Comparing the Enclosure Wall Performance of Low-Permeance Exterior Insulation to High-Permeance Exterior Insulation in the Pacific Northwest

Abstract

As the energy efficiency of buildings improves, the amount of insulation on the exterior of the enclosure will increase because of both energy- and durability-related benefits. There are a large number of papers that have addressed the misconception that exterior insulation increases the durability risk of the enclosure. However, an argument remains concerning the relative performance of low- and high-permeance exterior insulations. RDH Building Science Laboratories has collected nearly two years of field data of full-scale exterior-insulated commercial and residential wall assemblies in side-by-side comparisons. The walls were insulated on the exterior with either low-vapor-permeance extruded polystyrene or high-vapor-permeance mineral wool exterior insulation. The walls were constructed on the north and south orientations of a test hut in a Pacific Northwest climate and were subjected to typical cool, high-humidity wintertime temperatures. The test hut interior relative humidity (RH) was kept at a constant level of 40 % or 50 % (slightly above average for the climate zone) to help determine the performance differences in the assemblies. The test walls were also subjected to intentional controlled wetting events to the exterior surfaces of the structural oriented strand board sheathing to simulate various construction defects and determine the relative rates of drying for the two assemblies. The test walls were compared using measured RH, measured wood moisture content, and the Finnish VTT Technical Research Centre’s mold index. Generally speaking, all of the walls (both mineral wool and extruded polystyrene) performed quite well under normal operating conditions, considering the higher than recommended interior RH conditions. Following the wetting events, the commercial and residential wall assemblies with higher permeance mineral wool continuous insulation generally had improved drying rates.