A three-year study in Champaign, Illinois, was designed to measure the thermal and moisture performance of various typical wall assemblies, especially OSB sheathing, under field conditions. The variables that were studied included:

• insulation material (fiberglass and cellulose),
• vapor barriers (none, polyethylene, and facing material),
• encapsulation materials,
• means of attachment of facing material, and
• location within the wall cavity.

Measurements of temperature and moisture content, using Duff probes, were gathered at hourly intervals over three winters, and plugs made of the OSB sheathing material were regularly weighed for gravimetric moisture content measurements. The interior was maintained at constant humidity-50%-55% for one year and 40% for two subsequent years. Following the study, the walls were disassembled and inspected for sheathing condition and mold growth. Two samples with no vapor diffusion protection, one cellulose and one fiberglass, showed unacceptably high levels of mold growth. All of the samples with vapor barriers or encapsulation remained at a safe level of moisture content. Some of the samples with encapsulated and faced material showed mold growth at the tops of the cavities, particularly where cabling pierced the interior drywall. The construction and building pressure management for this study were such that the moisture performance is a consequence of the diffusion/capillary regime, not the air leakage regime. The cases that were unacceptably bad in this study were affected by diffusion and capillary moisture, not air leakage, under the interior and exterior loads imposed by the test.

AUTHOR: William B. Rose, David J. McCaa, Ph.D.
CITATION: Thermal Performance of the Exterior Envelopes of Buildings VII
KEYWORDS: December, Florida, 1998
YEAR: 1998