Do I Need A Vapor Retarder

New Green Building Advisor article here.

He is showing a picture of double vapor barrier.

In that case I would agree with him that it is not needed.

Saying that a vapor barrier in this cold climate is not needed, I disagree.

Vapor barriers are also air barriers when properly installed and serve two purposes.


I did not open the link but I do agree with you Marcel.

i go with marcel point

Interesting article. I tend to agree with his point about being very careful about using poly as a vapor retarder. I’ve seen first hand what poly on the interior of the wall and building wrap over plywood on the exterior can do, lots of trapped moisture buildup and rot. In most cases the damage I’ve seen is moisture that seemed to be trapped between the building wrap and the plywood. This was a while ago-20yrs so maybee the building wrap is better quality these days, but not sure on that. I have used it as many have as tarps to keep lumber dry and it works great, and when you lift it off the lumber it is soaked on the inside…huuummm…

Anyway here is a little more reading from NAIMA for those with an interest in the matter.

"Insulation and 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. It is important to control water vapor movement in homes. A vapor retarder helps prevent water vapor from condensing to liquid water within the structure. Liquid water can accumulate inside exterior walls and in roof and attic spaces. If enough water is present, wood rot and decay can cause significant damage. The kraft facing on insulation is a vapor retarder. It can reduce moisture movement through your walls, floors, and ceilings. Call the local building permitting office to find out about the local building codes and practices with regard to the need for vapor retarders.

On What Side Should the Vapor Retarder be Installed?
In areas where the climate is cold in the winter, the vapor retarder should be installed inward toward the warm living 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
Any material that has a perm rating of 1 or less is considered to be an adequate vapor retarder for residential construction. (A perm rating is a measure of the diffusion of water vapor through a material.) 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

Insulation Facing, Kraft
1/4 inch Plywood (douglas fir, exterior glue)
Insulation Facing, Foil Kraft Laminate
Vapor Retarder Latex Paint, 0.0031 inch thick
0.002 inch Polyethylene Sheet
0.004 inch Polyethylene Sheet
0.0006 inch Polyethylene Sheet
Aluminum foil 0.00035 inch thick
Aluminum foil 0.001 inch thick
Not Vapor Retarders Perm Rating

3/8 inch gypsum Wall Board (plain)
4 inch Unfaced Mineral Wool
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)
When is a Vapor Retarder Required?
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 (ICC) building codes are summarized below:

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.
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 - (refer to the climate zone map below.) 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.

Class III vapor retarders can be used on the interior side of the wall in the following climate zones when any one of the conditions in the table below apply:

Climate Zone
1, 2, 3, and 4
All wall assemblies
Marine 4
Vented cladding* over OSB
Vented cladding* over plywood
Vented cladding* over fiberboard
Vented cladding* over gypsum
Insulated sheathing with R-value ≥ 2.5 over 2x4 wall
Insulated sheathing with R-value ≥ 3.75 over 2x6 wall

Vented cladding* over OSB
Vented cladding* over plywood
Vented cladding* over fiberboard
Vented cladding* over gypsum
Insulated sheathing with R-value ≥ 5 over 2x4 wall
Insulated sheathing with R-value ≥ 7.5 over 2x6 wall
Vented cladding* over fiberboard
Vented cladding* over gypsum
Insulated sheathing with R-value ≥ 7.5 over 2x4 wall
Insulated sheathing with R-value ≥ 11.25 over 2x6 wall
7 & 8

Insulated sheathing with R-value ≥ 10 over 2x4 wall
Insulated sheathing with R-value ≥ 15 over 2x6 wall

  • 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.

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."