Moisture & mold
Basement humidity: ideal levels and how to measure
Short answer
Try to keep your basement under 60% relative humidity, and ideally between 30% and 50%. That’s what the EPA recommends for the air in your home. In my experience working in basements here in Virginia, I like to see them between 45% and 55% during the summer months, and many drop into the 30% to 45% range in the winter. I’m not worried if I see one reading of 60%. If I see it over several days, I’m concerned. Once a basement stays in the 65% to 70% range for an extended period, I start digging to find out why, even if there’s no mold yet.
On this page
Why basements get humid
Your basement is usually one of the coolest areas in your home, and cool air holds less moisture than warm air. When warm, moist air comes in through a door, a window or the rim joist area, it cools down and the relative humidity goes up. You can also get moisture coming up through the concrete slab, or water leaking into the basement after it rains.
How to measure it
- Use a digital thermo-hygrometer that shows temperature and relative humidity, ideally with a min/max or history feature. Look for a stated accuracy of about ±2–3% RH. You don’t need a professional instrument.
- Put it in the right spot: at about waist height, away from the walls and the floor, and not in front of a supply vent or next to the dehumidifier.
- Leave it for at least 24 to 48 hours. For a seasonal or hard-to-pin-down problem, leave it several days.
- Check that it reads correctly. Put it next to another meter you trust for a few hours. If one says 51% and the other 53%, that’s fine. If one says 48% and the other 61%, one of them has a problem. A salt test in a sealed container also works, but comparing two meters is simpler.
I’d rather see a trend than one reading. A hygrometer that records over a day or two gives you a much better idea of what’s going on in your basement than a glance at the display right after someone left the outside door open.
Mistakes I see
- Measuring right next to the dehumidifier
- Looking at a single reading
- Measuring right after the outside door has been open
- Setting the meter directly on the concrete floor
- Assuming that 70% for ten minutes means the whole house has a problem
- Using a cheap hygrometer that has never been compared with another one
What your basement humidity reading means
Scroll sideways to see the whole drawing.
| Reading over several days | What it means | What to do |
|---|---|---|
| 30–50% | The EPA’s ideal range | Nothing to fix |
| 50–55% | Normal for a basement in a humid summer; my target for finished basements is about 50% | Keep an eye on it |
| 55–60% | Still under the EPA’s limit, but close | Check whether it keeps creeping up |
| Above 60% for days | Higher than the EPA recommends | Look for the source and consider a dehumidifier |
| 65–70% or more for long stretches | Something is adding moisture, even if there’s no mold yet | Look for air leaks and water coming in before anything else |
Summer vs. winter
Most of the time, the trouble shows up in the summer. The air outside is hot and humid, your basement is cool, and any air that comes in from outside raises the humidity. That’s why opening the basement windows to let it “breathe” does more harm than good on a hot, humid day in Virginia. During the winter the outside air is drier, and many basements stay in the 30% to 45% range.
Finished vs. unfinished basements
If your basement is finished and hooked up to your HVAC system, it usually stays at a more constant temperature, and humidity can be easier to control. You just can’t see as much. In an unfinished basement I can see the foundation walls, the sill plate, the joists and the floor. In a finished one, the drywall can look great and still have a moisture problem behind it, and carpet over concrete can hide a lot too. I like to keep finished basements at around 50% humidity.
What to do next
- Humid, but no water coming in: how to size and drain a basement dehumidifier
- Walls, pipes or floors sweating: dew point and condensation
- A musty or sewer smell: basement smells
- Visible mold: mold in basements
- Water coming in after rain: basement water
- A crawl space instead of a basement: crawl space humidity and moisture
In this section
Basement dehumidifiers: size, drainage and placement
How to size a basement dehumidifier with ENERGY STAR's chart, why ratings changed in 2019, and how to drain and set it so it actually works.
Dew point and condensation on basement walls
Check whether a basement wall, pipe or floor will condense with your readings, and how to tell condensation from a leak.
Mold in basements: causes, cleanup and when to call a pro
How to tell mold from efflorescence, clean small areas safely, and know when a basement mold job needs a professional.
Mold inspection and testing: when it's worth it
When a mold inspection or lab test is worth paying for, what it should include, what it costs, and why home test kits tell you little.
Mold remediation cost in crawl spaces and basements
What mold remediation costs in basements and crawl spaces, what a complete job includes, and the red flags in a quote. Prices from Virginia jobs, 2026.
Musty and sewer smells in the basement
How to tell a musty smell from a sewer smell, where each one usually comes from, and what actually gets rid of it.
Sources
- EPA, A Brief Guide to Mold, Moisture and Your Home (updated February 18, 2026)
Humidity ranges for Virginia basements come from Robert Stulpt’s own inspections. Other climates and houses may differ.