baths and vents. Check it against the salt test if you doubt the reading.

2. Log a Week of Readings

   Write down morning and evening readings for a week. Note the highest numbers and which rooms hold them.

3. Fix Fans and Vents

   Test bath fans with a tissue and check that the range hood and dryer vent exhaust outside. Add a timer switch so the bath fan runs about 20 minutes after each shower.

4. Fix the Outside Water

   Clean gutters, extend downspouts at least 4 to 6 feet from the house and aim sprinklers away from walls.

5. Add a Dehumidifier Where Needed

   If a room or basement still sits above 60 percent, add a dehumidifier set to about 45 to 50 percent with a continuous drain if possible.

6. Tune the AC

   Set the thermostat fan to Auto, change the filter and confirm the condensate line drips when the AC runs.

7. Adjust for the Season

   In winter, lower your target and watch for window condensation. Use a humidifier only if readings fall below about 30 percent.

Mistakes That Make It Worse

• Buying a dehumidifier before measuring. You may be fixing the wrong room or a problem you do not have.
• Running the thermostat fan on nonstop in humid weather. It can blow moisture from the coil back into the house.
• Venting a bath fan or dryer into the attic. The moisture condenses on the roof deck and can grow mold there.
• Letting a humidifier run all winter at a high setting. Wet windows and moldy frames follow.

Where You Live Matters