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Evaluation of the Potential for Corrosion, Mold Growth, and Moisture Accumulation within Typical Brick Veneer Wall Assemblies Designed per 2006 International Energy Code in a Mixed Humid Climate
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With respect to the prescriptive minimum insulation requirements and the mandatory interior vapor retarder requirements for building envelopes in Climate Zone 4A, there are major changes included in the 2009 International Energy Conservation Code (IECC) relative to the 2006 IECC that help reduce the potential for moisture accumulation within the stud cavity. The bidirectional drying potential of walls constructed in mixed-humid climates is typically critical for preventing moisture accumulation within exterior walls. The 2006 IECC includes mandatory requirements for a vapor retarder installed on the warm-in-the-winter side of the insulation in Climate Zones 4 to 8. The climate in Zone 4A is considered mixed humid and varies greatly from the other northern climate zones. Hygrothermal models show that interior vapor retarders can create the potential for condensation and moisture accumulation related to issues within the interior portions of the stud cavity during cooling seasons in Zone 4A climates, because the vapor retarder restricts drying to the interior. This vapor retarder requirement was removed from the 2009 IECC. The 2006 IECC includes prescriptive requirements for the minimum insulation within light-gauge steel stud walls but does not require any continuous insulation within the exterior air cavity; this is required by the 2009 IECC. Hygrothermal modeling shows that when the interior vapor retarder is omitted, the continuous insulation within the exterior air cavity becomes important in mixed-humid climate zones during the heating season, when building envelopes dry to the exterior. The continuous insulation shifts the thermal gradient and location of the dew point temperature exterior of the stud cavity where condensation can be properly managed. This paper compares results of a field study to results of hygrothermal models to evaluate the potential for moisture accumulation, corrosion, and mold growth within a wall designed per the 2006 IECC. This comparison of the field results to the modeling is also used to assess the general effectiveness and overall accuracy of the tools commonly used by designers to predict moisture accumulation in wall assemblies. An eight-month-long field study of the hygrothermal behavior of a typical wall assembly designed in accordance with the 2006 IECC was conducted. The wall assembly consisted of brick veneer, exterior air space, exterior water-resistive barrier, exterior sheathing, foil-faced batt insulation in a 6-in. steel stud cavity, and interior gypsum board. Data loggers were used to record the temperature and relative humidity at several locations within the wall assembly. The field measurements were compared to transient hygrothermal models of this wall assembly. The modeling was conducted in general accordance with ASHRAE 160 using WUFI Pro 5.1. When the simulations were performed to account for the interior and exterior ambient conditions and the initial material moisture contents recorded during the field study, the simulation closely matched the collected data. This provides validation of the methodology and calculations used in the simulation. However, it also highlights the need for accurate assumptions regarding the climates to which the building assembly will be subjected, as well as accurate assumptions regarding the initial moisture contents of materials, particularly when the assembly includes a vapor retarder.
hygrothermal analysis, energy codes, mixed-humid climates, continuous insulation, interior vapor retarder
Wagner, Carly M.
Project Engineer, Whitlock Dalrymple Poston & Associates, Charlottesville, VA
Cyphers, Rex A.
Senior Associate, Whitlock Dalrymple Poston & Associates, Charlottesville, VA
Whitlock, Rhett A.
Principal, Whitlock Dalrymple Poston & Associates, Manassas, VA