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. 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.
Choosing by how the space is used
The realistic question is how often someone will be down there and on what surface.
- 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
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.