Insulation & Energy
Vapour Barrier Calculator
Calculates rolls of 6-mil poly vapour barrier for walls and ceilings
Updated June 26, 2026 · Live
What this tool does
Calculates the number of rolls of 6-mil polyethylene vapour barrier needed for a wall or ceiling, from the area, roll coverage and a lap allowance.
Formula Used
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How the vapour barrier calculator works
This calculator takes the wall or ceiling area, adds a percentage for lapping sheets over the framing and sealing around penetrations, divides by the area one roll covers, and rounds up to whole rolls of 6-mil polyethylene.
Where the vapour barrier goes
In a cold Canadian climate the vapour barrier goes on the warm (interior) side of the insulation, behind the drywall. It slows interior moisture from reaching the cold sheathing where it would condense. Seal the laps, and seal around boxes, pipes and wires with sheathing tape (tuck tape) or acoustical sealant.
Poly grades and air-barrier details
Six-mil polyethylene is the common choice and doubles as the air barrier when its laps and edges are sealed. Some assemblies use a smart membrane instead to allow drying. Follow NBC 9.25 and your provincial code for the correct placement and permeance.
What this tool does not do
It estimates poly quantity only. It does not select the membrane type for your assembly, design the air barrier continuity, or assess the dew point — those depend on your wall build-up and climate zone.
Adjusting the defaults
Every input is editable. Enter your own area, roll coverage and lap allowance and the roll count and cost recalculate instantly.
On-site considerations
Lap sheets over a stud or plate, never in a bay, and seal the lap so the poly is continuous as both a vapour and air barrier. Detail the tricky spots — electrical boxes, plumbing penetrations, the rim joist and around windows — with sealant or a poly box, because those gaps leak the most air. Do not double up vapour barriers in a way that traps moisture.
Using this alongside other BuildMetricLab tools
Pair this with the wall insulation, attic insulation and drywall calculators when finishing a wall or ceiling, and with the material cost calculator to price the poly and tape. Every BuildMetricLab tool runs in your browser — no sign-up, no data sent anywhere, and every formula is shown so you can audit the math.
Sources & methodology
Calculates poly rolls as the area plus a lap/waste allowance, divided by the coverage per roll and rounded up. Every result is calculated from the values you enter, and all inputs are editable.
Frequently asked questions
Are waterproofing membrane calculator results accurate enough to order materials?
Use them as a starting estimate only. Verifying the final quantity with your supplier or contractor before ordering is good practice — site conditions, wastage and cut-offs all affect the true figure.
What wastage percentage should I use?
The calculator defaults to the typical Canadian trade allowance for bathrooms. Increase it for complex cuts, awkward shapes, or first-time DIY. The default wastage allowance reflects common trade practice; values lower than the default may underestimate offcuts.
Does this replace professional advice?
No. This tool is a planning estimator. For works that affect structure, the National Building Code (NBC), property-line or shared-wall, gas, electrics, drainage to a sewer, or similar, consult a suitably qualified professional.
Can I change the unit prices?
Yes — every price field is editable. Plug in your supplier's quote to get a total that matches your project.
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