Use of Vapor Retarders

GreenFiber manufacturers GreenFiber Cocoon Insulation products for thermal and sound control application in walls, floors and attics. When applied within exterior walls, we do not recommend the use of a vapor retarder on the warm-in-winter side, primarily due to GreenFiber's mass density and hygroscopic properties. This application without a vapor retarder is considered an alternate method under the provisions of Section 106 of the 1999 National Building Code (NBC), relating specifically to Chapter 13 and Section 723.3.1 and additionally to Sections 101.2 and 103.1 of the International Energy Conservation Code.

The 1997 ASHRAE Fundamentals Handbook, page 23.5 defines a heating climate of having 4000 or more annual heating degree-days. The climate of Michigan would fall under this definition, since heating degree-days range from a low of 6179 to a high of 9670 and an average of 7605 (ANSI/ASRAE Standard 90.2-1993, Section 9). The purpose of a vapor retarder is to retard moisture movement into an exterior wall where it might condense on the exterior sheathing during heating periods or the interior wall covering during cooling periods. In heating climates, it would be located on the inner side (warm-in-winter side) of the exterior wall.

According to the ASHRAE Handbook, page 23.8, the use of a vapor retarder at both exterior and interior locations is undesirable because it can restrict not only the entry of moisture into the insulation but the escape of any moisture as well. Section 1406.3.6 of the NVC requires a water-resistive barrier of 15-pound asphalt felt, which is classified as a vapor retarder (ASHRAE Handbook, page 24.17), to be attached to the sheathing behind the exterior wall veneer. This creates a technical problem between the requirements of building code and energy code. A vapor retarder on both sides of the exterior wall was determined to be one of the aggravating causes for EIFS system failures in North Carolina, Washington and Vancouver, B.C.

Because of its mass density (high airflow resistance) and hygroscopic properties (moisture holding), GreenFiber Cocoon Insulation has the capacity to store up to 30% of its weight in moisture vapor while retarding its passage by air movement. In other words, it retards the flow of moisture vapor, and performs best when moisture vapor is allowed to enter or exit from either side. The chance of any significant amount of moisture vapor actually passing completely through from one side of the insulation to the other is highly unlikely given the dynamics of our daily environment. Even if enough moisture vapor were present to freeze on the exterior sheathing, GreenFiber Insulation would absorb it again as it melted. Make no mistake, GreenFiber Insulation is not classified as a vapor retarder, since it cannot be tested by ASTM E96; the only recognized test method for classifying vapor retarders. On the other hand, GreenFiber Insulation does meet the criteria on an exception since it creates an alternative construction where moisture or its freezing will not damage the materials.

Additionally, since the intent of the energy code regarding the use of vapor retarders is to avoid creating "conditions of accelerated deterioration from moisture condensation", it would not be prudent to risk the possibility that moisture or water could become trapped within the wall by vapor retarders on both sides.