Use reinforced 12–20 mil for a crawl space anyone enters. 6 mil unreinforced builder sheet is not suitable at any price. Reinforcement matters more than thickness: reinforced 12 mil outlasts unreinforced 20 mil, because the scrim layer stops a puncture becoming a tear.
What a mil actually is
A mil is one thousandth of an inch. A 12 mil liner is 0.012 inches thick — about three sheets of printer paper. On its own that number tells you about material volume and very little about whether the sheet survives service.
Puncture resistance and tensile strength are the properties that matter, and ASTM E1745 sets minimums for both alongside permeance — which is why the buyer's guide starts from reinforcement, not thickness.
The permeance myth: thicker does not mean drier
The intuition that 20 mil "blocks more moisture" than 12 mil is wrong, and ASTM E1745 is the proof: the standard holds all three of its classes to the same 0.1-perm permeance ceiling, and every liner that meets it — at any thickness — has already settled the moisture question. Polyethylene is simply a very good vapor retarder by the time it is a few mils thick.
What thickness and class actually buy is strength: tensile and puncture minimums rise with class, and scrim reinforcement multiplies both. So the mil decision is a durability decision — how hard the liner's life will be — not a dryness decision. The spec and standards reference lays out the class table in full.
Choosing by how the space is used
The realistic question is how often someone will be down there and on what surface. A crawl space with equipment needs servicing; one used for storage takes repeated traffic; one that is only inspected annually sees almost none. Match the liner to that life rather than to the biggest number on the shelf.
- 6 mil unreinforced — not suitable for a crawl space
- 10–12 mil reinforced — occasional service access, smooth ground
- 16 mil reinforced — regular access, equipment to maintain
- 20 mil reinforced — frequent access, storage use, or sharp/rocky ground
Thickness on the Cape Fear coast, practically
Coastal crawl spaces earn the heavier liners more often than inland ones. If there is a dehumidifier or sump pump down there, someone crawls to it for filter changes and float checks several times a year — that is regular access, and it argues for 16–20 mil on the traffic paths. And the ground under older Wilmington homes is often sandy fill mixed with shell and builder debris, which attacks liner from below; reinforcement matters more than mil, but 20 mil buys margin over rough ground.
What it does to the budget is modest. Installed, the liner component runs $1,500–$4,000 in this area either way; as material, reinforced sheet runs roughly $0.50–$0.90 per square foot, so the step from 12 to 20 mil is a few hundred dollars on a typical footprint — small against replacing a torn liner. The encapsulation cost guide shows where the liner sits among the other components, and the installation page shows the standard it should go in to.
The bottom line
Reinforced 12 mil meeting ASTM E1745 is the sensible default for most Wilmington crawl spaces. Step up to 20 mil if the space gets used rather than just inspected.
For a number against your own crawl space, use the cost estimator, or call (910) 886-2018 and we will come and measure it.