Vapor Barrier Calculator

A vapor barrier blocks moisture from migrating through building assemblies where it causes rot, mould, and structural damage. The Vapor Barrier Calculator estimates poly sheeting, seam tape, and fasteners for the four most common residential applications: crawl space floors, crawl space full encapsulation, under-slab installations, and wall cavities.

Ground moisture is the primary driver for crawl space and under-slab barriers. Soil holds moisture year-round, and that moisture evaporates upward through any uncovered surface. An uncovered crawl space floor can release 10–15 gallons of water vapour per day into the space beneath your house. That moisture condenses on cold surfaces — floor joists, subfloor sheathing, ductwork, plumbing — creating the conditions for mould growth, wood rot, and insect damage. Covering the crawl space floor with poly sheeting reduces moisture emission by 90–95%.

Under-slab barriers serve the same purpose but with an additional structural consideration. Water vapour that passes through concrete reacts with floor coatings (epoxy, paint), lifts adhesive under tile and VCT, and damages laminate and hardwood flooring installed over the slab. ASTM E1745 classifies under-slab vapor retarders by three performance levels (A, B, C) based on puncture resistance and water vapour permeance. Class A (10-mil or thicker) is required for occupied spaces; Class C (6-mil minimum) is acceptable for non-critical applications.

Wall cavity barriers prevent warm, humid interior air from reaching the cold exterior sheathing during winter, where it condenses and wets the structural framing. In cold climates (zones 5–7), the IRC requires a vapor retarder on the warm side of exterior walls. In mixed climates (zones 3–4), the requirement depends on the wall assembly. In hot-humid climates (zones 1–2), the vapor retarder goes on the outside because the moisture drive is inward. Getting the placement wrong traps moisture inside the wall assembly and accelerates rot. Check the insulation requirement calculator to match your climate zone before placing a wall vapor barrier.

The table below matches poly thickness to application. Thicker poly costs more but resists punctures from foot traffic, gravel, and construction debris.

Application	Minimum Thickness	Recommended Thickness	Code Reference
Wall cavity (warm side)	6 mil	6 mil	IRC R702.7
Under-slab (non-habitable)	6 mil	10 mil	IRC R506.2.3
Under-slab (habitable)	10 mil (ASTM E1745 Class A)	15 mil	ASTM E1745
Crawl space floor (vented)	6 mil	10 mil	IRC R408.3
Crawl space floor (encapsulated)	6 mil	20 mil	IRC R408.3.2
Crawl space walls (encapsulation)	6 mil	20 mil	IRC R408.3.2

Crawl space encapsulation — covering both the floor and walls with continuous poly, sealed at all seams and penetrations — requires the heaviest material because people walk on it during inspections and maintenance. A 6-mil poly in a crawl space tears within a year of regular access. A 20-mil product like Stego Wrap or Americover Premium survives decades of foot traffic, tool dragging, and HVAC technician visits.

For under-slab installations, the poly must survive the concrete pour: workers walking on it, rebar chairs poking through, wheelbarrow traffic, and the weight of 4–6 inches of wet concrete. A 10-mil product handles this if the gravel base is properly graded (no sharp rocks protruding). For guaranteed protection, 15-mil Class A is the professional standard.

Wall cavity barriers are always 6-mil because they face no mechanical stress after installation. The poly gets stapled to the stud faces before drywall goes up and is never touched again. Thicker poly in wall cavities is wasteful and harder to work with — it does not conform to stud bay corners as easily.

Crawl space vapor barrier installation is straightforward but physically demanding. You are working in a confined space with limited headroom, often on your back or hands and knees. Plan the job when the crawl space is dry — installing poly on wet or standing-water ground just traps the water and creates a worse problem.

1. Clear the crawl space. Remove debris, rocks, old insulation scraps, and anything that could puncture the poly from below. Grade the soil flat if there are humps or depressions — a smooth surface extends the poly lifespan and prevents standing water puddles under the barrier.

2. Roll out the poly from the far end. Start at the point farthest from the access opening and roll toward the exit. Unrolling in this direction means you are always working on clean poly, not tracking dirt across previously covered areas. Run the poly up the foundation walls at least 6 inches above grade (12 inches for full encapsulation). Hold the wall overlap in place with mechanical fasteners or butyl tape.

3. Overlap seams by 6–12 inches. Adjacent sheets must overlap to create a continuous barrier. Six inches is code minimum; 12 inches is recommended for crawl spaces where foot traffic can shift the material. Seal every overlap with butyl seam tape pressed firmly along the full length — no gaps, no skipped sections.

4. Seal around penetrations. Pipes, conduits, and support piers that pass through the poly must be sealed individually. Cut the poly to fit around the penetration, then seal the cut edge with butyl tape or mastic. A single unsealed penetration can admit enough moisture to undermine the entire barrier.

5. Secure the wall overlap. For full encapsulation, attach the poly to the foundation wall using mechanical termination bars — horizontal strips screwed into the concrete or block wall that clamp the poly edge. Butyl adhesive alone may peel off concrete over time, especially in humid conditions. Termination bars provide a permanent mechanical hold.

6. Install a dehumidifier in encapsulated crawl spaces. Sealing the crawl space traps whatever moisture is already inside. Without active dehumidification, humidity can rise above 60% and promote the mould growth you were trying to prevent. A commercial-grade crawl space dehumidifier (sized for the volume) maintains 50–55% relative humidity year-round. Drain the condensate to a sump pump or gravity drain.

For crawl spaces with spray foam insulation on the rim joists, the foam serves as both insulation and vapor barrier for the rim joist area. The poly barrier on the ground and walls handles the remaining surfaces.

The terms "vapor barrier" and "vapor retarder" are often used interchangeably, but they describe different levels of moisture resistance. The distinction matters for code compliance and for preventing moisture problems in wall assemblies.

Vapor barriers have a permeance rating of 0.1 perms or less. Polyethylene sheeting (6-mil and thicker) falls in this category at roughly 0.06 perms. Vapor barriers block virtually all moisture vapour from passing through. They are appropriate for crawl space floors, under-slab installations, and the warm side of walls in cold climates where you want to prevent interior moisture from reaching the cold sheathing.

Vapor retarders have a permeance rating between 0.1 and 10 perms. Kraft paper facing on fiberglass batts is a Class II vapor retarder at roughly 0.5 perms. Latex paint on drywall is a Class III retarder at roughly 5–10 perms. Vapor retarders slow moisture movement without blocking it completely, which allows some drying potential through the assembly.

When to use a vapor barrier (poly sheeting):

• Crawl space ground cover (always, regardless of climate)
• Under concrete slabs (always)
• Warm side of exterior walls in climate zones 5–7 (cold climates)
• Marine climate zone 4 (IRC R702.7)

When to use a vapor retarder (kraft facing or paint) instead of poly:

• Exterior walls in climate zones 3–4 (mixed climates). Full poly on the warm side can trap summer moisture driven inward by air conditioning, causing condensation inside the wall.
• Exterior walls in climate zones 1–2 (hot-humid climates). The vapor retarder goes on the exterior side here — the moisture drive is inward. Interior poly in a hot climate traps condensation on the drywall side.
• Any wall assembly with exterior rigid foam insulation. The foam is a vapor retarder on the cold side — adding poly on the warm side creates a double vapor barrier that traps moisture in the cavity.

Getting vapor barrier placement wrong is worse than not having one at all. A misplaced barrier traps moisture inside the wall where it cannot dry in either direction. If you are unsure about your climate zone or wall assembly, the insulation requirement calculator identifies your zone and the associated code requirements, including vapor retarder placement.

Vapor barrier decisions generate more confusion than most building science topics. These are the questions that come up most often during renovation projects.

Can I use Tyvek as a vapor barrier? No. Tyvek (and similar house wraps) are weather-resistant barriers, not vapor barriers. Tyvek has a permeance of roughly 58 perms — it blocks liquid water but allows water vapour to pass through freely. That is by design: house wrap goes on the exterior to shed rain while letting wall cavity moisture escape outward. A poly vapor barrier on the interior warm side serves the opposite function — it prevents interior moisture from entering the cavity.

Do I need a vapor barrier in my basement? For the floor (under a slab or over a dirt floor), yes. For the walls, it depends on how you are finishing them. If you are framing and insulating interior basement walls, a vapor barrier on the warm side of the insulation is standard practice in cold climates. If you are using rigid foam insulation directly against the concrete wall, the foam itself acts as a vapor retarder and no additional poly is needed — in fact, adding poly between the foam and the concrete can trap moisture.

What happens if I overlap seams without tape? Untaped seams allow moisture to pass through the overlap gap. In a crawl space, this can reduce the barrier effectiveness by 30–50% depending on how many seams are exposed and how well they overlap. Tape costs $12 per roll and takes 10 minutes per seam. There is no rational reason to skip it.

Is thicker poly always better? For crawl spaces and under-slab applications, yes — thicker poly resists punctures and lasts longer. For wall cavities, no — 6-mil is the standard and any thicker material is harder to work with in stud bays without adding meaningful benefit. The poly in a wall cavity faces no mechanical stress after the drywall goes up, so puncture resistance is irrelevant.