Crawl Space Humidity Levels: What Is Normal and What Is Too High?

Humidity is the number that matters most in a crawl space, and it is easy to measure. A small inexpensive hygrometer will tell you more about the health of your crawl space than almost anything else. Here is how to read it.

What Relative Humidity Means

Relative humidity (RH) is how much moisture the air is holding compared with the most it could hold at that temperature. Warm air can hold more moisture than cool air. That is why humid summer air condenses on cool surfaces in a crawl space, such as ductwork, pipes and the ground.

Target Ranges

• Under about 50-55%: A good target range for a crawl space. Mold and wood decay are unlikely to be a problem here.
• Around 55-60%: The upper edge. Worth watching, especially if it is trending up.
• Above about 60% for extended periods: Conditions where mold can begin to grow on wood and other organic materials, and where wood can slowly take on moisture.
• 70% and higher: A clear sign that moisture is getting in and not leaving. This is common in vented crawl spaces during Tennessee summers.

The EPA recommends keeping indoor humidity between 30% and 50% to limit mold growth. Crawl space air mixes with the air in your home, so what happens below affects what you breathe above.

Why Vented Crawl Spaces Run Humid in Tennessee

Foundation vents were designed around the idea that outside air would dry out the crawl space. In a humid climate it often does the opposite. Warm, moist summer air comes in through the vents, meets cooler surfaces and drops its moisture. Over a season, that adds up to damp wood, wet insulation and a space that stays humid for months.

How to Check Your Own Crawl Space

1. Buy a digital hygrometer (many are under $20).
2. Place it in the crawl space, ideally near the middle and away from the access door.
3. Check it at different times of day and again after rain. A single reading is only a snapshot.
4. Look at the wood while you are there: dark staining, white or green growth, or a damp feel are all warning signs regardless of the number.

If you do not want to go into the crawl space yourself, we are glad to do it for you as part of a free inspection.

How Humidity Gets Brought Down

Lowering humidity in a lasting way means controlling where the moisture comes from, then removing what remains:

• Seal the ground with a heavy-duty liner so soil moisture stays out of the air.
• Seal the vents and gaps so humid outdoor air stops flowing in.
• Manage water with proper gutters, downspout extensions, grading and drainage.
• Add a dehumidifier sized for the space to hold humidity in the target range through summer.

See how this fits together on our dehumidifier, vapor barrier and vent sealing pages.

Related Reading

• Tennessee Summer: How to Beat Crawl Space Humidity
• Musty Smell in Your House? Why the Crawl Space Is Often the Source
• Moisture Control Service Details