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 have classified vapor retarders as having a water vapor permeance of 1 perm or less when tested in accordance with the ASTM E96 desiccant, or dry cup method. Vapor-retarding materials are generally categorized as: Impermeable (≤1 US perm, or ≤57 SI perm) – such as asphalt-backed kraft paper, elastomeric coating, vapor-retarding paint, oil-based paints, vinyl wall coverings, extruded polystyrene, plywood, OSB; Semi-permeable (1-10 US perm, or 57-570 SI perm) – such as unfaced expanded polystyrene, fiber-faced isocyanurate, heavy asphalt-impregnated building papers, 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 or some polymer-based exterior air barrier films.
Materials
Vapor diffusion retarders are normally available as coatings or membranes. The membranes are technically flexible and thin materials, but sometime includes thicker sheet materials named as "structural" vapor diffusion retarders. The vapor diffusion retarders varies from all kinds of materials and keep updating every day, some of them nowadays even combined the functions of other building materials.
Materials used as vapor retarders: Elastomeric coatings can provide a vapor barrier and water proofing with permeability ratings of .016 perm rating with 10 mils/min. of coating and can be applied on interior or exterior surfaces. Aluminum foil, 0.05 US perm (2.9 SI perm). Paper-backed aluminum. Asphalt or coal tar pitch, typically hot-applied to concrete roof decks along with reinforcement felts. Polyethylene plastic sheet, , 0.03 US perm (1.7 SI perm). Advanced Polyethylene vapor retarders that pass the ASTM E 1745 standard tests ≤0.3 US perm (17 SI perm). Asphalt-coated kraft paper, often attached to one side of fiberglass batts, 0.40 US perm (22 SI perm). Metallized film Vapor retarder paints (for the air-tight drywall system, for retrofits where finished walls and ceilings will not be replaced, or for dry basements: can break down over time due to being chemically based). Extruded polystyrene or foil-faced foam board insulation. Exterior grade plywood, 0.70 US perm (40 SI perm). Most sheet type monolithic roofing membranes. Glass and metal sheets (such as in doors and windows).
Building construction
Moisture or water vapor moves into building cavities in three ways: 1) With air currents, 2) By diffusion through materials, 3) By heat transfer. Of these three, air movement accounts for more than 98% of all water vapor movement in building cavities. A vapor retarder and an air barrier serve to reduce this problem, but are not necessarily interchangeable.
Vapor retarders slow the rate of vapor diffusion into the thermal envelope of a structure. Other wetting mechanisms, such as wind-borne rain, capillary wicking of ground moisture, air transport (infiltration), are equally important.