Vapor barrier

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The phrase vapor barrier is often used to refer to any material, typically a plastic or foil sheet, that resists passage of moisture through wall, ceiling and floor assemblies of buildings. Technically many of these materials are only vapor retarders as they have varying degrees of impermeability. This article will use vapor barrier although increasingly vapor retarder is used in building codes. Vapor barriers resist moisture from penetrating through the barrier and the moisture will instead remain on the side the barrier originating the moisture.

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.

Permeability, rated in perms, is a measure of the transference of grains of water through a material at a standard vapor pressure and temperature. Vapor retarders have permeability ratings of 1.0 or lower.

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[edit] Materials used as vapor barriers:

  • Aluminum foil (perm = 0.05).
  • Paper-backed aluminum.
  • Polyethylene plastic sheet, 6 mil (600 gauge/0.15mm) or greater in thickness 10mm-20mm (perm = 0.06-0.0014 variable).
  • Advanced Polyethylene vapor retarders that pass the ASTM E 1745 standard tests
  • Kraft paper, often attached to one side of fiberglass batts (perm = 0.40).
  • Vapor barrier 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 do to being chemically based).
  • Certain types of rigid insulation.
  • Exterior grade plywood (perm = 0.70)
  • Most sheet type monolithic roofing membranes.
  • Glass and metal sheets (such as in doors and windows).

These are just a list of typical barriers. Note: Anything with a perm rate of 1 can be considered a air retarder. The best way to judge is to compare the perm rates. The lower the number the better it can guard against moisture and air penetration. For example the above mentioned Exterior Grade Plywood (perm .70) would fair well against air perm, however, being made of an organic material with many naturally occurring perforations it would fail when protecting against water.

[edit] Usage

In Traditional construction Vapor Barriers have become controversial and some out of date building codes may still require their use. Installing them on the outside of wall studs in a basement without connecting to a interior drain system can result in moldy basements. Current Building Science recommendations is to limit vapor barrier/retarder install on the above grade floors to limited circumstances. See sources in references.

In Basement situations the Vapor Barrier with a perm of .0014 (the smaller the number, the fewer grams of moisture can penetrate the substance) is very effective in stopping most moisture content from entering the space. The barrier would then have to be connected to a closed interior drain system which would then eject the water traveling in it outside. Without connecting the barrier to a closed drain system the moisture would collect behind the barrier and "belly" out. Mold and mildew would survive but most likely not cross the barrier do to their particle size and affinity for water and dark spaces.

A low perm Vapor Barrier can help not only to waterproof the basement which in turn would help the air quality of the house, but would also help to improve the efficiency of any dehumidifiers running. It would help to lessen the amount of moisture present, thus providing less work for the dehumidifier as well as (with Thermal rated Vapor Barriers) helping to regulate temperature (decreasing the energy the dehumidifier has to use to cool the air to extract the moisture from it).

Vapor Barriers with a low perm rate are also key factors in helping to fight Radon Leeks and V.O.C's from contaminating your air space.

[edit] Basements

Use of high perm vapor barriers in basement is especially not recommended when dealing with installation outside (facing to the interior) of wood studs walls installed in a basement before finishing with organic drywall materials. When dealing with any basement installation it's highly recommended by the NAWSRC to not use any organic material.

Low perm rate vapor barriers are often found through professional waterproofing companies that are trained to install them in conjunction with a drain system. In a case like these the vapor barrier could be used to encapsulate a crawl space or to just cover the walls in order to completely mitigate moisture from the basement.

[edit] Under concrete slabs

A slab-on-grade or basement floor should be poured over a cross-laminated polyethylene vapor barrier over 4" of permeable fill to prevent wicking of moisture from the ground (and radon)(concrete is hydrophilic). The use of a vapor barrier below concrete will prevent moisture in the concrete from drying towards the ground, and may lead to differential curing of new concrete. Modification of the concrete mix design, adjustment of reinforcement, or other measures may be necessary to keep the slab from cupping as it cures.

[edit] See also

[edit] External links

[edit] References

Sources for the section on basements and crawlspaces:

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