Basement Humidity Level: What to Aim For and When to Act
Keep a basement at 50 percent relative humidity or a little below, and treat anything over 60 as a problem to fix. This page covers where those numbers come from, how to check a cheap hygrometer with table salt, and what each reading calls for.
Hold 50 percent or a little below through the damp months, and 30 to 40 in a cold winter, lower still in a hard freeze. Sixty is the ceiling for mold in EPA’s guidance, and 50 is the dust-mite limit on North Carolina’s public health indoor air page; the winter range is ENERGY STAR’s, set to keep condensation off the windows. Once a checked hygrometer reads 60 or more on the wettest days of a week, buy a dehumidifier. Above 90 percent, or once water seeps through the walls, stop the leak first.
| Reading on the wettest days | Where it falls | What to do | Buy |
|---|---|---|---|
| 30 to 50% | EPA’s ideal range | Leave it; read the hygrometer again after a wet week | Nothing yet |
| 51 to 59% | Under EPA’s ceiling, over the dust-mite limit | Set an existing unit to 50; vent the dryer outside and keep the windows shut in humid weather. If it smells musty, treat it as the next row | Not yet; buy if it reaches 60 |
| 60 to 75% | ENERGY STAR’s slightly to moderately damp row: 20 to 30 pints a day under 2,000 sq ft, 30+ over | Buy a dehumidifier; a 35-pint unit covers it under 2,000 sq ft at 60°F or warmer | Frigidaire 35-pint at Abt, $229.00; in the 50s or over 2,000 sq ft, Midea Cube on Amazon |
| 76 to 90% | Very damp row: 25 to 40 pints under 2,000 sq ft, 40+ over | Buy a 50-pint unit, since a 35 falls short of the 40 at the top of the row, and look for damp spots on the walls | Midea Cube 50-pint on Amazon |
| Over 90%, walls sweat or water seeps in | Wet row: 30 to 50 pints under 2,000 sq ft, 50+ over | Stop the water first; see the wet basement guide | Once the water’s stopped and under 2,000 sq ft: Midea Cube on Amazon |
Why 50 percent
EPA’s mold course says indoor relative humidity should stay below 60 percent, ideally between 30 and 50, and lists high humidity, at 60 percent, among the common moisture problems. North Carolina’s indoor air page, which uses ASHRAE Standard 55 for its comfort ranges, draws two lines: below 60 to control mold and below 50 to control dust mites, at all hours, including when nobody is home.
A basement needs the lower of the two because its walls and floor run colder than its air. NDSU Extension’s rule of thumb is that a 20-degree drop in temperature doubles the relative humidity: air at 70°F and 35 percent reaches about 70 percent when it cools to 50°F. The air an inch from a cold foundation wall is wetter than the reading in the middle of the room, and EPA notes that mold often turns up in cold corners behind furniture, where condensation forms. Minnesota’s Center for Energy and Environment suggests setting a basement dehumidifier to 55 percent. I’d set it to 50 because of that cold-wall effect, and lower in winter as long as the unit keeps running.
The room’s temperature also decides which unit can hold that number. Household dehumidifier capacity is rated at 65°F under the Department of Energy’s test and drops in colder air, so a 35-pint unit’s 5-pint margin over the 30 at the top of ENERGY STAR’s moderately damp row holds at 60°F or warmer but not in a basement that sits in the 50s. That’s why the table sends a damp basement in the 50s to the 50-pint Midea Cube.
Summer and winter targets
Summer
NDSU notes that opening a basement that’s cooler than the outdoors lets in air that rises in relative humidity as it cools. In humid weather, keep the windows shut and let a dehumidifier or air conditioner do the drying.
Winter
In winter the limit drops with the outdoor temperature, because cold windows and outside walls condense moisture at lower readings, and a basement’s foundation walls and small windows are among the coldest surfaces in the house. ENERGY STAR suggests 30 to 40 percent during the heating season in colder climates. Minnesota CEE gives a finer scale:
| Outdoor temperature | Keep indoor humidity under |
|---|---|
| 20 to 40°F | 40% |
| 10 to 20°F | 35% |
| 0 to 10°F | 30% |
| -10 to 0°F | 25% |
| -20 to -10°F | 20% |
| Below -20°F | 15% |
The Frigidaire 35-pint and the Midea Cube are rated down to 41°F and the Hisense 35-pint to 38°F; below those, the unit stops pulling water no matter what the humidistat says. If your basement drops below 41°F, the cold-basement guide picks the Hisense from 38 up to 41°F and the AlorAir HD55S, rated down to 33.8°F, below that.
How to measure it
EPA says a gauge that reads both temperature and humidity costs less than $50 at a hardware store. Check it against salt before you rely on it: a teaspoon of table salt dampened with a few drops of water, sealed in a bag or plastic box with the hygrometer, holds the air inside at about 75 percent once it has sat sealed for a day. The University of Hawaii’s extension service measured that reference at 75.29 percent at 25°C (77°F) and let its test enclosures settle for 24 hours. Three of the five low-cost sensors it tested read within their rated accuracy without adjustment; the other two were off by 5.6 to 6.8 points. I’d keep a gauge that reads within 3 points of 75 either way and replace one that’s further off. The check tests only that one point, so it can’t prove the gauge is right at 50 or 60; it does catch one that’s badly off.
If your dehumidifier shows its own humidity reading, put the hygrometer in the middle of the basement at about waist height, away from the dryer, and compare the two. If the unit has no readout, ENERGY STAR suggests hanging the hygrometer near it so you can set the unit by it; keep it out of the exhaust stream. Read it morning and evening for a week, including the day after a heavy rain, and go by the wettest days; that’s the number the sizing guide works from.
When to act
A reading that jumps after every rain means water is getting in from outside, and the fix starts at the gutters and the grading. A dehumidifier that runs around the clock while the reading stays above 60 is too small for the space or is running below its rated temperature; the basement roundup has a pick for each size and temperature. A musty smell while the gauge reads 50 usually means something is still growing on a damp surface somewhere, and the steps for finding and cleaning it are in the musty smell guide.
Is 60 percent humidity too high for a basement?
Yes, if the wettest days read 60 or more. The cold foundation walls sit wetter than the air in the middle of the room, so the surfaces where mold starts are already past 60.
Can a basement be too dry?
Not from a dehumidifier set sensibly. EPA’s ideal range starts at 30 percent, and in a hard freeze Minnesota CEE’s limits for the house drop below that to keep windows from condensing. If your dehumidifier is holding the basement under 30, turn it up or off; the winter air is already doing the drying.
What should I set my dehumidifier’s humidistat to?
Start at 50 and check the hygrometer after a week. The built-in reading is taken at the unit, so if the hygrometer across the room reads higher, set the unit a few points lower.
Buy the hygrometer first
Check the gauge against salt, then read it for a week that includes a heavy rain. If the wettest days stay at 50 or below, you don’t need a dehumidifier, and between 51 and 59 the dryer vent and the windows come before buying one. From 60 to 75 in a basement under 2,000 square feet that holds 60°F or warmer, I’d buy the Frigidaire 35-pint. For readings over 75 I’d buy the Midea Cube, and the same goes for a basement over 2,000 square feet or one that sits in the 50s. Over 90, fix the water before buying anything.
Humidity targets come from EPA’s mold course (chapter 2, updated November 2026), North Carolina DHHS’s indoor air quality page (its comfort table from ASHRAE Standard 55-1992), ENERGY STAR’s dehumidifier guidance, and the Minnesota Center for Energy and Environment’s humidity page (July 2026); the temperature rule of thumb from NDSU Extension’s “Controlling Home Moisture”; the salt reference and sensor results from the University of Hawaii’s extension publication FST-70 (December 2023). All checked November 2026. This page contains affiliate links. If you purchase through these links, Dehumidifier Advisor may earn a commission at no additional cost to you. See our affiliate disclosure for details.