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

Abstract

The thermal resistance of a wall assembly is one of the key factors that is considered when 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 also 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 do 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 and modeling methodologies. Field data were collected for existing mass masonry wall assemblies using an assortment of data logging instrumentation to measure the in situ thermal resistance and moisture profile across the wall assembly. These data were analyzed to determine the impact that periods with elevated moisture levels within the wall assembly had on the measured R-value.