Quick answer: A crawl space vapour barrier is a polyethylene (or similar) membrane laid over the soil to stop ground moisture evaporating into the space. To install one: (1) fix leaks, drainage and standing water first; (2) choose a durable membrane, at least 6-mil per the US DOE weatherization baseline, and thicker where people crawl over it; (3) plan near-complete coverage, with overlapped seams and the sheet run up the walls and around piers; (4) lay, tape and fasten it; (5) check it's continuous and monitor humidity afterwards. A barrier controls soil vapour, not leaks or humid outdoor air.
Quick answer
- Fix water first; a sheet must not hide a leak.
- Membrane at least 6-mil; continuous coverage; overlaps taped; up walls and piers.
- Monitor humidity afterwards; add other measures if it stays high.
What a ground vapour barrier does and doesn't do
| Does | Doesn't |
|---|---|
| Reduces moisture from exposed soil | Stop plumbing leaks or surface water |
| Acts as a capillary break | Resist groundwater pressure |
| Helps a sealed crawl space design | Dry humid outdoor air entering through vents |
| Reduces soil gas entry when sealed | Count as radon mitigation by itself |
1. Inspect water sources first
Repair plumbing leaks, gutter discharge toward the foundation, grading problems and standing water. Don't block existing drains. Note where piers, pipes, ducts, access openings and equipment will interrupt the sheet. Decide whether the crawl space stays vented or will be sealed; see crawl space humidity control.
2. Choose the material
- Vapour-retarding performance stated by the maker.
- Puncture resistance for the ground and expected foot traffic.
- Compatible tape or sealant.
- The US DOE weatherization job aid uses a minimum 6-mil material and near-complete ground coverage as its baseline (US DOE job aid, install or repair crawl space vapor retarder). Local codes or product systems may require more.
3. Plan coverage, seams and wall rises
- Sketch the floor, piers, walls and penetrations.
- Plan seam overlaps as the product specifies (commonly about 150–300 mm), taped or sealed.
- Plan how far the sheet runs up the walls and piers, and how it is fastened.
- Leave an inspection strip at the top of walls if required for termite inspection.
4. Prepare, place and secure
- Remove sharp debris; rake rough ground.
- Lay the sheet from the far end toward the access.
- Overlap and tape seams.
- Run the sheet up walls and piers; fasten mechanically and seal the edge.
- Seal around pipes and posts with tape or boots.
- Protect high-traffic paths with extra material.
5. Check and monitor
- Walk the space: no gaps, tears or loose seams.
- Confirm drains and sump pits remain accessible.
- Log RH and temperature for a season; check wood moisture.
- If RH stays above about 60 %, review vents, air sealing and dehumidification.
DOE's Building Science Education Solution Center explains the role of a capillary break under crawl space floors (DOE BSESC).
Radon and health claims
A sealed barrier is part of soil-gas control, but radon results need testing; follow EPA and local radon guidance.
Handover checklist
- [ ] Water sources repaired
- [ ] Membrane type and thickness recorded
- [ ] Coverage near-complete; seams overlapped and sealed
- [ ] Wall and pier terminations fastened and sealed
- [ ] Penetrations sealed
- [ ] Drains and inspection access kept
- [ ] Monitoring plan
FAQ
Frequently asked questions
What thickness vapour barrier for a crawl space?
At least 6-mil per the DOE weatherization baseline; thicker (10–20 mil) resists damage where people crawl.
Should a crawl space vapour barrier go up the walls?
Yes in most sealed designs; how far depends on code and inspection requirements.
Will a vapour barrier stop crawl space humidity?
It stops soil moisture; outdoor air, leaks and residual humidity may still need treatment.
Sources
- US DOE, Install or repair crawl space vapor retarder (weatherization job aid).
- US DOE Building Science Education Solution Center, Crawlspace capillary break.
- US EPA, radon guidance.
