Is there plastic in your wall?

By Megan Kramer, Performance Homes Consultant


Vapor retarders are one of the most confusing aspects of building any home. There are state, county and sometimes city codes to follow, multiple inspectors to please and potentially a high performance certification program specification to meet.

I was surprised to see plastic in the wall assemblies of almost all the homes I have rated and wanted to find out more. Here are the various codes, what building science professionals have to say and some alternate products.

What does the code say?

WA State Prescriptive Checklist

2012 International Residential Code: R702.7 Vapor retarders. Class I or II vapor retarders are required on the interior side of frame walls in Climate Zones 5, 6, 7, 8 and Marine 4. Exceptions include:

1. Basement walls.

              2. Below grade portion of any wall.

3. Construction where moisture or its freezing will not damage the materials. 

Washington Climate Zones

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What Class is Your Vapor Retarder?

Class I Vapor Retarder: ≤ 0.1 perm (called impermeable). Also referred to as a vapor barrier, examples include: Sheet Polyethylene, on perforated aluminum foil.

Class II Vapor Retarder: 0.1 to 1.0 perm (called semi-impermeable. Examples include: Kraft-faced fiberglass batts.

WA State Energy Code

Vapor retarder. Wall assemblies in the building thermal envelope shall comply with the vapor retarder requirements of Section R702.7 of the International Residential Code or Section 1405.3 of the International Building Code, as applicable. Water-resistant gypsum board shall not be installed over Class I or II vapor retarder in shower or tub compartment.

Washington Administrative Code

R402.1.5 Vapor retarder. Wall assemblies in the building thermal envelope shall comply with the vapor retarder requirements of Section R702.7 of the International Residential Code or Section 1405.3 of the International Building Code, as applicable.


What do building scientist have to say?

Matt Risinger a leading building scientist discusses vapor barriers and sites a few resources.

Those resources along with some additional articles on the topic an be found below.

Energy Vanguard (one of the best blogs covering building science, heating & air conditioning, energy efficiency, and home energy audits) states, “You Don’t Need a Vapor Barrier (Probably).” Allison also introduces a few scenarios including: “In cold weather, a sheet of poly on the interior side of a wall probably won't cause any problems.” But, “Plastic on the outer surface of a wall in cold weather could cause problems.”

Building Science Corporation presents an article on vapor barriers by Joseph Lstiburek where he states interior vapor barriers such as polyethylene should never be installed in an air conditioned building — even one located in very cold climate.

Incorrect use of vapor barriers is leading to an increase in moisture related problems. Vapor barriers were originally intended to prevent assemblies from getting wet. However, they often prevent assemblies from drying. Vapor barriers installed on the interior of assemblies prevent assemblies from drying inward. 

Martin Holladay sums it all up in an article in Green Building Advisor:

  • The main reason to install an interior vapor retarder is to keep a building inspector happy.

  • If a building inspector wants you to install a layer of interior polyethylene on a wall or ceiling, see if you can convince the inspector to accept a layer of vapor-retarder paint or a “smart” retarder (for example, MemBrain or Intello Plus) instead.

  • Although most walls and ceilings don’t need an interior vapor barrier, it’s always a good idea to include an interior air barrier. Air leakage is far more likely to lead to problems than vapor diffusion.

What are the options?

Since there seems to be some question among leading professionals about the use of plastic in walls. We should start investigating and implementing alternate strategies. The most important thing a builder can do is make sure there is no air leakage through the walls in the first place. If there is no air leakage there is no way for bulk moisture to move through your wall system and no condensation potential in your wall.

After you have sealed the home, there are 3 ways to ensure you have a barrier against vapor.

  1. You can paint on your barrier for the same cost or possibly even less than primer. There are no additional labor costs and you get complete coverage without seams or gaps according to Applegate Insulation.

  2. Use a membrane technology such as Intelllo Plus or MemBrain. These products achieve the same goal as plastic without the possible complications and work well when combined with a blown-in insulation product.

Walls have the greatest surface area by far as compared to the floor or ceiling and therefore have the greatest impact on shell efficiency. As our homes get tighter, any moisture created has a greater chance of causing problems. We must be diligent with spot moisture control, air sealing and vapor retarders. We need to look at the whole system and ensure building integrity and occupant health.