Quick answer: Control crawl space humidity in order. First stop liquid water: leaks, poor grading, gutters, standing water. Then cover the soil with a continuous ground vapour barrier, because exposed soil is often the largest moisture source. Then deal with the air. In humid climates, vents bring in summer air that condenses on cool crawl space surfaces. Sealing the crawl space and conditioning or dehumidifying it is often more effective, but it needs a local specialist's design. A dehumidifier handles the moisture left after those steps; it isn't a substitute for them. Aim to keep the space below about 60 % RH, the level EPA guidance gives for limiting mould growth.
Quick answer
- 1: Fix water entry. 2: Ground vapour barrier. 3: Seal or ventilate by climate. 4: Dehumidify what remains.
- Keep below about 60 % RH.
- Check combustion appliances and radon before sealing.
Start with water and safety
Look for standing water, wet soil, plumbing leaks, blocked drains, poor grading, condensation on ducts and pipes, damaged insulation and damp wood.
Safety before work:
- Don't enter with exposed wiring, sewage or unstable structure; call a qualified contractor.
- If the crawl space contains a natural-draft furnace or water heater, have an HVAC professional assess combustion air before sealing. Dehumidifiers don't remove carbon monoxide.
- Test for radon before a sealing retrofit and follow local guidance; a dehumidifier is not radon mitigation.
Choose the route by moisture source
| Observed condition | First action |
|---|---|
| Standing water or recurring leak | Repair the source and drainage; dry materials |
| Exposed soil | Continuous ground vapour barrier; see vapour barrier installation |
| Humid outdoor air reaching cool surfaces | Consider sealing and conditioning with a local specialist |
| Residual humidity after the above | Dehumidifier or supply of conditioned air |
Related: vapour barrier installation.
The US DOE's Building America programme explains why closing a vented crawl space needs a source and safety assessment (DOE Building America, vented crawlspace guide) and covers capillary breaks for crawl space floors (DOE BSESC).
Vented or sealed?
| Vented crawl space | Sealed (encapsulated) crawl space | |
|---|---|---|
| Idea | Outdoor air dries the space | Space treated as part of the building |
| Works where | Dry climates and seasons | Humid climates |
| Risk | Summer outdoor air condenses on cool surfaces | Needs careful sealing, insulation, conditioning or dehumidification |
| Requirements | Vent area per local code | Vapour barrier up walls, sealed vents, insulation, moisture control |
Local codes decide what is allowed.
When a dehumidifier helps
- After water entry is fixed and the soil is covered.
- In sealed crawl spaces in humid climates.
- Choose a unit rated for low temperatures if the space is cool, with a drain or condensate pump.
- Size from the crawl space volume and moisture load; small spaces usually need a compact unit.
EPA guidance on mould and moisture recommends keeping indoor humidity below 60 % (EPA, A brief guide to mold, moisture and your home).
Inspection and decision sequence
- Inspect for water, leaks and safety hazards.
- Repair water sources and drainage.
- Install or repair the ground vapour barrier.
- Decide vented or sealed with a local specialist.
- Monitor RH and wood moisture for a season.
- Add dehumidification if humidity stays high.
See also basement and below-grade dehumidification and our commercial dehumidifiers.
FAQ
Frequently asked questions
What humidity should a crawl space be?
Below about 60 % RH to limit mould; lower is better for wood moisture.
Do crawl space vents reduce humidity?
In dry weather they can; in humid summers they often bring in moisture that condenses on cool surfaces.
Is a vapour barrier enough?
It controls soil moisture, but leaks, outdoor air and residual humidity may still need treatment.
Sources
- US DOE Building America, Guide to closing and conditioning ventilated crawlspaces.
- US DOE Building Science Education Solution Center, Crawlspace capillary break.
- US EPA, A brief guide to mold, moisture and your home.
