Spray Foam as a Vapor Retarder
Approximately 99% of the water vapor that travels through a building envelope enters because of air leakage. Icynene spray foam insulation controls air leakage so effectively that a separate vapor retarder is often not necessary. In fact, when a commercial building code requires it, all of Icynene’s closed-cell, medium density, spray foam insulation officially qualifies as a class II vapor retarder.
When open-cell, low density, spray foam insulation is used, a separate vapor retarder may or may not be necessary. In U.S. climate zones 1 through 4 (except for Marine 4), a vapor retarder is not required by code in exterior walls, and Icynene’s building science engineers do not recommend it. Exceptions include places with high interior relative humidity, such as interior swimming pools.
When a separate vapor retarder is needed, it should be applied to the warm side of the wall. Vapor retarder primer or paint is often the simplest solution. This kind of coating can be applied to the surface of drywall or approved versions can be sprayed directly onto cured Icynene open-cell spray foam insulation.
The following primers can be applied directly over Icynene open-cell spray foam insulation. They achieved a permeance rating of less than 1 perm (< 60 ng/Pa.s.m2) and qualify as class II vapor retarders according to the building code.
- Benjamin Moore® Fresh Start Primer 024
- CORO-BOND™ Universal Primer 1147