Crawl space ground cover usually starts at 6 mil for a code minimum, but 10 mil is the practical pick for durability and 20 mil is the heavy-duty grade for full encapsulation.
A crawl space barrier fails for one boring reason most of the time: something poked a hole in it. Plastic sheeting that’s too thin for the job tears during install, punctures on gravel, and rips the first time a plumber crawls under the house to fix a pipe. The thickness you pick decides whether that barrier lasts 20 years or gets patched every spring, so here’s how the three common gauges compare and which one fits your crawl space.
How Thick Is 6 Mil, 10 Mil, And 20 Mil Poly?
The number on the label is the sheet’s thickness in thousandths of an inch, so a bigger number means a thicker, tougher barrier. That’s the entire scale, and it’s the thing most shoppers get confused by.
- 6 mil polyethylene = 0.006 inch thick
- 10 mil polyethylene = 0.010 inch thick
- 20 mil polyethylene = 0.020 inch thick
Thickness also changes how well the plastic blocks moisture vapor. Sellers and manufacturers publish permeance ratings that drop as the sheet gets thicker: about 0.13 perms at 6 mil, 0.076 perms at 10 mil, and 0.038 perms at 20 mil. Lower perm numbers mean less vapor moves through the plastic over time. A 6 mil sheet still works as a vapor barrier, but the 10 mil and 20 mil options cut that movement roughly in half and then half again.
Which Thickness Do You Actually Need?
Pick by traffic and purpose: 6 mil for a sealed, untouched crawl space, 10 mil for occasional access, and 20 mil for full encapsulation or anywhere you’ll walk on it. Building codes draw the floor, and real-world use draws the ceiling.
| Thickness | Permeance | Best For |
|---|---|---|
| 6 mil | 0.13 perms | Minimum code cover in sealed, low-traffic crawl spaces |
| 10 mil | 0.076 perms | Occasional access, moderate puncture risk |
| 20 mil | 0.038 perms | Full encapsulation, foot traffic, wet heavy-duty jobs |
The IRC sets 6-mil polyethylene as a minimum for crawl space ground cover, and that’s what a lot of builders install. It passes inspection and it’s cheap. The problem is durability: 6 mil tears more easily, so if anyone will crawl through that space again, you’re gambling. Thinner plastic is fine when it sits protected behind framing inside a wall cavity, but a floor cover lives right where boots, tools, and gravel sit on top of it.
10 mil is the number most installers recommend for a normal crawl space. It resists punctures far better than 6 mil, still handles a worker kneeling on it, and it doesn’t cost much more. If you want to replace the liner once and forget about it, this is the honest answer for most homes.
20 mil is the encapsulation-grade sheet. It’s built for crawl spaces that get serviced, where the barrier doubles as a floor, and where moisture is a chronic problem. It’s also the right call under slabs in some non-habitable uses, where sources recommend 10 mil as a minimum and thicker sheeting holds up better against the concrete pour. A heavier sheet makes more sense once you plan to walk on it, and a tested roundup of heavy-duty 20 mil vapor barrier options is worth checking before you buy, since quality varies between brands even at the same gauge.
The catch: 20 mil isn’t automatically necessary. If your crawl space is dry, rarely entered, and sealed up, spending the extra money buys durability you’ll never stress-test. It earns its price through traffic, wetter conditions, or a genuine encapsulation goal.
How Do You Install It So The Seams Don’t Leak?
Overlap every seam by 6 inches and seal it with tape — a sloppy seam defeats even the thickest sheet. The install details matter as much as the gauge you choose.
- Clear the ground fully. Sweep out debris, rocks, and dust before anything else, because grit under the plastic is what punctures it from below. Clean surfaces also let the tape actually adhere.
- Roll out the sheeting across the floor, running it up the walls and beyond joint ends where the design calls for it. Fold and crease the material as the layout requires at those junctions.
- Overlap adjoining sheets by 6 inches and seal each overlap with joint tape or a compatible sealant. For wall poly, one published building guide calls for overlapping rows by at least 6 inches and sealing with acoustical sealant or approved tape.
- Seal every penetration — pipes, piers, and posts — and patch any tear immediately. A cut you leave for later becomes a moisture channel you’ll never find.
When it’s done, the floor reads as one continuous sheet with taped seams and no visible gaps around pipes or piers. If you can see daylight through a seam, that’s the leak.
Which Mistakes Make A Barrier Fail Early?
Most failures trace back to buying the wrong gauge for how the space is used, not to bad plastic. Watch for these three.
- Choosing 6 mil for a crawl space you’ll access later, then tearing it within a season
- Picking a thickness without accounting for foot traffic, debris, or future service work
- Assuming 20 mil is required everywhere when a dry, untouched space doesn’t justify it
The rule of thumb that holds up: match the sheet to the abuse it’ll take. Sealed and ignored means 6 mil can work. Entered at all means 10 mil. Walked on means 20 mil.
FAQs
Is 6 mil plastic enough for a crawl space?
It can be, if the space is dry and nobody goes in it. The IRC lists 6-mil polyethylene as a minimum for crawl space ground cover, so it passes code. The risk is physical: 6 mil punctures and tears more easily than thicker sheeting, so it’s a poor fit for any crawl space you’ll service or walk through later.
Does thicker vapor barrier mean better moisture control?
Yes, up to a point, because thicker sheets have lower permeance ratings. The published numbers drop from about 0.13 perms at 6 mil to 0.076 at 10 mil and 0.038 at 20 mil. Each step lets less vapor through. Past the thickness you actually need, you’re paying for puncture resistance more than for moisture blocking.
Can I use 10 mil for full encapsulation?
You can, and plenty of installers do for moderate spaces. Full encapsulation usually pushes toward 20 mil because the liner gets walked on and has to survive service visits. If your crawl space sees regular foot traffic or chronic moisture, step up to 20 mil. For a sealed, lightly used space, 10 mil holds up fine.
References & Sources
- Crawl Space Energy Institute. “Vapor Barriers.” Covers crawl space barrier materials and thickness recommendations.
- Global Plastic Sheeting. “Choosing The Right Crawl Space Vapor Barrier: 6 Mil vs. 10 Mil vs. 20 Mil.” Source for permeance values and use guidance by thickness.
- Stego Industries. “Products.” Under-slab and below-grade vapor barrier product specifications.
