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Fix a crawl space that has moisture

We'll check ventilation, vapor barrier, grading and downspouts, and plumbing—then add a dehumidifier or tell you when to call a pro.

Troubleshooting · Home maintenance
1–4 hours
What you'll need
  • 6-mil polyethylene sheeting (if adding vapor barrier)
  • Tape for vapor barrier seams
  • Hygrometer (to measure humidity)
  • Dehumidifier (if needed)
  • Mask and gloves
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Steps

Goal: Confirm the moisture, then fix ventilation, vapor barrier, and drainage.

  • Enter the crawl space (wear a mask and gloves). Check for damp soil, condensation on joists, or a musty smell.
  • Good: Damp soil or condensation—proceed to Check ventilation.
  • Bad: Standing water—see fix-sump-pump-will-not-run or call a pro.

Check ventilation

Goal: Confirm crawl space vents are open and not blocked.

  • Check each vent from outside. They should be open (not closed for winter in most climates).
  • Clear shrubs, mulch, or debris blocking vents.
  • Good: Vents open and clear. Proceed to Check vapor barrier.
  • Bad: Vents closed or blocked—open and clear them, then recheck.

Check vapor barrier

Goal: Confirm the ground is covered with a vapor barrier.

  • Look for plastic sheeting on the soil. If missing, ground moisture evaporates into the space.
  • If present but torn, patch with overlapping plastic and tape.
  • Good: Ground covered. Proceed to Check drainage.
  • Bad: Add 6-mil polyethylene. Overlap seams 12 inches, tape, run up walls 6–12 inches.

Check drainage

Goal: Confirm water drains away from the foundation.

  • Check that soil slopes away from the house (at least 6 inches over 10 feet).
  • Check that downspouts extend 4–6 feet from the foundation. See fix-downspout-is-disconnected if needed.
  • Good: Water drains away. Add a dehumidifier if humidity stays high.
  • Bad: Fix grading and downspouts first.

Add a dehumidifier

Goal: Lower humidity if ventilation and vapor barrier are in place but moisture persists.

  • Place a crawl space dehumidifier on a platform above the floor.
  • Set it to keep humidity below 50%. Connect a drain hose if possible.
  • Good: Humidity drops over a few days.
  • Bad: Consider encapsulation—call a crawl space professional.

When to get help

Call a crawl space or foundation professional if:

  • You have standing water.
  • The space is hard to access for vapor barrier work.
  • You want full encapsulation (sealing vents and conditioning the space).
  • You suspect structural damage.

Confirm you have checked ventilation, vapor barrier, and drainage first.

Verification

  • Crawl space vents are open and unobstructed.
  • The vapor barrier covers the ground with no large gaps.
  • Soil slopes away from the foundation and downspouts extend 4–6 feet.
  • Humidity is below 60% (ideally 50%) when measured with a hygrometer.
  • No condensation on joists or ductwork.

Escalation ladder

Work from the device outward. Stop when the problem is fixed.

  1. Confirm moisture Verify damp soil, condensation, or musty smell—not standing water.
  2. Ventilation Open vents and clear blockages.
  3. Vapor barrier Add or repair plastic sheeting on the ground.
  4. Drainage Fix grading and downspouts so water drains away.
  5. Dehumidifier or pro Add a dehumidifier if moisture persists; call a pro for standing water or encapsulation.

What to capture if you need help

Before calling support or posting for help, have these ready. It speeds everything up.

  • Vent condition (open, blocked)
  • Vapor barrier presence and condition
  • Grading and downspout condition
  • Plumbing leak check results
  • Humidity readings
  • Steps already tried

Is there standing water in the crawl space?

Standing water is different from general moisture. It usually indicates a leak, flooding, or drainage failure.

Check the crawl space. Standing water: see fix-sump-pump-will-not-run. Damp soil or condensation: proceed with this guide. Verify you should see either standing water or dampness/condensation.

Yes No (damp or condensation)

You can change your answer later.

Call a pro or fix drainage

Standing water requires different steps. Check fix-sump-pump-will-not-run. If the cause is unclear or the water is deep, call a crawl space or foundation professional.

Done

Are the vents open and not blocked?

Crawl space vents must be open for cross-ventilation. Blocked vents trap moisture.

Check each vent from outside. Confirm they are open (not closed for winter in most climates). Clear shrubs, mulch, or debris blocking vents. Good: vents open and clear. Bad: vents closed or blocked—open and clear them. Verify you should see vents open and unobstructed.

Yes No

You can change your answer later.

Open vents and clear blockages

Open any closed vents. Clear debris, shrubs, or mulch blocking vents. Recheck the crawl space in a few days. If moisture persists, proceed to check the vapor barrier.

Done

Is there a vapor barrier covering the ground?

A vapor barrier blocks ground moisture from evaporating into the crawl space.

Look for plastic sheeting on the soil. If missing or torn, ground moisture enters the space. Missing: add 6-mil polyethylene, overlap seams 12 inches, tape, run up walls 6–12 inches. Torn: patch with overlapping plastic and tape. Good: ground covered. Bad: add or repair. Verify you should see plastic covering the soil.

Yes (intact) No or torn

You can change your answer later.

Add or repair vapor barrier

Lay 6-mil polyethylene over the soil. Overlap seams 12 inches and tape. Run plastic up foundation walls 6–12 inches. Patch tears with overlapping plastic and tape. Recheck moisture in a few days.

Done

Does water drain away from the foundation?

Grading and downspouts must direct water away from the house.

Check that soil slopes away (6 inches over 10 feet). Check that downspouts extend 4–6 feet from the foundation. See fix-downspout-is-disconnected if needed. Good: water drains away. Bad: fix grading and downspouts first. Verify you should see water draining away from the house.

Yes No

You can change your answer later.

Fix grading and downspouts

Regrade soil so it slopes away from the foundation. Extend downspouts 4–6 feet from the house. Fix disconnected downspouts. Recheck moisture after rain.

Done

Is humidity still high after ventilation and vapor barrier fixes?

Use a hygrometer. Aim for below 60% (ideally 50%).

Check humidity with a hygrometer. If above 60% after ventilation and vapor barrier are in place, add a crawl space dehumidifier. Set to keep humidity below 50%. If moisture persists or you want full encapsulation, call a pro. Good: humidity drops. Bad: call a crawl space professional for encapsulation.

Yes No

You can change your answer later.

Add dehumidifier or call a pro

Add a crawl space dehumidifier on a platform. Set to 50% humidity. Connect drain hose if possible. If you prefer full encapsulation (sealing vents, conditioning the space), call a crawl space professional.

Done

Moisture is under control

Ventilation, vapor barrier, and drainage are in place. Monitor humidity with a hygrometer. If moisture returns, recheck vents, vapor barrier, and drainage.

Done

Reviewed by Blackbox Atlas

Frequently asked questions

Why would a crawl space have moisture?
Common causes: poor ventilation (blocked or closed vents), no vapor barrier on the ground, water pooling from grading or downspouts, plumbing leaks, or high humidity. Fix ventilation and drainage first, then add a vapor barrier if missing.
Can I fix crawl space moisture myself?
Yes. You can open vents, add or repair a vapor barrier, fix grading and downspouts, and add a dehumidifier. Full encapsulation (sealing vents and conditioning the space) usually requires a pro.
When should I call a pro for crawl space moisture?
Call a pro if: you have standing water, the vapor barrier is missing and the space is hard to access, you need full encapsulation, or you suspect structural or plumbing issues.

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