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Home / Code Guides / Vapor Retarders

Vapor Retarder Requirements by Climate Zone — Class I, II & III

A vapor retarder is only correct for a specific climate and a specific wall assembly — the same sheet of poly that protects a wall in Minnesota traps moisture in one in Houston. The IRC sorts this out by climate zone and by what else is in the wall.

RequirementModel Code StandardSection
Where requiredClass I or Class II vapor retarders are required on the interior side of frame walls in Climate Zones Marine 4, 5, 6, 7, and 8IRC R702.7
Warm zonesIn Climate Zones 1, 2, and 3, an interior vapor retarder is not required — and in those zones an interior poly sheet is actively harmful because it blocks inward dryingIRC R702.7
Class definitionsClasses are set by permeance: Class I is 0.1 perm or less (sheet polyethylene, foil), Class II is greater than 0.1 up to 1.0 perm (kraft facing), Class III is greater than 1.0 up to 10 perms (latex vapor-retarder paint)IRC R702.7, R202
Vented cladding optionA Class III vapor retarder is permitted in colder zones where the wall has vented cladding — vinyl, polypropylene, or horizontal aluminum siding, or brick veneer with a clear airspaceIRC Table R702.7(3)
Continuous insulation optionA Class III or Class II retarder is also permitted where continuous insulating sheathing keeps the sheathing above the dew point, with minimum R-values that climb by climate zone and wall depthIRC Tables R702.7(3), R702.7(4)
Crawl space ground coverExposed earth in a crawl space is covered with a Class I vapor retarder with joints lapped not less than 6 in.; in an unvented crawl space the retarder is sealed at joints and turned up the wallsIRC R408.2, R408.3.1
Under-slab retarderA vapor retarder of not less than 10 mil is placed between the base course or subgrade and slabs in conditioned spaces, with seams and penetrations sealedIRC R506.2.3

Worth knowing

  • The rule of thumb: vapor retarder toward the warm-in-winter side, and never a vapor-impermeable layer on both faces of a wall — moisture that gets in has to be able to dry somewhere.
  • The 10 mil under-slab requirement replaced the older 6 mil standard, so specs and details copied forward from a previous project frequently still call for the wrong material.
  • Vapor retarder and air barrier solve different problems. Air leakage carries far more moisture into an assembly than diffusion ever will.
  • Mixed-humid climates and air-conditioned houses in humid zones are where poly does the most damage; a vapor-retarder paint is usually the right tool there.
  • Climate zones come from Figure R301.1 and Table R301.1 by county, and some states amend those boundaries — confirm the zone rather than guessing from latitude.
Sections cite the 2021 International Residential Code and 2023 National Electrical Code as model standards. This is BuildSpec’s own summary — not ICC or NFPA text; read the authoritative language at codes.iccsafe.org. Your state may be on a different edition with amendments — check your jurisdiction page or get an Address Report.
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