What Humidity Should a Basement Be?
Below 60 percent relative humidity, and ideally between 30 and 50 percent. That is the EPA's indoor guidance, and a basement is held to the same number as the rest of the house. It just has a harder time meeting it.
What a basement reading means
| Reading | What it means for a basement |
|---|---|
| 30 to 50% | The EPA's ideal range. Nothing to do except keep checking through the seasons. |
| Above 50%, up to 60% | Still under the EPA's ceiling, but outside its ideal range. Watch whether it keeps climbing. |
| Above 60% for days at a time | Over the line this site treats as a mold-risk signal. Find the moisture source, then plan on a dehumidifier. |
| Below 30% | Below the EPA's ideal range, on the dry side. Not a mold concern. |
One reading on a humid afternoon doesn't tell you much; a few days of readings show where the number usually sits.
Why a basement reads higher than the floors above
Relative humidity is a ratio: how much water vapor the air holds compared with how much it could hold at that temperature. Basement air is cooler than the floors above, and cooler air holds less, so the same amount of moisture shows up as a higher percentage downstairs. Add moisture coming through the slab and foundation walls, and a basement can sit well above the rest of the house while the thermostat upstairs reads fine.
Cool walls, cold water pipes and the slab itself are also where condensation forms. The EPA treats moisture collecting on walls or pipes as a sign of high humidity, so a sweating pipe is worth a reading even if the room doesn't smell.
How to get a reading you can trust
- Use a digital hygrometer rather than guessing from feel or smell.
- Put it in the middle of the space at about chest height, not on the floor, against an outside wall or next to the dryer.
- Give it time to settle after you move it, then read it morning and evening for several days.
- Check again in a different season: a basement that is fine in winter can sit above 60% all summer.
What your humidity percentage means covers the same thresholds for the rest of the house.
What to do when the number stays above 60%
Look for the water first: leaks, gutters, ground that slopes toward the wall, a dryer that isn't vented outside. Then bring the humidity down with a dehumidifier that drains on its own, sized with the 2019 rating change in mind. If the basement already smells musty, start with finding the source of the smell and choosing a dehumidifier for a mildew smell, and see how to prevent mold for keeping it dry.
Where this comes from
U.S. EPA, A Brief Guide to Mold, Moisture, and Your Home (page updated February 18, 2026; read October 4, 2026). The explanation of relative humidity and temperature is general physics, not a figure from that guide.