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What the EPA recommends
The EPA's advice is to keep indoor humidity below 60%, and ideally between 30% and 50%, to help prevent mold. Basements drift above that range in summer because cool walls and floors raise the relative humidity of the air next to them.
| Reading | What it means | What to do |
|---|---|---|
| Below 30% | Drier than needed | Nothing for mold; may feel dry |
| 30%–50% | EPA ideal range | Keep it here |
| 50%–60% | Above ideal | Run a dehumidifier, improve air flow |
| Above 60% | Above EPA upper limit | Dehumidify, look for leaks and condensation |
Getting it down
- Run a dehumidifier sized for the area; see the size calculator.
- Fix gutters and grading so water drains away from the foundation.
- Vent dryers and bathrooms outdoors.
- Close windows on humid days; warm humid air cools and its relative humidity rises in a basement.
Moisture comes first
Finishing materials hide water problems until they become mold problems. Before framing, tape a 2 × 2 ft sheet of plastic to the slab and to a wall for two days: water under it means moisture is coming through the concrete; water on top means humid air is condensing. The first calls for drainage or sealing, the second for dehumidification. Keep a hygrometer in the finished space afterward and aim for the EPA's 30%–50% range.
- Extend downspouts several feet from the foundation.
- Slope soil away from the house.
- Use materials that tolerate moisture below grade: foam board, treated bottom plates, vinyl plank.
- Leave access to the sump pit, the radon fan and the main water shutoff.
Where basement moisture comes from
| Source | Sign | Usual fix |
|---|---|---|
| Rain and snowmelt | Wet floor edges after storms | Gutters, downspout extensions, grading |
| Groundwater | Water through cracks or under walls | Interior drain and sump pump |
| Humid outdoor air | Musty smell, sweating pipes in summer | Dehumidifier, keep windows shut on humid days |
| Vapor through concrete | Damp under a plastic test sheet | Vapor barrier, foam board, sealer |
| Plumbing and appliances | Local stains | Repair leaks, vent the dryer outside |
Relative humidity and cold walls
Relative humidity is how much water vapor the air holds compared with the most it can hold at that temperature. Cool air holds less, so the same air that reads 50% upstairs can read much higher next to a cool basement wall. When the surface is cold enough, moisture condenses on it. That is why basements smell musty in summer even with no leak, and why a hygrometer in the basement itself, not upstairs, is the reading that matters.
- Place the hygrometer away from the dehumidifier outlet.
- Read it at different times of day for a week.
- Check the coldest corners and behind furniture against outside walls.
Common questions
What humidity should a basement be?
Below 60%, ideally 30%–50%, according to the EPA.
Is 65% humidity too high for a basement?
Yes. It is above the EPA's 60% upper limit.
Related calculators
Sources
- https://www.epa.gov/node/78417