- Q. I'm building a house in North Carolina, which has a mixed-humid climate (climate zone 3). My local building inspector — backed up by his boss — requires a polyethylene vapor retarder on the interior side of cellulose-insulated walls. My insulation contractor says that interior polyethylene is not only unnecessary in this type of climate, but it may create condensation problems. What should I do?
A.Martin Holladay, editor of Energy Design Update,responds: Your insulation contractor is correct: An interior polyethylene vapor retarder should not be installed in North Carolina. During the summer months, when interior drywall is cooled by a home's air conditioner, condensation can occur on the back (exterior side) of the polyethylene. This phenomenon — called inward solar-vapor drive — occurs after a rainstorm, when sun shines on damp siding. As the siding warms up, water vapor is driven into the wall, allowing condensation to form on the cool polyethylene. This is particularly a problem in homes with so-called "reservoir" claddings — such as brick, stucco, and adhered stone veneer — and in homes without foam sheathing.
Evidently, your local building code does not reflect recent changes adopted by the international codes. The 2004 supplements to the IRC and the International Energy Conservation Code (IECC) abolished all vapor-retarder requirements in climate zones 1, 2, 3, and 4. Moreover, the most recent versions of these codes (the 2007 supplements) provide new levels of flexibility in the remaining requirements for vapor retarders in climate zones 5 through 8.
Even older versions of most residential codes never required interior polyethylene. Until recently, codes defined a vapor retarder as "a material having a permeance rating of 1.0 or less when tested in accordance with ASTM E96," a requirement that can be met with a layer of vapor-retarding paint applied to the drywall. Show your local building inspector a can label; you may be able to satisfy him and conform to current national energy codes with this low-cost compromise.
If you can't change your inspector's mind with education or logic, it might necessary to follow the path of least resistance and cover your walls with a so-called "smart" vapor retarder like CertainTeed's MemBrain (800/233-8990, www.certainteed.com). Since the permeance of MemBrain varies with the relative humidity, it will allow a wall to dry to the interior during North Carolina's hot, humid summers.