The best crawl space vapor barrier is a reinforced liner of 12–20 mil meeting ASTM E1745, with a permeance below 0.1 perms. Reinforcement matters more than the mil number: reinforced 12 mil outlasts unreinforced 20 mil in any crawl space someone actually enters.
Reinforced vs unreinforced
Reinforced liner has a scrim layer — a woven mesh — sandwiched between plies of polyethylene. That mesh is what stops a puncture becoming a tear that runs the length of the sheet.
Unreinforced sheet of the same thickness punctures and then splits. This is why comparing products on mil rating alone gets people the wrong material: the construction matters more than the number.
What ASTM E1745 guarantees
ASTM E1745 sets minimums across water vapor permeance, tensile strength and puncture resistance. A product either meets it or it does not, and it is the only meaningful shorthand for whether a liner is fit for the job.
A Class I vapor retarder is one at 0.1 perms or below. That is the property doing the actual work of stopping soil moisture entering as vapour.
Which thickness for which situation
12 mil reinforced is appropriate for most crawl spaces that see occasional service access. 20 mil is worth it where the space is entered regularly, where there is equipment down there needing maintenance, or where the ground is rough and sharp.
- Occasional access, smooth ground — reinforced 12 mil
- Regular access or equipment servicing — reinforced 16–20 mil
- Storage use — 20 mil, and consider a walkway layer
- Any thickness unreinforced — not suitable
The bottom line
Ask two questions of any quote: is the liner reinforced, and does it meet ASTM E1745. If the answer to either is unclear, the mil number on its own tells you very little.
For a number against your own crawl space, use the cost estimator, or call (910) 886-2018 and we will come and measure it.