Vapor Retarder Use

GreenFiber® manufactures products for thermal and sound control application in walls, floors and attics.

Facts about Vapor Retarders

  • Increased knowledge regarding climate driven moisture vapor transport has found its way into the model building codes. The 2004 ICC Supplement to the 2003 IRC and IECC exempts jurisdictions in Zones 1 through 4 from the requirement of a vapor retarder in unvented above-grade frame walls, floors and ceilings.
  • "Vapor barriers were originally intended to prevent assemblies from getting wet. However, they often prevent assemblies from drying. Vapor barriers installed on the interior prevent assemblies from drying inward. This can be a problem in any air-conditioned enclosure, in any below grade space, or when a vapor barrier is also on the exterior" (Lstiburek, ASHRAE Journal Aug. 04).
  • The intent of the Model Energy Code regarding the use of vapor retarders is to avoid creating "...conditions of accelerated deterioration from moisture condensation." Therefore, it is prudent to reduce the risk of the possibility that moisture or water could become trapped within the wall by use of vapor barriers on both sides.
  • According to the 2005 ASHRAE Handbook, section 24.7, 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.
  • Application without a vapor retarder is considered an alternate method under the 2006 International Building Code (IBC) section 104.11.

Facts about GreenFiber® Insulation

  • GreenFiber® Insulation has been tested to assure normal variations in temperature and relative humidity will not adversely affect thermal performance. GreenFiber® Insulation meets the requirement of ASTM C-739 with less than 15% maximum weight gain under the specified test conditions.
  • GreenFiber Insulation reversibly binds moisture vapor and it is also vapor permeable. Because of this combination of properties GreenFiber Insulation has the capacity to store and release moisture vapor. The product also has a relatively high airflow resistance due to its mass density. The combination of these three properties means GreenFiber Insulation has the capacity to reversibly store moisture vapor and thus acts as a hygric buffer.
  • GreenFiber Insulation is not classified as a vapor retarder. This insulation has been used in Climate zones 5 and 6 for years without a vapor retarder with no adverse effects from moisture issues. GreenFiber insulation with no vapor retarder on the warm-in-winter side is considered an exception under the 2006 IECC section 402.5. Architects, Builders and Code Officials need to ascertain the appropriateness of the Code exception to the specific location.

Summary

GreenFiber® Insulation will absorb and release moisture vapor or condensation without degradation to itself or adjacent building materials when installed by a trained insulation contractor according to the manufacturer's instructions. GreenFiber does not recommend the use of a vapor retarder on the conditioned side of exterior walls in Climate Zone 5.* In Climate Zones 6-8 GreenFiber recommends vapor retarders on the conditioned side of exterior walls.

These are general recommendations, and they do not supersede GreenFiber's warranty statement found on all printed literature. No wall should be designed with a double vapor barrier. It is the responsibility of architects, engineers and builders to fully understand the function of the design, the materials and the operation of the structures that are built to prevent accumulation of moisture vapor in building assemblies. Architects and builders must also understand that interior flush mounted components, such as cabinet backs and mirrors, can act as vapor retarders or barriers and must be accounted for in the design and build of the structure.

*Climate Zones as determined by the Department of Energy www.energy.gov

Considerations

  • State and local jurisdictions may have different requirements for use of vapor retarders than the recognized National and International Model Building Codes.
  • The current International Building Code (and its companion codes) defines a vapor retarder as being 1.0 perm or less when tested with the desiccant method using Procedure A of ASTM E 96.
  • There are various ways to achieve perm ratings where vapor retarders are required.