A crawl space vapor barrier should be a reinforced sheet of 12–20 mil meeting ASTM E1745, with a water vapor permeance of 0.1 perms or below, which classifies it as a Class I vapor retarder. Reinforcement, not thickness, is what determines whether it survives service.
Vapor retarder classes
Permeance is measured in perms. The class system is defined by that number and nothing else.
- Class I — 0.1 perms or less. Impermeable. Crawl space ground liner.
- Class II — greater than 0.1 up to 1.0 perms. Semi-impermeable.
- Class III — greater than 1.0 up to 10 perms. Semi-permeable.
What ASTM E1745 sets minimums for
The standard covers plastic water vapor retarders used in contact with soil under concrete slabs and in crawl spaces, and it sets minimum requirements across three properties rather than one: water vapor permeance, tensile strength, and puncture resistance.
That combination is why it is a more useful shorthand than any single figure on a product label.
Permeance vs mil: which number does which job
ASTM E1745 holds all three of its strength classes — A, B and C — to the same 0.1-perm permeance ceiling. Only the tensile and puncture minimums change between classes. In other words, every liner that meets the standard has already won the moisture argument; what class and mil buy is durability.
That is the single most useful fact when comparing quotes. A pitch comparing 12 mil against 20 mil on “moisture protection” is comparing the wrong property — both sit far below the permeance ceiling. Compare them on puncture resistance and on how the space will be used, which is the whole subject of the mil thickness guide and the buyer's guide.
Reinforcement, and why it dominates
A reinforced liner carries a woven scrim between polyethylene plies. The scrim arrests a puncture so it does not propagate into a tear.
In a space that anyone crawls through, this is the property that decides service life. Reinforced 12 mil will outlast unreinforced 20 mil, which is why comparing products on mil alone leads people to the wrong material.
Where the liner terminates
In a sealed crawl space the liner runs across the floor, is wrapped around each pier, and terminates on the perimeter wall above the historic high-water line, mechanically fastened to a cleaned surface as well as sealed.
In a vented crawl space, floor-only is normal, because the walls sit outside the thermal and air boundary.
The Cape Fear spec: what changes on this coast
Two local conditions move the specification here. The first is the water table: much of Wilmington, Leland and the beach towns sits low over sandy soil, and after sustained rain groundwater can rise under a crawl space that was bone dry in October. That is why termination height matters — the liner runs up the wall above the historic high-water line — and why drainage goes in first wherever that line exists.
The second is the air. A ground liner only solves the soil half of the moisture problem; open foundation vents keep re-loading the space from outside for most of the year. A DOE-funded North Carolina field study measured it: wall-vented crawl spaces ran above 80% relative humidity through spring and summer while sealed ones held below 65%, against a mold-growth threshold near 70%. The spec that works here is the liner as one part of a sealed, conditioned system — the encapsulation vs vapor barrier guide covers that decision.
One coastal caution: in an AE or VE flood zone, sealing a crawl space must never block required flood openings. Read the NFIP flood vent requirements before any vent is closed.
What the spec costs installed
In the Wilmington area a reinforced 12–20 mil liner installed to this spec — seams taped, piers wrapped, perimeter mechanically fastened — runs $1,500–$4,000 as a standalone component. As part of a full encapsulation with vent sealing and a dehumidifier, budget $4,200–$14,000; the encapsulation cost guide breaks that down line by line.
Material is the smaller share of that. Bought yourself, reinforced liner runs roughly $0.50–$0.90 per square foot — the DIY guide covers what the saving actually buys and where DIY installs fail. Either way the install standard is what you are paying for: our vapor barrier installation service photographs every seam and termination before the space is closed.
The bottom line
Reinforced, 12–20 mil, ASTM E1745, Class I. Those four facts are the whole specification, and any quote should be able to state them.
For a number against your own crawl space, use the cost estimator, or call (910) 886-2018 and we will come and measure it.