What Does Mil Thickness Mean for Crawl Space Liners?
A mil is one-thousandth of an inch (0.001 in), and "mil thickness" is how manufacturers and contractors describe the thickness of a crawl space vapor retarder or liner.
Explanation
A mil is not a millimeter. One mil is about 0.0254 mm, so a 6-mil sheet is roughly 0.15 mm thick. North Carolina's Residential Code uses exactly that figure when it sets the minimum ground vapor retarder for a closed crawl space: 6-mil (0.15 mm) polyethylene or equivalent.
Common liner thicknesses you'll see in quotes:
| Thickness | Inches | Typical use |
|---|---|---|
| 6 mil | 0.006 | Code minimum ground cover; basic builder-grade plastic |
| 10 to 12 mil | 0.010 to 0.012 | Mid-grade liners, often reinforced |
| 15 to 20 mil | 0.015 to 0.020 | Heavy-duty encapsulation liners for crawl spaces with regular foot traffic |
Thickness is only one part of quality. Two liners with the same mil rating can perform very differently. Ask about:
- Reinforcement: A woven scrim or cord grid resists tearing and punctures.
- Material: Virgin polyethylene or blended polyolefin, compared with recycled content.
- Permeance: To count as a Class I vapor retarder, the material must be rated at 0.1 perm or less.
- Installation: Overlapped and taped seams, sealing around piers, and the termite inspection gap at the top of the walls.
A thicker liner makes the most sense where people will crawl over it often, for example to reach HVAC equipment, plumbing or a dehumidifier.
When to seek help
- Quotes list different mil ratings and you can't tell which is the better value. Our quote comparison guide helps.
- Your existing thin plastic has torn or slid off the soil.
- A home inspection flagged the vapor barrier as damaged or incomplete.
See vapor barrier services or estimate costs with the cost calculator.