Moisture movement is affected by seasonal changes as well as climate characteristics, which will define your best strategies and options. Even in cold climates, moisture will move from hot exteriors to cooler interiors in the summer, which places new emphasis on proper selection and placement of vapor retarders.

Best Practices

It’s not just builders who are confused. Many building code officials misunderstand or misinterpret the requirements. Just because it was done that way 20 years ago, doesn’t mean it will work the same way in today’s modern wall systems. Some jurisdictions are becoming more flexible, and that is a welcome change.

The Numbers

The current code has the following definitions:
Class I Vapor Retarder: 0.1 perm or less
Class II Vapor Retarder: 1.0 perm or less and greater than 0.1 perm
Class III Vapor Retarder: 10 perm or less and greater than 1.0 perm
The current proposals are to define them this way:
Vapor impermeable: 0.1 perm or less
Vapor semi-impermeable: 1.0 perm or less and greater than 0.1 perm
Vapor semi-permeable: 10 perms or less and greater than 1.0 perm
Vapor permeable: greater than 10 perms

Current IBC/IRC specifications (see “The Numbers,” left) for VDRs factor the classification of the material’s perm rate with the location’s climatic conditions. Future code directions will most likely specify the use of more permeable products in building assemblies: the use of paint, rather than poly, for instance. Until all codes reflect current building science knowledge, and provide climate-specific material specifications, you need to be wary of selecting a VDR with a perm rating that’s too low or placing it in the wrong location.

If in doubt, I recommend using Class I and Class II VDRs only in the coldest Zones, 7 and 8. In these zones, use a product like MemBrain from CertainTeed. This innovative product looks and installs like poly, but it behaves like a vapor diode, allowing the perm rate to increase as the vapor pressure increases. This means that if installed on the interior side of the wall system, it will meet the Class II requirements by reducing the amount of vapor entering the wall in the winter months. But should the wall system be subjected to a larger flow of moisture from outside or from wet materials inside the wall system, the product becomes more vapor open. The perm rate rises, based on the increased vapor pressure. This sounds like magic, but in the world of materials science it’s actually quite simple.

For other zones, I recommend applying a good quality wall primer to all unprimed drywall, in lieu of traditional VDRs. Prior to priming, drywall has a perm rate of about 50. After a couple coats of latex paint, that perm rate is reduced to 2 or 3. In most climates, this degree of vapor control will be quite adequate and meets the Class III requirements shown in “The Numbers.”

For more information, visit the IRC Web site at, where you can purchase and read the latest code book. Otherwise, get a copy of the Builders Guide Series for your climate from Building Science Press (

A Final Note

Though I advocate a more liberal approach to VDRs, this doesn’t mean I am unconcerned about moisture control. On the contrary, I believe it is absolutely critical that we control and manage the movement of moisture in structures. Used correctly and in the right climate, vapor diffusion retarders can be an important part of a comprehensive solution to effective moisture management.

To manage moisture effectively, we need to control both air movement and moisture flow. The two are always intertwined. Sometimes one product, such as spray foam, addresses both issues. In other cases, we can achieve success with a well installed and properly sealed weather resistive barrier like housewrap or an alternate system of spray-on coatings, or even SIPs. Remember: Always give careful consideration to the location and quality of the air barriers and water management details you use, and train the people on your crews to properly install them.

Joseph Lstiburek, Ph.D., principal of Building Science Corp., contributed to this article.