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Insulation & Efficiency

Vapor Barrier Calculator

Estimate how many rolls of 6-mil poly vapor barrier you need for a crawlspace or basement.

  • Answers as you type
  • Every formula cited
  • Calculated in your browser
SettingsSettings for this calculationUS
Market
Imperial · sales tax
The total crawlspace floor or basement wall area to cover.

Measure the ground area (for a crawlspace vapor barrier) or wall surface area (for a basement wall barrier).

Sheet spent where seams overlap before they are taped.

On a plain rectangular crawlspace the seam laps, taped, use roughly a tenth of a standard roll. Wider overlaps, narrower rolls, or a footprint that forces many short strips take more. Sheet turned up the walls and wrapped round piers is extra area rather than overlap, so measure it and add it to the area.

Vapor barrier rolls needed

2 rolls

High confidence
Area to cover (with overlap allowance)
1,188 sq ft
Then change the inputs to see how far the answer moves.

Show calculation logic

How this was calculated

Formula source(s)

  • A standard 6-mil poly vapor barrier roll (10 ft x 100 ft) covers approximately 1,000 sq ft

Inputs used

Floor or Wall Area
1080 sq ft
Seam Overlap Allowance (%)
10

Intermediate steps

Area to cover (with overlap allowance)
1,188 sq ft
Final result2 rolls

What this calculation does not cover

  • The roll is fixed at a 10 ft x 100 ft (3 m x 30 m), 1,000 sq ft (93 m²) sheet. The calculator never asks what you are buying, so a 12, 16 or 20 ft wide roll, a 50 ft roll, or a reinforced 10-20 mil barrier will not divide into this count. Take the area figure from the breakdown and divide it by your own product's stated coverage.
  • The overlap allowance is for flat-plane seams only. Nothing is added for turning the sheet up the foundation wall, wrapping piers and columns, sealing around penetrations, or the off-cuts an irregular crawlspace footprint produces. Measure and add those separately.
  • Only the sheet is counted. Seam tape, mastic, mechanical fasteners and termination bar are not in this estimate.
  • This is a quantity take-off, not a vapour-control design. It says nothing about the permeance, thickness or puncture class the barrier has to meet, or which face of the insulation it belongs on. In a cold store or an unvented crawlspace the wrong side traps moisture inside the assembly no matter how many rolls you order.
  • It is not a radon or ground-gas membrane specification. Those are designed, jointed and verified systems with their own material, welding and testing requirements, and a 6-mil poly roll count does not substitute for one.

Estimated cost — your price

This site holds no price list for this material — local prices vary too much to publish honestly. Enter your supplier's price and the result is costed with it.

32.86 ft10.02 m32.86 ft10.02 mequivalent area1,080 sq ft100.34 m²10 ft5 m

Part of bigger jobs

This trade is one line of several job takeoffs. Run the whole job and every other trade comes back with it, off the same measurements.

Or plan the space itself: measure it once, doors and windows included, and this figure comes back worked out on that space, with what goes with it.

Computed in your browser — nothing you enter is uploaded. Presented in US customary units and US trade terminology. Where a formula follows a published standard, that standard and its edition are cited beside it on this page; where none governs, the page says so. Local amendments override model codes — verify against the code in force where you build.

Sources checked 2026-09-17 · v1.1.0

Regulatory standards & verification citations1
  1. A standard 6-mil poly vapor barrier roll (10 ft x 100 ft) covers approximately 1,000 sq ft
Cite this page

Your workspace

Most jobs need more than one number. Add the calculators you need next and they open right here, underneath this one — your figures stay on screen and nothing is lost to a page change.

Tools and safety for this job

To fix a vapour retarder. Generic types, no brands, no prices.

Protection this work requires

  • Most fatal falls on small jobs are from under three metres: use a tower or a ladder tied at the top, and never work off the top two rungs.
  • Knives cause more site injuries than any power tool: cut away from your body, change blades often, and wear cut-resistant gloves to EN 388 level C for repeated cutting.
Show the 4 tools this job needs

Essential

  • Staple gun

  • Tape measure

  • Utility knife

Recommended

  • Step ladder

Also needed as materials: replacement blades, staples.

what each insulation tool is for, and the spec that decides which to buy where one does.

Now that you have the number

These guides cover the work this quantity is for — the first ones run this calculator inside the section that raises the question.

  • Building the Floor of a Cold Store So It Does Not Heaveuses this calculator

    The frost front under a freezer never retreats, so the insulation, the vapour barrier and the sub-floor heating grid all have to be right in one pour.

  • Converting an Integral Garage Into a Roomuses this calculator

    Building up from a slab poured to drain: the floor stack, the infill under the old lintel, and holding the damp line unbroken across the threshold.

  • Draining a Wet Basementuses this calculator

    Wet basements are a pressure problem: read the head, relieve it at the slab and footing, and pump only what the relief path actually delivers.

Called something else where you work? Damp-proof course / membrane · Damp-proof membrane under a floor · Vapour control layer — the term in each market, how close the equivalence really is, and the standard that governs it.

Still deciding? Vapour Barrier vs Air Sealing · Crawl Space Encapsulation vs Ground Barrier — the factors that actually differ, with no invented prices.

Already have some? How big a crawlspace will my vapor barrier rolls cover?

Worked examples: Flat roof recovered — step 8 of 8 · Garden room slab — step 4 of 9

The data behind it: Production rates by operation

How to calculate vapor barrier in 3 steps

  1. Floor or Wall AreaThe total crawlspace floor or basement wall area to cover.
  2. Seam Overlap Allowance (%)Sheet spent where seams overlap before they are taped.
  3. Vapor barrier rolls neededThe tool computes the vapor barrier rolls needed from those figures and shows the formula, its sources, and a confidence rating alongside it.

Vapor barrier rolls needed by floor or wall area

Page defaults, not your figures above.

Floor or Wall AreaVapor barrier rolls needed (rolls)
500 sq ft1
1,000 sq ft2
1,500 sq ft2
2,000 sq ft3

Frequently asked questions

Why is this measured as an area rather than a perimeter?
Because it is fighting DIFFUSION, which happens across every square metre of the surface at once. Water vapour moves from warm and humid towards cold and dry through the whole plane of a floor or a wall, driven by the pressure difference across it, and the material's job is to present a low permeance over that entire plane. That is why continuity is the property that matters and why a lap that is not taped is not a small defect: vapour finds the gap the way water finds a hole, and a barrier with a 2% opening performs far worse than 98% of a whole one. It is a fundamentally different problem from a thermal edge detail, which fights CONDUCTION along a line and is quantified by running metre.
Why does a crawlspace need a vapor barrier at all?
Bare soil constantly releases moisture into the crawlspace air, which can lead to wood rot, mold, and higher humidity in the living space above — a vapor barrier blocks that moisture at the source.
Do seams need to overlap?
Yes — seams are typically overlapped at least 6-12 in and often taped, which is what the seam overlap allowance in this estimate accounts for, 10% unless you change it.
Is 6-mil poly the standard thickness?
6-mil is a common standard for crawlspace vapor barriers, though some installers use heavier 10-20 mil reinforced barriers for better puncture resistance in high-traffic crawlspaces — coverage area math is the same regardless of thickness.
Preliminary estimate, not certified engineering. This tool produces an indicative quantity calculation for planning purposes only — it is not a certified structural analysis, a guaranteed material takeoff, or a substitute for building department approval. Always verify measurements on-site and have a licensed contractor or structural engineer review any load-bearing, code-sensitive, or safety-critical work before purchasing materials or starting construction. Spotted an arithmetic or standards error? Report it to contact@craftquantities.com with your inputs — a confirmed fix gets a permanent check of its own, so the same mistake cannot come back.