Vapour Barrier Calculator
Calculate how many poly vapour barrier rolls you need for a concrete slab, with overlap allowance and optional material cost.
Vapour Barrier
Poly vapour barrier / moisture protection
Inputs
Results
How to Use the Vapour Barrier Calculator
Enter the slab length and width, the roll width and length (from the product label), and your required overlap distance. The calculator computes the net coverage per roll after overlap deduction, total rolls needed, and optional material cost.
How It Calculates
Runs = ceil(Slab Width / Effective Roll Width)
Length per Run = Slab Length + Overlap (end laps)
Total Area Covered = Runs × Effective Roll Width × Slab Length
Rolls = ceil(Total LF Required / Roll Length)
What to Know Before You Calculate
- Roll dimensions vary by supplier: Common roll widths are 10 ft, 12 ft, and 16 ft. Roll lengths are typically 100–200 ft. Enter the actual roll dimensions from the product spec.
- Tape all seams: This calculator estimates material area, but seams must be taped. Budget for vapour barrier tape — typically 1 roll per 200–300 lf of seam.
- Wall upturns: Vapour barrier should turn up the wall 6–12 inches. Add perimeter linear footage × upturn height to your total area calculation.
- Don't walk on it excessively: Once placed, minimize foot traffic on the vapour barrier. Punctures must be patched with tape before concrete is placed.
Common Mistakes
- Not accounting for wall upturns. The base calculation covers the flat floor area only. Upturns at walls require extra material — commonly 5–8% additional.
- Using torn or punctured material. Rebar, sharp aggregate, and foot traffic puncture poly. Inspect and patch before the pour.
- Forgetting seam tape. Overlapping without taping does not provide a continuous barrier. All seams and penetrations must be sealed.
- Under-ordering to save money. Running short mid-install means splicing at an awkward location. Order 10% extra — poly is inexpensive relative to the risk.
Frequently Asked Questions
What thickness of vapour barrier is required under a concrete slab?
Most building codes require a minimum 6-mil (0.006 inch / 0.15 mm) polyethylene vapour barrier under concrete slabs on grade. Many residential builders use 10-mil for added durability during concrete placement. Industrial applications may require 15–20 mil.
How much overlap is required for vapour barrier seams?
Building codes typically require a minimum 6-inch (150 mm) overlap at seams. Many contractors use 12 inches for added protection. Seams should also be taped with a compatible vapour barrier tape to maintain a continuous barrier.
Should the vapour barrier go above or below the rigid foam insulation?
In most slab-on-grade assemblies, the vapour barrier goes on top of the gravel subbase, below the rigid foam insulation. Some energy codes require the vapour barrier directly under the concrete (above insulation) for optimal moisture control. Follow your local code and engineer's specification.
Do I need a vapour barrier under an interior concrete floor?
Yes, virtually all concrete floors on grade require a vapour barrier to prevent ground moisture from migrating through the slab. Without it, moisture causes flooring failures, efflorescence, and mold. Only concrete placed above grade (on elevated decks or suspended slabs) typically does not require a sub-slab vapour barrier.
Related Calculators
- Concrete Slab Calculator
- Rigid Foam Insulation Calculator
- Gravel Subbase Calculator
- Wire Mesh Calculator
Disclaimer: Vapour barrier quantities are estimates. Material requirements vary by roll dimensions and site conditions. Confirm minimum thickness and installation requirements with your local building code before installation.
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