Vapor Barrier for a Crawl Space: Spec & Standards Guide (2026)
Which spec actually matters when you choose a crawl space vapor barrier — the ASTM E1745 class, the mil thickness, and the perm rating — and why the number most people fixate on (mil) isn’t the one that blocks moisture. This is a standards reference, not a sales page: we lay out the code and the lab data so you can spec it right, then have it installed.
Last updated July 19, 2026 · ~8 min read
Under ASTM E1745, all three vapor-retarder classes (A, B, and C) share the same 0.1-perm ceiling. What changes with thickness is puncture and tensile strength — not moisture-blocking. Mil governs durability; permeance governs moisture.
Source: ASTM E1745-17 · cross-verified with the IRC vapor-retarder classes & manufacturer data sheets
Key takeaways
- 0.1 perms is the permeance ceiling for every ASTM E1745 class — A, B, and C are identical on moisture; they differ only in strength. (ASTM E1745)
- 0.04–0.06 perms is where plain 6-mil builder poly already sits — it’s a Class I vapor retarder by code. The weakness is durability, not permeance. (Insulation Institute)
- 475 g → 2,200 g is the puncture jump from Class C to Class A. That toughness — the tear and puncture rating — is what a higher mil actually buys. (ASTM E1745)
- 1:150 → 1:1500 — covering the ground with a Class I liner cuts a vented crawl’s required vent area tenfold. The barrier is the code’s own lever. (IRC R408.1)
- >80% vs <65% RH — in a NC field study, vented crawl spaces ran humid all summer while sealed ones stayed dry. On the coast, mil won’t fix that; sealing will. (Advanced Energy / DOE)
1Permeance vs. mil: the one thing to get straight
A vapor barrier’s job is to stop water vapor, and the property that measures that is permeance, in perms — lower is tighter. Mil is just thickness. The catch: essentially every real crawl-space liner already blocks vapor far below the code threshold, so the mil number you’re comparing is buying you durability, not a drier crawl space.
Here is the proof in three numbers. Plain 6-mil polyethylene tests around 0.05 perms. A 15-mil reinforced liner tests at 0.0086. A 20-mil liner tests at 0.005. All three are a Class I vapor retarder — the tightest class building code defines, which only requires 0.1 perms or less. Tripling the thickness roughly halves an already-negligible permeance, while it more than doubles the puncture resistance. That’s the whole story: mil is a toughness spec. It decides whether the liner survives being crawled over, wrapped around piers, and left in place for years — which is exactly why a 6-mil builder sheet fails where a reinforced liner doesn’t.
A 20-mil liner has roughly ten times the puncture resistance of a Class C material, but its permeance differs from a 15-mil by only thousandths of a perm — both are trivially below the 0.1-perm ceiling.
Stego Wrap 15-mil + W.R. Meadows PERMINATOR 20-mil data sheets
2ASTM E1745: Class A, B, and C are strength tiers, not moisture tiers
ASTM E1745 is the material standard for below-grade plastic vapor retarders — the one a quality crawl-space liner cites. It rates three classes. All three must meet the same 0.1-perm permeance limit; the only things that change between them are tensile strength and puncture resistance.
| ASTM E1745 class | Min. tensile | Min. puncture | Max. permeance |
|---|---|---|---|
| Class A | 45 lbf/in | 2,200 g | 0.1 perms |
| Class B | 30 lbf/in | 1,700 g | 0.1 perms |
| Class C | 13.6 lbf/in | 475 g | 0.1 perms |
Source: ASTM E1745-17 (via Stego Industries). Puncture per ASTM D1709.
The practical read: a liner marketed as “Class A” isn’t drier than a Class C — it’s tougher. For a crawl space that people will enter to service HVAC, plumbing, or a dehumidifier, that toughness is the point.
3Mil-thickness decision table
Because permeance is a solved problem across the board, the mil you choose is really a match between the liner’s toughness and how the crawl space will be used. Here is how the common thicknesses line up on the two numbers that actually vary — permeance (barely) and puncture (a lot).
| Liner | Permeance | Puncture | Typical use |
|---|---|---|---|
| 6-mil builder poly | 0.04 – 0.06 perms | Not rated (low) | Code-minimum vented ground cover; radon soil-gas retarder |
| 10-mil reinforced | 0.0183 perms | — | Light-duty encapsulation liner |
| 15-mil reinforced (Class A) | 0.0086 perms | 2,266 g | Standard serviced-crawl liner |
| 20-mil reinforced (Class A/B/C) | 0.005 perms | > 3,500 g | Heavy-traffic / walk-on liner |
Sources: Insulation Institute (6-mil poly), W.R. Meadows PERMINATOR 10/15/20-mil, Stego Wrap 15-mil.
A useful rule of thumb: 6-mil is a code-minimum ground cover; 12–15 mil reinforced is the standard for a crawl space you’ll actually seal and use; 20 mil is the walk-on tier for spaces serviced often. The step up from 6-mil isn’t about the moisture math — it’s that a reinforced liner stays intact where builder sheet tears, which is what keeps a barrier working for the long haul instead of failing in a few years.
4Class I / II / III: the code’s permeance language
When code or a spec sheet says “Class I vapor retarder,” it’s describing permeance, not a product. The IRC sorts every vapor retarder into three bands by perm rating. A crawl-space liner needs to be the tightest one.
| Retarder class | Permeance | Typical material |
|---|---|---|
| Class I | ≤ 0.1 perm | Polyethylene sheeting (crawl-space liner) |
| Class II | > 0.1 to ≤ 1.0 perm | Kraft-faced batt insulation |
| Class III | > 1.0 to ≤ 10 perm | Latex or enamel paint |
Source: Insulation Institute (IRC vapor-retarder classification).
Note the collision of terms: ASTM E1745 Class A/B/C (a strength grade) is a different axis from the IRC Class I/II/III (a permeance grade). A crawl-space liner is almost always Class I on permeance and then graded A, B, or C on toughness. Polyethylene hits Class I easily — which is why the argument on a spec sheet is never really about whether the plastic blocks vapor, but about how well it survives the crawl space.
5The IRC code baseline for a crawl-space vapor barrier
Building code sets the floor, and it treats the barrier as central. Whether the crawl space is vented or sealed, a Class I vapor retarder over the ground is required — the details are the overlap, the up-wall run, and, for a sealed crawl, how the air gets conditioned.
| Requirement | Value | Code |
|---|---|---|
| Vented crawl — free vent area | 1 sq ft per 150 sq ft floor (1:150) | IRC R408.1 |
| Reduced ratio with Class I ground cover | 1 sq ft per 1,500 sq ft (1:1500) | IRC R408.1 |
| Unvented liner — joint overlap | 6 in, sealed or taped | IRC R408.3 |
| Unvented liner — up-wall extension | ≥ 6 in up stem wall, sealed | IRC R408.3 |
| Unvented — dehumidification option | 70 pints/day per 1,000 sq ft | IRC R408.3 |
| Unvented — exhaust option | 1 cfm per 50 sq ft | IRC R408.3 |
| Radon soil-gas retarder | min 6-mil poly, lapped ≥ 12 in | IRC Appendix AF |
Sources: International Residential Code R408.1, R408.3, and Appendix AF (radon).
Two details catch people out. First, radon control (IRC Appendix AF) asks for a 12-inch lap on the ground membrane — stricter than the moisture code’s 6-inch lap — so if radon is on the table, spec the tighter lap. Second, a sealed crawl space isn’t “just seal the vents”: code requires the air to be conditioned, and the common path is a dehumidifier sized to at least 70 pints/day per 1,000 sq ft. A barrier with no humidity control isn’t a code-compliant closed crawl space — it’s half of one.
6Coastal & high-water-table spec: why thickness isn’t the answer
On the Cape Fear coast and anywhere with a high water table, the moisture problem isn’t mainly coming up through the ground — it’s the humid outside air pouring through open vents. No mil of liner seals that. The spec question shifts from “how thick?” to “vented or sealed?”
The landmark data here is a 2005 North Carolina field study by Advanced Energy and the U.S. Department of Energy: across twelve identical homes, vented crawl spaces exceeded 80% relative humidity through spring and summer, while sealed crawl spaces held below 65% — some under 60% all summer — and the sealed homes used 15–18% less heating and cooling energy. That study is what drove North Carolina’s closed-crawl code. On the coast, the takeaway for spec is blunt: buy the reinforced liner for durability, but recognize that a bare barrier over an open, vented crawl space leaves the air half-uncontrolled. The fix is an unvented Class I liner plus dehumidification — and, where the lot takes on water, drainage and a sump pump under the barrier first.
This is also where a musty smell or visible growth changes the spec: if there’s active mold to remediate, that’s handled before the liner goes down, and the whole case for sealing (not just covering) gets stronger. Whether the full system pencils out is its own question — the worth-it math comes down to what the crawl space actually needs, which an on-site look settles.
The component-by-component cost guide breaks down where the money goes — barrier, vent sealing, dehumidifier, and any drainage or mold work, each on its own line.
Same permeance ceiling for ASTM Class A, B, and C
Class A puncture minimum — vs 475 g for Class C
A 15-mil reinforced liner — already far under the ceiling
Vented vs sealed crawl-space summer humidity (NC study)
Vent area a Class I ground cover lets you cut, tenfold
Code dehumidification per 1,000 sq ft for a sealed crawl
Sources: ASTM E1745-17 · IRC R408 · Advanced Energy / U.S. DOE (2005)
7Spec picker: match the liner to your crawl space
Answer two questions — how the space gets used, and how humid it runs — and this returns the spec the data points to. It’s a starting point for the conversation, not a substitute for an on-site look.
Every option is a Class I vapor retarder — the difference is toughness and the system around it, not moisture-blocking.
- 1
Treat the class as a toughness grade
Every crawl-space liner is Class I on permeance — moisture-blocking is a given. ASTM Class A/B/C only tells you how tough it is.
- 2
Match the mil to the traffic
6-mil for a bare, leave-alone ground cover; 12–20 mil reinforced for a crawl space anyone will enter and service. Thickness buys durability, not dryness.
- 3
Humid or musty? Seal and dry, don’t just cover
Seal the vents and add a dehumidifier (code: ≥70 pints/day per 1,000 sq ft). A bare barrier over open vents leaves the air uncontrolled.
- 4
Coastal or high water table? Add water management first
On the Cape Fear coast, an unvented Class I liner plus drainage and a sump pump where the lot takes on water — installed before the barrier goes down.
Framework built from ASTM E1745, IRC R408, and the Advanced Energy / DOE field study.
How we sourced this
Every figure on this page traces to a primary standard, building code, national-lab field study, or manufacturer data sheet — not to other blogs. The class and permeance numbers come from ASTM E1745-17 and the International Residential Code (R408.1, R408.3, Appendix AF). The per-mil permeance and puncture values come from published Stego and W.R. Meadows PERMINATOR data sheets. The vented-versus-sealed humidity and energy figures come from the 2005 Advanced Energy / U.S. Department of Energy field study of twelve matched homes in the humid Southeast. Two per-mil PERMINATOR values (10-mil and 15-mil permeance) are drawn from single manufacturer sources and are directionally consistent with the verified 20-mil figure. Reviewed July 2026; we refresh on the next IRC code cycle and when a cited data sheet is revised.
Frequently asked questions
What mil vapor barrier do I need for a crawl space?
Code-minimum is 6-mil polyethylene, which already qualifies as a Class I vapor retarder (about 0.04–0.06 perms). But mil governs durability, not moisture-blocking: a reinforced 12–20 mil liner survives the foot traffic and pier-wrapping a real crawl space sees, where 6-mil single-sheet tears. For a crawl space anyone will service, 12–20 mil reinforced is the practical spec.
Does a thicker vapor barrier block more moisture?
Barely — and not in a way that matters. Every Class I liner sits at or below 0.1 perms; 6-mil poly is already about 0.04–0.06, a 15-mil at 0.0086, a 20-mil at 0.005. All are far below the ceiling. The real difference thickness buys is puncture resistance (ASTM E1745 Class C is ≥ 475 g, Class A ≥ 2,200 g) and tear strength — durability, not dryness.
Is 6-mil plastic enough for a crawl space vapor barrier?
For meeting code, yes: 6-mil polyethylene is a Class I vapor retarder and satisfies the IRC ground-cover requirement, and it's also the radon soil-gas-retarder minimum. Its weakness is physical — single-sheet 6-mil isn't scrim-reinforced, so it punctures and slides under traffic. It's acceptable as a bare vented-crawl ground cover but under-specced for an encapsulation you'll walk on.
What is a Class I vapor retarder?
A Class I vapor retarder is a material rated at 0.1 perms or less — the tightest class the IRC defines. Polyethylene sheeting qualifies. Class II is > 0.1 to 1.0 perm (kraft-faced batt), Class III is > 1.0 to 10 perm (latex paint). The IRC's crawl-space rules call specifically for a Class I retarder over the ground, which is why crawl-space liners are poly.
What does ASTM E1745 Class A, B, or C mean?
ASTM E1745 is the material standard for below-grade plastic vapor retarders. All three classes must hit the same 0.1-perm permeance ceiling; they differ only in strength. Class A needs ≥ 45 lbf/in tensile and ≥ 2,200 g puncture, Class B ≥ 30 lbf/in and ≥ 1,700 g, Class C ≥ 13.6 lbf/in and ≥ 475 g. So a class rating is really a toughness rating, not a moisture rating.
Does building code require a vapor barrier in a crawl space?
Yes. For a vented crawl space, IRC R408.1 requires a Class I vapor-retarder ground cover (and covering the ground drops the required vent area from 1:150 to 1:1500). For an unvented crawl space, R408.3 requires a continuous Class I retarder with joints lapped 6 in and sealed and edges run 6 in up the wall — plus conditioning of the air.
What mil vapor barrier for a coastal or high-humidity crawl space?
Going thicker won't fix humid air — that's the key spec insight on the coast. A North Carolina field study found vented crawl spaces ran above 80% relative humidity spring through summer, while sealed ones held below 65%. On the Cape Fear coast, the answer isn't a heavier liner, it's an unvented Class I liner plus code-level dehumidification (70 pints/day per 1,000 sq ft). Thickness protects the liner; sealing and drying protect the house.
Vapor barrier or full encapsulation — which does the spec point to?
The material spec is the same either way (a Class I, reinforced liner); what differs is the system around it. If your crawl space is genuinely dry, a sealed reinforced barrier can stand alone. If it's humid, musty, or has had mold — the common coastal case, where sealed crawl spaces held below 65% RH versus over 80% vented — the barrier needs sealed vents and a dehumidifier to actually control the air. An on-site look settles which.
Sources
- ASTM International. “E1745-17, Standard Specification for Plastic Water Vapor Retarders Used in Contact with Soil or Granular Fill under Concrete Slabs.” (Class A/B/C tensile, puncture, permeance.) Accessed 2026-07-19.
- Stego Industries. “What Is ASTM E1745,” and Stego Wrap 15-mil Vapor Barrier Data Sheet. Accessed 2026-07-19.
- W.R. Meadows. PERMINATOR 20-mil, 15-mil, and 10-mil Underslab Vapor Barrier data sheets. Accessed 2026-07-19.
- Insulation Institute (NAIMA). “Moisture Control: Utilizing Vapor Retarders” (Class I/II/III perm ranges). Accessed 2026-07-19.
- International Code Council. International Residential Code, R408.1, R408.3, and Appendix AF (Radon Control Methods). Accessed 2026-07-19.
- Davis, B., Dastur, C., et al. “A Field Study Comparison of the Energy and Moisture Performance Characteristics of Ventilated Versus Sealed Crawl Spaces in the South.” North Carolina Advanced Energy Corporation / U.S. DOE, 2005 (OSTI 850459). Accessed 2026-07-19.
- Advanced Energy. “Taking the Vents Out of Vented Crawl Spaces” (15–18% energy savings). Accessed 2026-07-19.
Last updated: July 19, 2026
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