Vapor barrier

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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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 (perm = 0.06).
• 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).
• 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).

Usage

Vapor Barriers have become controversial and some out of date building codes may still require their use resulting in moldy basements. Current Building Science recommendations is to limit vapor barrier/retarder usage to limited circumstances. See sources in references.

Basements

Use of vapor barriers in basement is especially not recommended.

• ICAA Technical Bulletin re vapor retarders "it is recommended that a vapor retarder not be used in a wall that is partially or fully below grade."]

• http://www.buildingscience.com/documents/primers/plonearticlemultipage.2006-12-05.5229931729/section-2b-recommendations2014air ("One of the worst assemblies for basement walls from the perspective of mold and moisture problems is a foundation wall that is internally framed and insulated with fiberglass cavity insulation and covered with a plastic vapor barrier."

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).

See also

• Infiltration
• HVAC

References

Sources for the section on basements and crawlspaces:

• http://www.buildingscience.com/bsc/resources/foundations/basement_insulation_systems.pdf
• http://www.buildingscience.com/bsc/resources/foundations/Understanding_Foundations.pdf
• http://www.buildingscience.com/resources/3-Understanding_Vapor_Barriers.pdf
• http://www.buildingscienceseminars.com/2006/handouts/BSD-103_Understanding_Basements.pdf
• http://www.eere.energy.gov/buildings/building_america/pdfs/db/35398.pdf
• http://www.buildingfoundation.umn.edu/OCBasementSystem/ProjectReview.htm
• http://ths.gardenweb.com/forums/load/basements/msg1013404110282.html?4
• http://www.newsday.com/features/home/nyp-hsdr-082505,0,452831.column
• http://www.housingzone.com/proremodeler/article/CA6358797.html
• http://www.huduser.org/Publications/pdf/moisturehomes.pdf (p. 54, 84)
• Fine Homebuilding No. 169 March 2005 p. 78
• Fine Homebuilding No. 162, May 2004 p. 52
• http://www.housingzone.com/topics/pr/build/pr04ca007.asp
• http://www.housingzone.com/proremodeler/article/CA6404480.html
• http://www.askthebuilder.com/VaporBarrier.shtml