How to lower humidity in your house

Too much humidity in a house usually shows up in a few ways: the air feels sticky, the windows fog up, or the basement smells musty. Left that way, it can lead to condensation and mold. Most of the fixes cost nothing, and a dehumidifier takes care of the rest.

Quick ways to lower humidity

The fastest fixes are to send moisture outside at the spot where it’s made and to keep humid outdoor air from coming in.

Showers, cooking, dishwashing and laundry all put water into the air, and a fan or vent can send most of it outside instead. Run the bathroom fan or open the window when you shower, and make sure the fan exhausts outdoors; shorter showers mean less steam, too. Use the range hood when you cook or run the dishwasher, but check where it sends the air. A hood that blows it back into the kitchen through a filter takes no moisture out of the house.

If your clothes dryer has a vent, make sure it goes outside. All the water from a wet load leaves through it, so a loose, crushed or disconnected duct puts it in your laundry room instead. In humid weather, skip the indoor drying rack.

On warm days, the air conditioner dries the air as well as cooling it. Its indoor coil is colder than the dew point of the room air, so water condenses on it and drains away, as long as the drip pan is clean and the drain line is clear. Keep windows and doors closed on muggy days, because outdoor air only helps when its dew point is lower than the one inside.

Finally, dry spills and wet spots fast. If wet materials are dried within 24 to 48 hours, in most cases mold won’t grow.

What humidity your house should be

Keep indoor relative humidity below 60%, and ideally between 30% and 50%.

Above that, higher humidity may promote mold growth, and moisture can condense on walls, floors and other surfaces, where bacteria, dust mites and mold can grow. Treat 60% as the point to act, not as a target.

In cold climates, aim lower in winter, around 30-40% during the heating season, because cold window glass fogs up at higher humidity. Our dew point article has a table showing how cold the glass can get before it does.

How to measure the humidity in your house

Use a hygrometer, a small digital gauge that shows relative humidity and temperature.

Most hardware stores carry one, sometimes as a moisture or humidity meter. A thermostat that shows indoor humidity works too, but only for one spot. The Govee H5075 is a Bluetooth model that keeps a history in the Govee Home app and can alert you when humidity passes a level you set; the TempPro TP50 is a plain battery-powered display with high and low records and a Dry, Comfortable or Wet indicator. Both are rated accurate to within 2 to 3%, which is plenty: you’re trying to tell 45% from 65%, not 48% from 50%.

Keep it away from the bathroom door, the stove, heaters and direct sun, which all skew the reading, and check a few rooms, including the basement, because humidity can differ around the house. Then put the temperature and humidity into our dew point calculator. The dew point it gives you is what tells you whether opening a window will help.

When opening windows helps, and when it makes it worse

Opening windows lowers indoor humidity only when the outdoor dew point is lower than the indoor dew point.

The humidity percentage in your weather app can’t tell you that, because it depends on temperature and doesn’t say how much water is actually in the air. The dew point does. Here’s how two kinds of outdoor air compare with a 75 °F living room at 55% humidity, which has a dew point of about 58 °F:

Most weather apps show the outdoor dew point, and so does the current-conditions box on weather.gov. In summer that often works out to windows open on cool, dry nights and mornings, and closed with the AC on through humid afternoons.

When you need a dehumidifier, and what size

Get a dehumidifier when humidity stays above 50-60% after the free fixes, or when a room keeps feeling damp or smelling musty, which is usually a basement.

Dehumidifiers are rated by how many pints of water they remove in 24 hours, and the size you need depends on how damp and how big the space is:

Between two sizes, take the bigger one; it’s better to oversize than undersize. If you’re replacing an old unit, don’t go by the number on the box. Since June 2019, portable dehumidifiers are tested at 65 °F instead of 80 °F, where air holds less water, so the same machine now gets a smaller rating. Size the new one by how much water you empty from the old one each day.

Close the doors and windows to the space while it runs. Units that don’t blow air out the top should stand away from walls and furniture. A drain hose to a nearby floor drain or sump pump saves emptying the bucket; keep it away from power cords. Since a dehumidifier may run all summer, pick an ENERGY STAR model, which is 20% more efficient than a non-certified one. Our guide to the cost of running an appliance shows how to work out what yours costs.

How to lower humidity in a basement

Basements are humid mostly because their walls and floors are cool, and when warm, humid air touches a cold surface, condensation can form.

That makes summer the tricky season. On a day when the outdoor dew point is 70 °F, air coming in through an open basement window is moister than a 65 °F wall can handle, so water collects on the wall, the floor and the cold-water pipes. Keep basement windows shut on humid days.

Run a dehumidifier sized from the table above. A basement where the walls or floor sweat counts as “wet,” which means 30-50 pints a day. Wrap cold-water pipes in foam insulation so their surface stays above the dew point and stops dripping. Then look outside: clean and repair the gutters and check that the ground slopes away from the foundation, because better drainage there can lower basement humidity.

Signs of a bigger moisture problem

If humidity stays high after all of this, look for water getting into the house rather than adding more equipment.

Condensation on windows, walls or pipes that keeps coming back is a sign of high humidity, and rust on pipes shows that condensation has been forming on them. A musty smell means you should suspect mold even when you can’t see any; walls in cold corners behind furniture, where condensation forms, are a good place to look. Water stains, drips or damp spots near plumbing, the roof or the foundation can point to a leak, and leaks are worth fixing as soon as you can.

You can usually clean up a patch of mold smaller than about 10 square feet (roughly 3 by 3 feet) yourself by following the EPA’s guide. For larger areas, a lot of water damage or mold you suspect is hidden inside walls, consider an experienced professional. Either way, fix the water problem behind it, or the mold comes back. If anyone in the house has health concerns, talk to a health professional before starting the cleanup.