Vapor Barrier Calculator — ft, rolls, cost
How to use this calculator
Pick the application — crawlspace floor, interior wall, under-slab, or a custom area. Choose poly thickness (6-20 mil) and roll size, then set seam overlap (6-12″) and a waste allowance for trimming around piers, ducts, and penetrations.
Rolls drive the order quantity; the area shown with overlap is what actually gets unrolled on site. Seam tape and acoustic sealant are the difference between a working Class I retarder and a barrier that fails the next time the dew point swings.
Saved Calculations
| Time | Application | Area ft² | Rolls | Cost |
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How to Calculate Vapor Barrier
Measure the surface to cover — crawlspace floor, interior wall, or under-slab. For crawlspaces, add the wall run-up height so the polyethylene wraps a short distance up the foundation walls and is sealed to them. The estimator converts that area into rolls after accounting for seam overlap losses and a trimming waste allowance. Under-slab membrane works alongside the below-grade waterproofing estimate and the gravel/sand bed beneath the slab.
Formulas
Base area = length × width. Crawlspace total = floor area + perimeter × run-up height. Overlap factor = roll width ÷ (roll width − overlap), since each strip loses its overlap to the adjacent strip. Adjusted area = base area × overlap factor × (1 + waste%). Rolls = ⌈adjusted area ÷ roll coverage⌉. Seam length drives the tape quantity; perimeter drives the acoustic sealant tubes.
Membrane Thickness Guide
6 mil — standard for most crawlspaces and under-slab (ASTM E1745 Class C, IRC R506.2.3). 10 mil — heavy-duty for high-traffic crawlspaces (Class B). 15 mil — premium, puncture-resistant (Class A). 20 mil — commercial grade, reinforced. A poly vapor barrier controls moisture but is not air or thermal insulation — pair it with wall batt or board insulation, an air-sealing spray foam layer, exterior house wrap, and the right attic/roof R-value for the assembly to perform.
FAQ
How much overlap? Minimum 6″ for floor applications and 12″ for wall seams per IRC R506.2.3; all overlaps are taped. Do I tape the seams? Yes — every seam and any tears or penetrations get approved vapor-barrier tape. Do I need sealant? Apply acoustic sealant at all perimeter edges where the membrane meets foundation walls, piers, columns, and pipe penetrations. Which side faces the conditioned space? In cold climates the barrier goes on the warm-in-winter side; under slabs it sits directly beneath the concrete on the gravel base. For finished floors over a crawlspace, coordinate with the floor covering takeoff.
On install day
Budget $0.08–0.20/ft² for 6 mil poly and $0.30–0.55/ft² for 10–15 mil reinforced membrane (StegoWrap, Viper, Husky) in 2025; encapsulated crawl jobs run $3–7/ft² installed once tape, mechanical fasteners, and termination strips are added. The single most common takeoff mistake is forgetting that overlap is a loss, not just a detail — a 1,200 ft² crawl with 12″ overlaps on a 10 ft roll actually consumes ~1,330 ft² before waste. For under-slab pours per ACI 302.2R and IRC R506.2.3 always order a Class A or B membrane (10–15 mil reinforced) — the 6 mil poly that survives an unheated shed pad will be shredded by rebar chairs and a power trowel on a finished basement. Tape every seam, every penetration, and pre-cut boots for plumbing before the concrete crew arrives.
