Dehumidifiers for Basements: Sizing, Setup and Upkeep

Short answer: Measure your basement's humidity first. EPA and NC State Extension say it should stay below 60%, ideally 30% to 50%.^{Source [1]Source [4]} If it's higher, choose a dehumidifier by capacity in pints per day, based on how damp the space is and how large it is, and lean toward the bigger size.^{Source [2]} Plan where the water will go, check whether the unit handles temperatures below 65°F, and clean it regularly so it doesn't become a mold source of its own.^{Source [2]Source [5]} A dehumidifier treats the air; it doesn't fix leaks or seepage, so deal with water coming in as well.^{Source [5]}

Basements are often the dampest part of a house. They sit against cool soil, they collect water when drainage is poor, and in summer they can pull in warm, humid air. This guide covers how to tell whether you need a dehumidifier, how to size and set one up, and how to keep it working.

Step 1: Measure before you buy

A musty smell is a clue, but a number is better. A humidity meter, or hygrometer, costs roughly $10 to $50 at hardware stores.^{Source [1]} Put it in the basement for a few days, including a humid summer stretch, and note the readings.

What the numbers mean:

• Below 50%: Generally fine. ENERGY STAR calls 30% to 50% the optimum range for a building.^{Source [2]}
• 50% to 60%: Borderline. Watch for musty odors and condensation.
• Above 60%: NC State Extension suggests considering a dehumidifier at this point.^{Source [4]} EPA also advises keeping humidity below 60%.^{Source [1]}

Other signs of excess moisture include condensation on windows, walls or pipes, which EPA says can signal high humidity.^{Source [1]} Our guide to humidity and moisture explains relative humidity in more detail. If you already see or smell mold, read when mold testing is worth it before you buy anything.

Step 2: Size it in pints per day

Dehumidifier "size" is the amount of water it can remove in 24 hours, measured in pints.^{Source [2]} The right capacity depends on two things: how big the space is and how damp it is without a dehumidifier.^{Source [2]}

ENERGY STAR's sizing chart for portable units uses these minimums:^{Source [2]}

ENERGY STAR says it's better to oversize than undersize.^{Source [2]} If you're replacing an old unit, you can also judge by how much water you empty each day.^{Source [2]}

Why pint ratings look smaller now

If you shopped for a dehumidifier years ago, today's numbers may look low. In 2019 the Department of Energy changed the test. Portable units are now rated at 65°F instead of 80°F, which is closer to real basement conditions, and cooler air yields less water.^{Source [3]} So a unit once sold as 50 pints is roughly a 30-pint unit under the new rating, and a former 70-pint unit is about 40 to 45 pints.^{Source [3]} When you compare models, make sure both use the new rating, and use the chart above, which reflects current ratings.^{Source [2]Source [3]}

Portable or whole-home?

Most people use a portable unit that sits in the space and can be moved. Whole-home dehumidifiers connect to your ductwork and are usually permanent.^{Source [2]} ENERGY STAR suggests a whole-home system may be the right choice if you live in a humid climate and have central air conditioning, and recommends having a contractor size and install it.^{Source [2]}

Step 3: Plan for the water

A dehumidifier collects a lot of water, and in a damp basement a bucket can fill quickly. Most portable units have a removable bucket with a full-bucket warning and automatic shutoff.^{Source [2]} Many also have a fitting for a hose that runs to a floor drain or sump pump, so you don't need to empty it by hand.^{Source [2]}

If you plan to use a hose:

• Put the unit close enough to the floor drain or sump that you don't need a long, awkward hose that someone could trip over.^{Source [2]}
• Keep the hose and any water away from electrical cords, connections and outlets.^{Source [2]}
• Plug the unit into a properly grounded outlet.^{Source [2]}
• Check that the sump pump works and the floor drain is clear. NC State Extension includes both on its moisture checklist.^{Source [4]}

Whole-home units usually drain only through a hose to a floor drain or condensate pump, set up by an HVAC professional.^{Source [2]}

Step 4: Watch out for a cool basement

Many basements are cool for much of the year, and that matters. Frost can form on a dehumidifier's coils when the air drops below about 65°F, which can make the compressor cycle on and off without removing much moisture.^{Source [2]} If that happens, ENERGY STAR says to switch the unit off and let it defrost.^{Source [2]}

If your basement is often below 65°F, look for a model specified for lower temperatures or one with an anti-frost sensor.^{Source [2]}

Step 5: Place it and set the target

A few placement details make a difference:^{Source [2]}

• If the unit doesn't blow air out the top, keep it away from walls and furniture so air can move around it.
• Keep basement doors and windows closed while it runs.
• Keep it away from dust sources such as a woodworking area, which can clog the coils.
• Expect the air right around it to be a little warmer. That's normal.

All ENERGY STAR certified dehumidifiers have a built-in humidistat, so you set a target and the unit cycles on and off to hold it.^{Source [2]} A setting around 50% keeps you inside the recommended range.^{Source [1]Source [2]} If your unit doesn't display humidity, hang a separate hygrometer nearby to check it.^{Source [2]}

Why summer can be the worst season

It seems logical to open basement windows on a warm day to "air it out," but that can make things worse. Building scientists at the National Research Council of Canada described how warm, humid summer air can condense on cool basement floors and walls, especially when the basement is ventilated with outdoor air.^{Source [6]} Dehumidification was one of the remedies they described for floor condensation.^{Source [6]} The paper is from 1984, but the physics hasn't changed. Running the dehumidifier with windows closed is the more reliable approach, which matches ENERGY STAR's advice.^{Source [2]}

Step 6: Keep the dehumidifier from becoming the problem

EPA recommends dehumidifiers, but warns that the appliances themselves shouldn't become sources of mold or bacteria.^{Source [5]} Standing water in a bucket or drip pan can do that.

• Empty and clean the bucket or drip pan regularly.^{Source [4]}
• Keep the coils and grilles free of dust.^{Source [2]}
• If any area gets wet from a leak or flood, dry it within 24 to 48 hours. EPA says mold usually won't grow in that window.^{Source [1]}

Step 7: Reduce the water coming in

A dehumidifier treats the air. It doesn't stop water from getting into your basement. EPA's first steps for moisture control are fixing leaks and seepage and making sure the ground slopes away from the house.^{Source [5]} ENERGY STAR adds several ways to cut the load on the machine:^{Source [2]}

• Extend downspouts away from the foundation and keep gutters clear.
• Don't overwater plants along the foundation.
• Vent the clothes dryer outdoors, and avoid hanging laundry to dry in the basement.
• Repair leaking outdoor faucets.
• If you have central air, consider an A/C vent in the humid space.

EPA also notes that carpet on concrete floors can absorb moisture and become a place for mold to grow.^{Source [5]}

Other basement checks while you're down there

A damp basement is a good time to look at the rest of the space. If your furnace or water heater is down there, make sure carbon monoxide alarm coverage is right; our carbon monoxide detector guide covers placement and testing. Basements are also where radon tends to build up, and a radon test is cheap and simple.

Frequently asked questions

What humidity should a basement be?

Below 60% relative humidity. EPA says ideally 30% to 50%, and NC State Extension suggests considering a dehumidifier once a basement goes over 60%. A $10 to $50 hygrometer will tell you where you are.

What size dehumidifier do I need for a basement?

It depends on how damp the space is and how big it is. ENERGY STAR's chart starts around 20 to 30 pints a day for a slightly damp space under 2,000 square feet and goes up to 50 pints or more for a wet one. If you're unsure, size up.

Why do new dehumidifiers have lower pint ratings?

The test changed in 2019. Portable units are now rated at 65°F, closer to basement conditions, instead of 80°F. A unit once sold as 50 pints would be labeled about 30 pints under the new test.

Can a dehumidifier work in a cold basement?

It can struggle. Below about 65°F, frost can form on the coils and the unit may cycle without removing much water. Look for a model rated for lower temperatures or one with an anti-frost sensor.

Should a basement dehumidifier run all the time?

Not necessarily. Set the built-in humidistat to your target, such as 50%, and the unit will cycle on and off to hold it. Check a separate hygrometer now and then to confirm the reading.