Hygrothermal risks of using exterior air barrier systems for highly insulated light weight walls: A laboratory investigation

The current paper presents the results of a detailed laboratory experiment to study the hygrothermal behaviour of vertical light weight walls with an exterior air barrier. Four independent test walls (each 2.3 m by 0.5 m) are placed between a newly developed hot and cold box, operating at controlled temperatures, humidities and air pressures. All four walls are insulated with 30 cm of standard mineral wool to which OSB is applied as interior vapour retarder. The test walls differ from each other by the physical properties of applied exterior air barrier; airtightness, moisture buffer capacity, vapour permeability and thermal resistance. An additional objective of the current investigation is to generate validation data for numerical HAM models. As a consequence, great care was given to create precise two dimensional conditions and to minimize air leakages. The results reveal that using an exterior air barrier instead of traditional interior one, may increase the moisture load as a result of buoyancy driven convection. This leads to an increased risk for mold growth and interstitial condensation against the upper position of the exterior sheathing in winter conditions.