Search for crawl space moisture control and you will see “vapor barrier” and “vapor retarder” used as if they were two products. They are not. In a crawl space they are the same job: a sheet of plastic over the dirt that slows ground moisture from rising into the air under your house. The real questions are which sheet to buy, how it has to be installed to count, and when no sheet will fix the problem.
This guide sorts out the two terms, then works from the code text and a U.S. Department of Energy weatherization job aid to show what a ground cover actually has to do. It compares the three products readers ask about most (a plain 10 mil film, a 20 mil woven sheet and seam tape), and works the numbers for a 30 by 40 foot crawl space, because the roll width you choose changes how many seams you have to tape. If your problem is water entering through the foundation wall, start with the foundation waterproofing sealant guide instead.
Barrier or retarder: the words do not agree, so read the perm number
You will find confident explanations that a “retarder” slows vapor and a “barrier” stops it, with a perm cutoff between them. The cutoffs depend on who is writing. Dorken’s Delta Academy defines a vapor barrier as a layer of 1 perm or less and a retarder as more than 0.1 perm and up to 1 perm. The international energy code defines a vapor retarder as a material of 1 perm or less and does not use the word barrier at all. A perm is a unit of how much water vapor passes through a material, so a lower number means a tighter sheet.
The residential building code avoids the argument by using classes. Its table for vapor retarders, measured by the ASTM E96 test, calls a material Class I at 0.1 perm or less, with sheet polyethylene as the standard example. Class II is above 0.1 up to 1 perm, and Class III is above 1 up to 10. For a crawl space ground cover, Class I is the one that matters. So when a listing says “vapor barrier” and another says “vapor retarder,” ignore the label. Ask for the perm rating or the class.
What the code and the Department of Energy actually ask for
The International Residential Code section on unvented crawl spaces (R408.3) says exposed earth is covered with a continuous Class I vapor retarder. Joints overlap by 6 inches and are sealed or taped, and the edges extend at least 6 inches up the stem wall and are attached and sealed to the wall or insulation. That section also requires a ventilation or conditioning method, such as continuously operated mechanical exhaust at 1 cubic foot per minute for each 50 square feet of crawl space floor. Older code editions said only “continuous vapor retarder,” and states amend this text, so check which edition your local building department has adopted. Notice what is missing: the code names no thickness.
The Department of Energy’s weatherization job aid Install or Repair a Crawl Space Vapor Retarder (June 2024) is stricter and more practical:
| Item | Building code (R408.3, unvented) | DOE job aid |
|---|---|---|
| Material | Continuous Class I retarder | 6 mil minimum, perm rating of 0.1 or less |
| Seam overlap | 6 inches, sealed or taped | At least 12 inches, lapped upslope, taped |
| Walls, piers, columns | 6 inches up the stem wall, sealed | At least 6 inches up walls, piers and columns, sealed with mastic or acoustical sealant |
| Before you start | Not addressed | Clear debris, aim for 100 percent coverage, do not block drainage |
If you are doing this yourself, follow the stricter column. A 12 inch overlap passes both. Martin Holladay of Green Building Advisor adds two points in that article’s comments that are worth keeping: a sheet with a few small holes still works as a diffusion retarder, and polyethylene does not stop liquid water, so gutters, downspouts and grading come first.
What thickness buys you
The Department of Energy minimum is 6 mil, and the code sets none, so every product here clears the paperwork bar. In practice, extra thickness mostly buys puncture resistance. That is my reading, not a code requirement, and it follows from how the sheet gets used: you crawl on it, drag tools across it, and lay it over gravel or rubble. A sheet that will be walked on for HVAC or plumbing service is worth more than one that will be installed and never touched. Woven (scrim-reinforced) sheeting resists tears running from a nick better than plain film.
One caution. Perm ratings come from the manufacturer’s data sheet, and not every listing provides one. The two sheeting listings below do not state a perm rating or ASTM class on the page, so confirm it with the seller before you rely on the product for a code-driven job such as a permitted encapsulation.
Which sheet fits which crawl space
| Your crawl space | Pick | Why |
|---|---|---|
| Dry, rarely entered, smooth or graded soil | 10 mil plain film | Clears the 6 mil minimum with margin, lowest cost per square foot |
| Regular service access, sharp gravel or old concrete rubble | 20 mil woven | Tear and puncture resistance where you will walk and drag tools |
| Standing water or a muddy floor after rain | None yet | Plastic does not stop liquid water. Fix gutters, grading or drainage first |
| Sealed (unvented) crawl space under a permit | Whatever the inspector approves | Ask for the perm rating and follow the ventilation or conditioning rule in R408.3 |
The three products
Spirecover Crawl Space Vapor Barrier, 10 Mil, 10 by 100 feet. A black and white film (the white side faces up to reflect light), rated 4.7 stars from 380 ratings and an Amazon’s Choice when I read the listing. It covers 1,000 square feet per roll and is made of polyethylene. The listing gives no perm rating and says nothing about reinforcement, so treat it as plain film. One verified buyer noted that it unfolds black side up, which means extra work flipping it in a tight space. Best for the dry, low-traffic row of the table above.
Check price on Amazon: Spirecover Crawl Space Vapor Barrier, 10 Mil. This is an affiliate link (see our disclosure for details).
Whitecap Woven Vapor Barrier, 20 Mil, 12 by 100 feet. A white woven polyethylene sheet, sold by CrawlSpace-DIY with a lifetime warranty named in the title, rated 4.6 stars from 109 ratings and an Amazon’s Choice. The roll covers 1,200 square feet and the white surface makes standing water and debris easy to spot on later visits. It costs well above a plain 10 mil roll, so it earns its place only where the floor is rough or you will be down there often. The listing states no perm rating, so ask the seller.
Check price on Amazon: Whitecap Woven Vapor Barrier, 20 Mil. This is an affiliate link (see our disclosure for details).
Crawlspace-DIY Vapor Barrier Tape, 12 Mil, 4 inch by 180 feet. A film is only as good as its seams, and both the code and the Department of Energy call for them to be sealed or taped. This polyethylene seam tape is rated 4.7 stars and was in stock.
Check price on Amazon: Crawlspace-DIY Vapor Barrier Tape, 12 Mil. This is an affiliate link (see our disclosure for details).
How much to buy: a 30 by 40 foot crawl space
The floor is 1,200 square feet. The Department of Energy wants the plastic to run at least 6 inches up the walls, so the sheet has to cover 31 by 41 feet. Running the strips along the 40 foot side, each strip is 41 feet long. Every extra strip adds only its width minus the 12 inch overlap, so the roll width changes the count:
| Roll width | Strips needed | Seams to tape | Seam tape | Film used |
|---|---|---|---|---|
| 10 feet | 4 (10 + 9 + 9 + 9 = 37 feet, 31 needed) | 3 | 123 feet | 1,640 sq ft |
| 12 feet | 3 (12 + 11 + 11 = 34 feet, 31 needed) | 2 | 82 feet | 1,476 sq ft |
Either way you buy two 100 foot rolls, because a roll gives only two 41 foot strips. A single 180 foot roll of seam tape covers the seams in both cases, with room to spare. The perimeter is 140 feet, and the Department of Energy seals that edge with mastic or acoustical sealant rather than tape. If you tape the edge instead, add 140 feet and you are at 263 feet of tape, which means a second roll. Posts and piers need extra film and sealing, so count them and add a margin before you order.
The wider roll is not cheaper in film, but it removes one 41 foot seam to tape on your knees. That is a reason to consider a 12 foot roll for a wide crawl space even before you consider its thickness.
When a liner is the wrong fix, and who to call
Skip the plastic for now if the crawl space floods after rain, if water runs in through the foundation wall, or if the soil stays muddy for days. A sheet laid over standing water traps it against the framing. Start outside: make sure the soil slopes away from the house, gutters are clean and downspouts discharge well away from the foundation. If water still gets in, a foundation or waterproofing professional can tell you whether you need drainage work or a sump, and that is a job for a contractor rather than a roll of plastic. Angi’s foundation pros are one way to find one. This is an affiliate link (see our disclosure for details).
For damp basement walls rather than a dirt floor, see our basement waterproofing paint guide, and for cracked slabs, the concrete crack filler guide.
Etienne Basson maintains and repairs his own home, and writes practical guides on choosing the right materials and products for repair and finishing jobs: fillers, sealants, adhesives, and similar. He focuses on what actually works for a specific job, not just a spec sheet.