Most crawl space and slab jobs land on 10-mil to 15-mil film; 6-mil is the code floor, and 20-mil is for encapsulated crawl spaces.
A 6-mil sheet saves a few dollars at the register, then rips open on the first crawl across a gravel floor. Water finds the tear, and the plastic under the house stops doing its job.
What sits on top of the plastic decides what mil vapor barrier you need underneath it. Slab pours, maintenance traffic, and rocks under the house push toward heavier film, and code sets a 6-mil minimum for crawl space ground cover. Most homes land in the 10-mil to 15-mil range, while an encapsulated crawl space with regular foot traffic leans toward 20-mil.
What Mil Vapor Barrier Do You Need for a Crawl Space?
Section R408.3 of the International Residential Code requires at least 6-mil polyethylene ground cover in a crawl space. That number is a compliance floor, not a durability target.
Field conditions punish thin film. A 6-mil barrier tears on rock, gravel, and debris, and it does not survive long in a crawl space anyone enters for plumbing or HVAC repairs. A 10-mil roll handles light-duty floors and general ground cover with far fewer punctures. A 12-mil liner adds another step of toughness to a crawl space floor, and 20-mil is the usual pick for full encapsulation or a floor that sees service traffic.
Thicker is not automatically better everywhere. Wall cavities and covered areas rarely need more than the code minimum, and a 20-mil roll hidden inside a finished wall buys no protection you can use. Basement walls face the same moisture and the same occasional contact as crawl space floors, so they usually land in the same 10-mil to 12-mil range.
Choosing Vapor Barrier Thickness: What Each Mil Level Handles
A mil is one-thousandth of an inch, so 6-mil film measures 0.006 inch, 10-mil measures 0.010 inch, 12-mil measures 0.012 inch, 15-mil measures 0.015 inch, and 20-mil measures 0.020 inch. The heaviest sheet is a little over three times the thickness of the lightest.
Under-slab work follows its own standard. Vapor retarders beneath concrete slabs fall under ASTM E1745, and the sheets specified for that job are typically 10-mil to 15-mil. A 6-mil film can meet a minimum on paper but struggles across an angular subbase and under construction traffic. Vapor retarder installation guidance keeps returning to the same failure point: punctures from equipment and construction abuse, which thin film cannot absorb.
| Thickness | Typical Job | What Matters |
|---|---|---|
| 6-mil (0.006 in) | Code-minimum crawl space ground cover, protected wall cavities | Tears easily; keep gravel and traffic away |
| 10-mil (0.010 in) | Light-duty floors, general ground cover | Common low end for slab retarders |
| 12-mil (0.012 in) | Mid-range crawl space liner | Sturdier than 10-mil, still easy to handle |
| 15-mil (0.015 in) | Under-slab pours, floors needing puncture resistance | Sits at the top of the usual ASTM E1745 range |
| 20-mil (0.020 in) | Encapsulated crawl spaces, floors with maintenance traffic | Heaviest common sheet; rarely needed behind walls |
Prices climb with thickness, and no authoritative manufacturer price list turned up for most of these rolls, so comparisons vary by retailer. If the middle of the range is where your house lands, the tested 12 mil vapor barrier picks we rounded up show which rolls arrive with the puncture resistance they promise.
Getting the Mil You Chose to Survive the Job
Installation, not the mil number, decides how long the barrier lasts. Even a heavy sheet fails when it goes down over sharp gravel or gets dragged by equipment during a pour.
Three mistakes show up in crawl spaces again and again. Thin 6-mil film gets used where service traffic is constant, so the cheap roll is bought twice. Thickness gets treated as a substitute for prep, even though a smooth, debris-free subbase matters more than the film’s rating. Non-ASTM E1745 plastic gets laid under a slab, where the spec exists for a reason.
