Vapor barrier

6-mil (0.15 mm) polyethylene plastic sheet as vapour barrier between insulation and gypsum board
Glass wool pipe insulation covering a steel pipe as it penetrates a mock-up concrete slab whose opening will be firestopped. In this manner, the vapor barrier (in this case made of foil/scrim/kraft paper, called ASJ, all-service jacket, aluminium on the inside, white paper on the outside) can remain intact as it penetrates the fire barrier.

A vapor barrier (or vapour barrier) is any material used for damp proofing, typically a plastic or foil sheet, that resists diffusion of moisture through the wall, floor, ceiling, or roof assemblies of buildings and of packaging to prevent interstitial condensation. Technically, many of these materials are only vapor retarders as they have varying degrees of permeability.

Materials have a moisture vapor transmission rate (MVTR) that is established by standard test methods. One common set of units is g/m2·day or g/100in2·day. Permeability can be reported in perms, a measure of the rate of transfer of water vapor through a material (1.0 US perm = 1.0 grain/square-foot·hour·inch of mercury ≈ 57 SI perm = 57 ng/s·m2·Pa). American building codes started classifying vapor retarders in the 2007 IRC supplement. They are Class I <0.1 perm, Class II 0.1 - 1 perm and Class III 1-10 perm when tested in accordance with the ASTM E96 desiccant, dry cup or method A.[1] Vapor-retarding materials are generally categorized as:

  • Class I, Impermeable (<0.1 US perm, or ≤5.7 SI perm) – such as asphalt-backed kraft paper, glass, sheet metal, polyethylene sheet, rubber membrane, vinyl wall coverings;
  • Class II, Semi-impermeable (0.1-1 US perm, or 5.7-57 SI perm) – such as unfaced expanded or extruded polystyrene, OSB, fiber-faced isocyanurate, 30 pound asphalt-impregnated building papers, exterior oil-based paints, unfaced expanded polystyrene, 30 pound asphalt coated paper, plywood, bitumen coated kraft paper;
  • Class III, Semi-permeable (1-10 US perm, or 57-570 SI perm) – such as unfaced expanded polystyrene, fiber-faced isocyanurate, plywood, 15 pound asphalt coated paper, some latex-based paints;
  • Permeable (>10 US perm, or >570 SI perm) – such as unpainted gypsum board and plaster, unfaced fiber glass insulation, cellulose insulation, unpainted stucco, cement sheathings, spunbonded polyolefin (building wraps) or some polymer-based exterior air barrier films.
  1. ^ Smart Vapor Retarders. Certain Teed Corporation. 2006. p. 2.