Characterization of the Moisture Performance of Energy Efficient and Conventional Light Frame and Wood Wall Systems
Published Tuesday, December 15, 2015
Phase 1
The recent changes in the minimum energy codes (2012 IECC) resulted in increased wall insulation levels and reduced wall air leakage for all light-frame wood systems (relative to 2009 IECC). The long-term moisture performance of these new wall systems is not well understood with regard to vapor drive, condensation risk, and drying capability. With moisture performance increasingly becoming a design consideration in the selection of wall systems, home builders and designers need practical guidance for construction of walls that ensure durability of wood buildings. This type of design guidance is particularly needed as various industry groups are advocating specific wall design solutions based on incomplete information.
The project this report is based upon involves monitoring of the moisture performance of wall assemblies with specific design characteristics using the Home Innovation Research Labs outdoor Test Hut facility located in Climate Zone 4. Six configurations of energy-efficient walls are studied: four with exterior rigid foam with the study variables including different levels of cavity insulation and types of water resistive barrier; and two with 2x6 framing with different interior vapor retarders (Kraft-facing vs. gypsum with interior paint only). In addition, six conventional 2x4 wood-frame wall systems with varying cladding materials including stucco, manufactured stone, vinyl siding, brick, and fiber cement siding are under continued monitoring as follow-up research to the previous studies. Specifically, the impact of the interior vapor retarder on the performance of these conventional systems in Climate Zone 4 is studied. Cedar siding has been added in this monitoring phase to expand the library of cladding systems under testing at Home Innovation.
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Phase 2
A field investigation of wood-framed wall assemblies was conducted to monitor the moisture performance of various exterior wall constructions with intentional air leakage and seasonal moisture injections between the cladding and sheathing over a 20-month period from November 2013 to June 2015. The research focused on four specific studies: moisture performance of Extended Plate and Beam (EP&B) walls, walls with air leakage versus air sealing, 2x4 walls with various types of exterior insulation, and baseline walls insulated with kraft-faced batts and unfaced batts. This report covers the field monitoring results and compares the performance of the 10 wall pairs.
The field study was conducted in a mixed-humid climate located in Upper Marlboro, Md., on the campus of Home Innovation Research Labs, located 20 miles east of Washington, D.C. Wall sections with each of the cladding systems were investigated in North and South orientations. Over the test period, indoor humidity conditions were controlled to 30 to 60 percent RH during the winter heating period. Stud bay temperature and humidity conditions were monitored, as was moisture content in the exterior sheathings and studs. During this winter portion of the testing, the wood-based components of the wall sections indicated moisture levels significantly higher for the stucco, stone, brick, and fiber cement walls relative to the walls with vinyl siding with or without exterior insulations. The winter peak moisture content in the sheathing for these walls ranged from 20 to 24 percent; higher than the generally accepted 20 percent Moisture Content (MC) level for wood products (Drumheller and Carll, 2010). Thus, they might have been at risk for moisture problems such as rot and mold growth.
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