Permeability Converter

In building science, "permeability" almost always refers to water vapor permeance — how readily water vapor passes through a material for a given difference in vapor pressure across it. This property determines whether a wall, roof, or packaging material will trap moisture (leading to condensation and mold) or let it escape safely, making it one of the most important specifications for vapor barriers, house wraps, insulation, and moisture-control membranes.

Our Permeability converter supports the metric (SI) perm — kilogram/pascal/second/square meter — alongside the traditional US customary "perm," historically defined as 1 grain of water vapor passing through 1 square foot of material per hour for each inch of mercury (inHg) of vapor pressure difference. Because US perm ratings are measured using two different ASTM E96 test procedures — the "dry cup" method (roughly corresponding to the 0°C reference values in this converter) and the "wet cup" method (the 23°C reference values) — the tool keeps these as separate units rather than treating all "perms" as interchangeable, since a material's actual permeance can shift with humidity.

The converter also includes "perm-inch" variants at both reference conditions. A plain perm rating describes a specific sample's permeance at whatever thickness it happens to have, while a perm-inch normalizes that value to a standard 1-inch thickness — giving a true material property (permeability) that lets you fairly compare, say, a 2-inch-thick insulation board against a thin plastic sheet, independent of how thick either sample actually was.

To use the converter, select your starting and target units from the dropdown lists and enter a value — the result is calculated instantly to up to 12 significant digits. This is especially useful when a US building product datasheet reports permeance in perms, but your project specification or a European standard calls for the metric SI perm value.

Vapor permeance ratings are central to building code classifications of vapor retarders (in the US, materials are grouped into Class I, II, and III vapor retarders based on their perm rating, which determines where in a wall assembly they may be safely installed), moisture control design (choosing a housewrap or barrier with the right permeance prevents both moisture intrusion and trapped condensation), and packaging engineering (moisture-barrier films for food and pharmaceutical packaging are specified by water vapor transmission rate, a closely related quantity).

All conversion factors used in this tool are drawn from the standard definitions used in ASTM E96 and related building science references, so results are dependable for material specification, building envelope design, and code compliance work. The converter is free, requires no account, and works equally well on desktop and mobile devices.

Because US building codes, manufacturer datasheets, and international standards each tend to favor a different one of these perm variants, this converter is especially useful when comparing a domestic product's rating against an imported material's specification, or when checking that a chosen vapor retarder actually meets the perm-class threshold required by a local building code.