Vapor Retarders

New provisions in the IRC and IECC are making houses greener

Posted on Dec 23 by Lynn Underwood, GBA Advisor

Some vapor retarders change with humidity Kraft facings on batt insulation and other products, such as Membrain (pictured), become more or less permeable to water vapor as the relative humidity changes.

9 Steps to A Greener Code

New homes built using the 2009 International Residential Code (IRCInternational Residential Code. The one- and two-family dwelling model building code copyrighted by the International Code Council. The IRC is meant to be a stand-alone code compatible with the three national building codes—the Building Officials and Code Administrators (BOCA) National code, the Southern Building Code Congress International (SBCCI) code and the International Conference of Building Officials (ICBO) code.) or International Energy Conservation Code (IECC International Energy Conservation Code.) will be more energy efficient than ever. As a consequence, a builder’s world may become a bit more complex and, in some cases, a bit more expensive.

STEP 9: INSTALLING VAPOR RETARDERS

Code: R601.3 Class-I or -II vapor retarders are required on the interior side of framed walls in zones 5, 6, 7, and 8 and in Marine 4, except on basement walls; in the below-grade portion of any wall; and in construction where moisture or frozen moisture will not damage the materials used in the assembly.

What it means to you: Since the last edition of the IRC, the vapor-retarder requirements in the IECC, IRC, and IBC were changed based on an understanding of building science. This change moves the new vapor-retarder requirements into their proper code
location because they are more reasonably related to the building shell. This means that in the prescribed zones, you’ll need to install a vapor retarder on the interior side of a wall. A class-III vapor retarder can be used in lieu of class-I and -II products in some cases when the claddingMaterials used on the roof and walls to enclose a house, providing protection against weather. is vented or insulated sheathingMaterial, usually plywood or oriented strand board (OSB), but sometimes wooden boards, installed on the exterior of wall studs, rafters, or roof trusses; siding or roofing installed on the sheathing—sometimes over strapping to create a rainscreen. is used. This code, which was previously only part of the IECC, is now integrated into the IRC.

The 2009 building codes reflect practices that not only increase energy efficiency, like air sealing measures and increased insulation, but also address sustainable building practices, like moisture control.