# Crawl Space Vapor Barrier Thickness: 6, 12 or 20 Mil? > Whether to use 6, 12 or 20 mil vapor barrier in a crawl space, what the mil rating does and does not tell you, and why reinforcement matters more. URL: https://910crawlspacepros.com/guide/crawl-space-vapor-barrier-thickness/ Home / Crawl Space Vapor Barrier Installation / # What Thickness Vapor Barrier Does a Crawl Space Need? Reference What Thickness Vapor Barrier Does a Crawl Space Need? The number on the box is not the specification. Updated 14 August 2026 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. Get an exact quote A free on-site measure beats any online range. Website What do you need? Select a service… Crawl Space Encapsulation Crawl Space Insulation Crawl Space Waterproofing Crawl Space Mold Removal Crawl Space Vapor Barrier Installation Crawl Space Dehumidifier Installation Crawl Space Drainage Crawl Space Sump Pump Installation Crawl Space French Drain Crawl Space Cleanup Crawl Space Structural Repair Sagging Floor Repair Crawl Space Door Replacement Flood Vent Installation Not sure — I need someone to look ZIP code Continue Your name Phone What are you seeing? (optional) Back Request my inspection Got it — your request is in. We'll be in touch with a quote. Related service Crawl Space Vapor Barrier Installation Related guides How to Choose the Best Vapor Barrier for a Crawl Space Vapor Barrier for a Crawl Space: Spec and Standards What Crawl Space Humidity Level Should You Aim For? Common Encapsulated Crawl Space Problems How Long Does Crawl Space Encapsulation Last? All guides → , not a heading: this is a CTA, not a section of the document outline. --> Get your crawl space measured and priced Free on-site inspection across Wilmington and New Hanover, Brunswick and Pender counties. Free quotes within 24 hours. Call (910) 886-2018 Get a free quote ## Questions Is 20 mil vapor barrier worth the extra cost? In a crawl space with equipment to service or regular access, yes. In one that gets entered once a year, reinforced 12 mil is the proportionate choice. Can I double up 6 mil instead of using 12 mil? No. Two unreinforced layers still have no scrim, still tear, and now have a moisture plane trapped between them. Does 20 mil vapor barrier block more moisture than 12 mil? Not meaningfully. Both sit far below the 0.1-perm Class I ceiling; permeance is settled long before thickness becomes interesting. The extra mil buys puncture and tear resistance — durability, not dryness. What thickness do installers actually use in Wilmington? Reinforced 12 mil is the working default for standard encapsulations here, stepped to 16 or 20 mil where a dehumidifier or sump needs regular service access, storage is planned, or the ground is rough. Unreinforced sheet is not used at any thickness.