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Introduction to Vapour Barriers |
A vapor barrier (or vapour barrier) is often used to refer to any material, typically a plastic or foil sheet, that resists diffusion of moisture through wall, ceiling and floor assemblies of buildings and of packaging. Technically, many of these materials are only vapor retarders as they have varying degrees of permeability.
Water vapor moves into building cavities by two mechanisms: diffusion through building materials and by air transport (leakage), which is usually far more significant and problematic. A vapor retarder and an air barrier, serve to reduce this problem but are not necessarily interchangeable.
Permeability, rated in perms, is 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). Vapor retarding materials are generally categorized as impermeable (=1 US perm, or =57 SI perm), semi-permeable (1-10 US perm, or 57-570 SI perm), and permeable (>10 US perm, or >570 SI perm). There are estabilished standards for measuring the Moisture vapor transmission rate.
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
In modern construction, vapor barriers use is legislated within the building code of some countries (such as Canada, Ireland, England, Scotland & Wales). Current building science recommendations are to locate the vapor retarder in the thermal envelope (exterior walls and ceiling/roof) depending on the climate zone. Heating-dominated climates require an interior vapor retarder. Cooling-dominated climates require an exterior vapor retarder. In mixed climates it is often better to have none. It is also important to allow water vapor to diffuse out of the building envelope (outward in heating climates, inward in cooling climates).
In areas below foundation level (or, subgrade areas), particularly those formed in concrete, vapor retarder placement can be problematic, as moisture infiltration from capillary action can exceed water vapor movement outward through framed and insulated walls. If the foundation walls are properly waterproofed and have appropriate sub-surface drainage, then a warm-side vapor retarder and an air space (capillary break) between framing and concrete should offer sufficient protection against moisture problems.
A slab-on-grade or basement floor should be poured over a cross-laminated polyethylene vapor barrier over 4 inches (10 cm) of granular fill to prevent wicking of moisture from the ground and radon gas incursion.
Inside a steel building, water vapor will condense whenever it comes into contact with a surface that is below the [dew point] temperature. Visible condensation on windowpanes and purlins that results in dripping can be controlled with ventilation.
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