What humidity should a basement be, and how to measure it

The target: 30 to 50 percent, never over 60

The EPA's guide to mold and moisture in homes puts it in one sentence: keep indoor humidity below 60 percent, ideally between 30 and 50 percent relative humidity. For a basement we aim at 50 in summer and let it fall toward 35 in winter. Below 30 the air is dry enough to crack wood and raise static; above 60 for days at a time is where mold grows on cardboard, the back of drywall and the dust on everything, and where the musty smell comes from. Dust mites need humidity above about 50 percent, which is the reason the band stops where it does.

Two more numbers matter alongside the percentage. Temperature, because relative humidity is relative to it; a basement at 62°F and a living room at 74°F holding the same air give very different readings. And dew point, the temperature at which the air in the room will start to condense on a surface. If the slab, a cold water pipe or a duct is below the dew point of the basement air, that surface is wet no matter what the hygrometer says. Most digital hygrometers show the first two; the table below does the third for you.

Why a basement is humid when the house is not

A basement is the coldest space in the house in summer, and humidity follows temperature downhill. Take the air from upstairs on a July afternoon, 75°F at a comfortable 50 percent. Its dew point is about 55°F. Carry that same air down the stairs into a basement sitting at 65°F and, with not one drop added, it now reads about 70 percent. Against a slab or a cold water line at 55°F it is at 100 percent, which is why pipes sweat and the floor feels clammy. Outdoor air on a muggy day is worse: 85°F at 60 percent has a dew point near 70°F, so letting it into a 65°F basement through an open window condenses water onto every surface in the room. Opening basement windows in summer makes a basement wetter, not drier.

When a dehumidifier is the answer, and how to run it

If the plastic stayed dry underneath and the humidity does not follow the rain, the basement's problem is air, and a dehumidifier is the whole fix. Three things make it work:

Size it for the space. Capacity on the box is pints per day at the DOE 2019 test condition, which is 65°F and 60 percent, close to a real basement. A 50-pint unit covers most basements up to about 1,500 sq ft of real floor area; a very damp or very large basement wants two units or a commercial one. The calculator below does the arithmetic from your floor area, ceiling height and how damp the space reads.

Set it to 50 and leave it. Lower setpoints cost more electricity for no benefit; 45 is as low as a basement needs in summer. Most units stop at their setpoint and restart a few points above it, so a 50 setting holds the room between 50 and 55.

Put it where the air can reach it and the water can leave. Open floor, roughly central, a foot of clearance all round, not in a corner or a closet, with the continuous drain hose falling to a floor drain or the sump so the bucket never stops it at 2 a.m. The drawing shows where and our drain setup guide covers the hose, the air gap and the pump option for a drain that is uphill.

The full dehumidifier size calculator explains the math and the 2012-versus-2019 pint ratings. For the units themselves, our basement dehumidifier picks are the ones below and their siblings; a crawl space is a different problem with a colder target range, covered in the crawl space roundup.

When it is a water problem, not a humidity problem

A dehumidifier removes water from air. It cannot remove the water that a wall or floor delivers every day, and a basement with a water source will hold a 50-pint unit at full output all summer and still read 65. These are the signs that the humidity is a symptom:

• The reading tracks the weather. Up a day after rain, down a few days later. Ground water is coming through the walls or the slab, or in at the footing.
• Water under the plastic sheet. Vapor is moving through the concrete from wet soil behind it.
• White crust on the walls or floor. Efflorescence is the mineral that water left behind on its way through the concrete.
• The bucket tells on the house. A 50-pint unit in a closed basement should taper off to a few pints a day once the space is dry. One that still fills its bucket daily in week three, in dry weather, is being fed.
• Damp to the touch at the base of the wall, a tide line, rust at the bottom of metal shelving, a sump that runs in dry weather.

The order of fixes is outside-in and mostly cheap: downspouts extended 6 ft or more from the foundation, soil graded to fall away from the house, window wells covered and drained, cracks sealed, and a sump pump with a battery backup if there is a pit. Interior drain systems and exterior membranes come after those, not before. The dehumidifier stays running through all of it; it just stops being the plan.

When to call a pro

Call a basement waterproofing contractor if the humidity tracks the rain and the outside-in fixes have not changed it, if there is water under the plastic sheet on more than one wall, if a crack weeps, or if the sump runs in dry weather. Call a restoration or remediation contractor if you find growth covering more than about 10 square feet, which is the EPA's threshold for professional mold remediation, or any growth inside ductwork or on insulation. A home inspector or a crawl space contractor with a moisture meter can read the framing if you do not own one. The form below reaches vetted local waterproofing, encapsulation and restoration contractors through HelpMeNow.com.