The numbers
| Relative humidity | What happens |
|---|---|
| Under 45% | Very dry. Fine, but rare in NC without a dehumidifier running hard |
| 45% to 55% | Target range. Wood stays dry, no mold growth |
| 55% to 60% | Borderline. Watch it in summer |
| 60% to 70% | Mold can start on wood. Wood moisture climbs |
| Over 70% | Active mold growth, condensation on ducts, rot over time |
Why 60% is the line
Mold spores are everywhere. They only grow when the surface they sit on is wet enough. At around 60% relative humidity, wood absorbs enough moisture from the air to feed mold. Above 70%, it grows fast. Wood moisture content above 19% is where rot fungi take hold too.
Charlotte in summer
Outdoor humidity in Charlotte sits above 70% most summer mornings. A vented crawl space pulls that air in. Then the air cools under the house, and cooler air holds less water, so the humidity goes even higher. It is common to measure 80% to 90% under a Charlotte home in July.
How to measure it
- Buy a cheap digital hygrometer. Some have a remote sensor you can read from inside the house.
- Put it in the middle of the crawl space, not near the door.
- Read it in the afternoon and again the next morning.
- Check a few times through the summer. One reading is not enough.
How to hold the range
Block soil moisture
A vapor barrier over the dirt floor stops gallons of water vapor a day from rising into the space.
Seal the vents
This is the step most people skip. As long as vents are open, you are trying to dry the whole outdoors.
Run a crawl space dehumidifier
A proper crawl space unit with a humidistat set to 50% holds the range year round. Household units are not built for it and fail fast. Expect $1,200 to $3,000 installed, as covered in our dehumidifier cost guide.
Fix water first
If water pools under the house, no dehumidifier can keep up. Drainage and grading come first.
Questions people ask
Is 65% humidity in a crawl space bad?
Should I run a fan in my crawl space?
What should crawl space humidity be in winter?
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