Vapor Barrier Film Calculator
Written by Thierno Sadou Diallo, formula verified per our methodology • Last checked on 10/10/2026
The number of vapor barrier rolls is calculated by increasing the surface to cover by an overlap between sheets (usually 10 to 15%), then dividing by one roll's area. For 120 m² with a 10% overlap and a 4 × 25 m roll (100 m²), you need 2 rolls.
Explanation
A vapor barrier is a plastic film installed to block water vapor from migrating through an insulated assembly (wall, ceiling, floor), to prevent it from condensing inside the insulation and degrading its performance over time. For this barrier to stay continuous and effective, each sheet of film must overlap the previous one by a sufficient width (generally 10 to 15%, sometimes more depending on the manufacturer's instructions), with that overlap then taped with a specific adhesive to ensure the assembly's airtightness. Unlike a wallpaper roll, whose standard European size (10.05 m × 0.53 m) is standardized, vapor barrier film rolls on the market vary greatly in width (commonly 2, 4, 6, or 8 m) and in length depending on the manufacturer and intended use (ceiling, wall, slab): this calculator therefore asks for the exact dimensions of the chosen roll rather than assuming a universal size, to avoid a wrong calculation if the product actually purchased differs. A vapor barrier differs from a vapor retarder (more permeable, letting a small, controlled amount of vapor through): the choice between the two depends on the insulation's thermal resistance and the climate, a decision generally made with a professional or according to applicable thermal regulations. Good airtightness achieved with this vapor barrier must also be balanced by sufficient ventilation of the home, or indoor moisture gets trapped: see our air change rate calculator for that second, inseparable aspect of good airtightness.
Example: 120 m² to cover with a 4 × 25 m roll
Inputs
Surface to cover: 120 m². Overlap: 10%. Roll: 4 m × 25 m.
Calculation
Surface with overlap = 120 × 1.10 = 132 m². Roll area = 4 × 25 = 100 m². Rolls needed = ceiling(132 ÷ 100) = 2.
Result
You need to order 2 rolls of this size to cover this surface with the planned overlap.
Frequently asked questions
What's the difference between a vapor barrier and a vapor retarder?
A vapor barrier is nearly impermeable to water vapor, while a vapor retarder lets a small, controlled amount through (described by its Sd value, expressed in meters, which measures its resistance to vapor diffusion). The choice between the two depends on the exact makeup of the insulated assembly and the local climate: a professional or the insulation's technical sheet usually specifies which is suited to your situation.
Why does the overlap between sheets matter so much?
Because a vapor barrier is only effective if it forms a truly continuous barrier, with no gaps: insufficient overlap between two sheets, or poorly applied joint tape, creates a path for water vapor that can then condense locally in the insulation at that exact spot, creating a risk of targeted moisture damage rather than damage spread evenly across the whole surface.
Why doesn't this calculator offer a fixed default roll size?
Because, unlike a wallpaper roll whose European size is standardized, vapor barrier film rolls really do vary a lot in width and length depending on the manufacturer and intended use: setting a default size would risk a wrong calculation if the product actually purchased has different dimensions. Always check the width and length printed on the roll's packaging before entering them into this calculator.