Moisture Control: Utilizing Vapor Retarders

A vapor retarder is defined as a material or system that adequately retards the transmission of water vapor under specific conditions. Building occupants, certain appliances, plants, and plumbing equipment generate moisture that is carried in the air as vapor.

Moisture control through limiting water vapor movement in commercial buildings is essential. A vapor retarder helps prevent water vapor from moving into building assemblies, like walls, where it can condense into liquid water within the structure. Liquid water can accumulate inside exterior walls and in roof and crawl spaces. If enough water is present, rot and decay can cause significant damage. The kraft facing on insulation with an asphalt coating serves as a vapor retarder. The vapor retarder can reduce the likelihood of condensation within building walls, floors, and ceilings.

Where should vapor retarders be installed?

In areas where the climate is cold in the winter, the vapor retarder should be installed on the inner side of the wall near the warm interior space — or on the warm side in winter. In humid climates or areas where there is extensive use of air-conditioning, if a vapor retarder is required, it should be installed on the exterior side of the wall.

Types of Vapor Retarders 

The table below shows the perm rating of some common building materials that are consistent with the ASHRAE Handbook of Fundamentals and other industry sources.

Vapor Retarders and Perm Ratings

Vapor Retarder Perm Rating
Insulation Facing, Kraft 1.0
¼ Inch Plywood (douglas fir, exterior glue) 0.7
Insulation Facing, Foil Kraft, Laminate 0.5
Vapor Retarder Latex Paint 0.0031 inch thick 0.45
0.002 inch Polyethylene 0.16
0.004 inch Polyethylene 0.08
0.0006 inch Polyethylene 0.06
Aluminum foil 0.00035 inch thick 0.05
Aluminum foil 0.001 inch thick 0.01

Not Vapor Retarders and Perm Ratings

3/8 inch gypsum Wall Board (plain) 50
4 inch Unfaced Mineral Wool 30
Typical Latex Paint — 0.002 inch thickness 5.5 to 8.6
4.4 lb/100ft2 Asphalt Saturated Sheathing Paper 3.3
1/4 inch Plywood (douglas fir, interior glue) 1.9

When is a Vapor Retarder Required?

The latest research on moisture performance of walls and vapor retarders has significantly changed the vapor retarder requirements in the building codes.

2009 and newer editions of the International Code Council (ICC) building codes, summarized:

  • The International Residential Code (IRC) 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.
    • 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.
    • 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 1.0 perms and less than or equal to 10 perms. Latex or enamel paint qualify as Class III vapor retarders.

Climate zones for Class III Vapor Retarders

Class III vapor retarders can be used on the interior side of the wall in the following climate zones in any of the conditions outlined.

Climate Zones Construction
1, 2, 3, 4, All Wall Assemblies

Vented cladding* over OSB

Vented cladding* over plywood

Vented cladding* over fiberboard

Marine 4 Vented cladding* over gypsum

Insulated sheathing with R-value ≥ 2.5 over 2×4 wall

Insulated sheathing with R-value ≥ 3.75 over 2×6 wall

5 Vented cladding* over OSB

Vented cladding* over plywood

Vented cladding* over fiberboard

Vented cladding* over gypsum

Insulated sheathing with R-value ≥ 5 over 2×4 wall

Insulated sheathing with R-value ≥ 7.5 over 2×6 wall

6 Vented cladding* over fiberboard

Vented cladding* over gypsum

Insulated sheathing with R-value ≥ 7.5 over 2×4 wall

Insulated sheathing with R-value ≥ 11.25 over 2×6 wall

7 and 8 Insulated sheathing with R-value ≥ 10 over 2×4 wall

Insulated sheathing with R-value ≥ 15 over 2×6 wall

Vapor Retarders in Warm Climate Zones 1, 2, 3, and 4

The IRC does not require or prohibit the use of vapor retarders in climate zones 1, 2, 3, and 4. NAIMA recommends using either a Class II or III vapor retarder in these warmer climate zones and avoid the use of Class I (very low perm) vapor retarders. Kraft-faced batts can be installed in all climate zones.

In the warmer climate zones, installing vapor retarder with a very low perm rating on the interior of a wall assembly can lead to moisture problems. Even vinyl wall paper, which has a low perm rating, can induce moisture problems in warm, humid climates where hot, moist conditions tend to drive moisture into the wall from the outside of the building.

In very warm, humid climates, if a vapor retarder is used, NAIMA recommends installing it to the exterior side of the wall.

Vapor Retarders in Cold Climate Zones (5, 6, 7 and Marine 4):

The International Residential Code (IRC) requires either a Class I or II vapor retarder on the interior side of frame walls in climate zones: 5, 6, 7, 8 and marine 4 (see climate zone map). A vapor retarder is not required for basement walls or on any portion of the wall that is below ground or on walls made of materials that can’t be damaged by moisture or by freezing.

Department of Energy Climate Zones

NAIMA has developed a map showing thermal recommended levels of insulation for various climate zones. They are based on recommendations from the U.S. Department of Energy (DOE) and the International Energy Conservation Code (IECC). The IECC is the model building code for the United States.


climate_map

*Vented Claddings include vinyl lap or horizontal aluminum siding installed over a weather-resistive barrier, typically housewrap or 15 lb. building paper, or Brick veneer with a minimum space of 1″ between the brick and the weather-resistive barrier.