The latest research on moisture performance of walls and vapor retarders has brought a greater understanding of how moisture moves through walls and where vapor retarders or needed. This research has significantly changed the vapor retarder requirements in the building codes. The 2009 editions of the International Code Council building codes are summarized below.
The International Residential Code defines vapor retarders as Class I,II or III based on how permeable they are to water vapor,the lower the permeability - the less water vapor that will pass through the vapor retarder.
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Class I - Very low permeability vapor retarders rated at 0.1 perms or less. Sheet polyethylene (visqueen) or unperforated aluminum foil (FSK) are Class I vapor retarders. |
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Class II - Low permeability vapor retarders - rated |
greater than 0.1 perms and less than or equal to 1.0 perms. The kraft facing on batts qualify as a Class II vapor retarder.
Class III - Medium permeability vapor retarders rated greater than 1 perm and less than or equal to 10 perms. Latex or enamel paint qualify as Class III vapor retarders.