Cliffs: The study points out that wall cavities (between the studs) in EIFS-clad buildings dried faster without the interior 4mil polyethylene barrier than with one.

This is an important point to take a note of, because the majority of homes constructed use a 4mil polyethylene barrier between the drywall and studs based on outdated building practices in warm climates (Toronto is considered a cold climate, further exasperating the problem).

Interior vapor control strategies (vapor barrier/vapor retarders) were introduced to reduce the flow of interior space water vapor moisture into the concealed wall cavity. The original intent was to reduce the inflow of moisture due to moisture generation from the interior of the building. In the 1940’s, American construction materials and wall systems were much different than the ones used today. There has been a lot of speculation and confusion among building envelope practitioners on the use of vapor barriers in constructions, such as the deployment of a sheet of polyethylene or asphalt coated kraft paper for interior vapor control.

Once a home is renovated with EIFS, a water-resistive barrier is typically added to the exterior sheating of the home as part of the full EIF system. This creates a condition where if water gets in the wall cavity (the space between the studs), it will dry slower – increasing the odds that mould or rot can develop. Ideally then, the interior 4mil polyethylene should be removed on homes where EIFS covers the entire exterior surface.

Article: http://www.wconline.com/Articles/Breaking_News/BNP_GUID_9-5-2006_A_10000000000000988992

A study for architects, specifiers and engineers found that in three diverse climates, EIFS with a vapor permeable Class III retarder, kept interior wall cavities warm even in the coldest months, thus preventing the possibility of condensation in the exterior sheathing.

Conducted in partnership with the Oak Ridge National Laboratory, the study measured the moisture performance of four EIFS wall configurations: EIFS with either 2-inch or 4-inch of EPS insulation board and a 4-millimeter polyethylene vapor retarder and EIFS with either 2-inch or 4-inch of EPS insulation board without the polyethylene vapor retarder. Researchers reached their conclusions after evaluating framed walls clad with EIFS without stud cavity insulation in three diverse climates (Boston, Seattle and Minneapolis).

The study’s methodology, approach and observations are outlined in a white paper entitled, “Evaluation of the Moisture Performance of EIFS Walls Without Interior Vapor Barriers” published by Dryvit Systems Inc. To read the white paper, visit http://tinyurl.com/4q88639.

Researchers discovered that for the assemblies and climates tested, EIFS’ monolithic exterior insulation properties eliminated the need for a polyethylene vapor retarder to prevent water condensation in a wall cavity. For the three climates investigated, results of the hygrothermal analysis indicate that use of a vapor permeable (IBC and IRC Class III) retarder on the outside face of the exterior sheathing outperforms the use of polyethylene (IBC and IRC Class I) retarder for diffusion control.

“The results show that the EIFS walls with the Class III vapor retarder and without the 4-mil polyethylene vapor retarder dried faster and maintain balanced conditions throughout the year,” said Bill Preston, Dryvit’s senior engineer/code specialist. “It is also clear that a vapor retarder is not required to prevent water condensation in the wall cavity for the assemblies and climates tested.”

White Paper: http://www.dryvit.com/fileshare/doc/architect/White_Papers_Barriers.pdf