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Measurement of heat, air and moisture (HAM) phenomena in building assemblies under both controlled conditions and field conditions are difficult to achieve with uniform accuracy and reliability. Care is needed in selecting the measurement sensors and instrumentation to achieve an acceptable degree of accuracy. When the experiments are planned to answer specific questions or to confirm expected responses, the degree of accuracy needed is pre-determined. Proper calibration coupled with appropriate selection of materials can improve the reliability of the measurements, enhance the accuracy achieved, and ensure that the installation has the durability to survive for as long as is needed by the experiment. The field and laboratory experience of the authors in undertaking HAM measurements, particularly those involving transient conditions arising from exposure to real weather, are the basis for the recommendations provided here. The limitations in both undertaking certain measurements and in the interpretation of some data are addressed. The complex of interactions related to the driving forces and changes in material properties prevents experimenters from attributing certain outcomes to particular theoretical assumptions. However, field studies are complementary to carefully executed laboratory studies. As the accuracy of theory increases, there will be an increased need for detailed and accurate field measurements.

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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.