Evaluating the Impact of Moisture Content on Thermal Resistance of Wall Assemblies

Please note: The attached file below contains a paper and a presentation in pdf format from the BEST5 conference. Open with Adobe Acrobat for best results.

The thermal resistance of a wall assembly is a key factor in evaluating the contribution of the building envelope to the overall energy performance of new and existing buildings. The conductivity and thickness of the building materials comprising the wall assembly are used to determine the theoretical thermal resistance of a given wall assembly. While the thickness of the materials is generally unchanged, the conductivity of the material will vary depending on its exposure to moisture. The moisture content of a building material can be impacted by direct exposure to bulk water, such as during a rain event, or through the transmission of water vapor. Because water will conduct heat at a greater rate than most building materials, an increase in moisture content within a wall assembly will result in a decrease in the overall thermal resistance. Published data for the thermal resistance of typical building materials does not account for changes in moisture content. Over time, the thermal resistance of a wall assembly will fluctuate and may not be captured accurately based on current design methodologies. Hygrothermal modeling can be used to predict moisture accumulation within a wall assembly over time and corresponding temperature profile through the wall assembly during periods of increased moisture levels. This information can then be used to evaluate the impacts of moisture content of building materials on thermal resistance of the overall assembly. Field data has also been collected for existing wall assemblies using an assortment of data logging instrumentation to measure the in-situ thermal resistance and moisture profile across the wall assembly. For this presentation, this data was analyzed and compared to the results of the hygrothermal analyses to further evaluate the impacts of moisture content on thermal resistance.