Moisture Convection Performance of External Walls and Roofs

Abstract

Full-scale laboratory measurements were taken and 2D simulations were performed to determine the moisture convection performance of the joint of an external wall and an attic floor. Both the measured and simulated results showed that the building fabric is locally sensitive to exfiltration airflow as moisture convection could cause a remarkable increase in the moisture accumulation rate on the inner surface of the sheathing. The results prove that in modern houses with balanced ventilation, where the positive pressure caused by the stack effect cannot be easily controlled, it is essential to control moisture convection by good airtightness and the use of assembly solutions with improved hygrothermal performance, such as the case with mineral wool sheathing that was tested. The simulated results showed that the use of a constant moisture excess value instead of the profile did not cause any inaccuracy in the moisture content. At the same time, the use of the constant outdoor temperature over a long time period did change the hygrothermal performance, which became significantly more critical when simulated with variable outdoor temperatures. The laboratory measurement and simulation results show that in a cold climate, for the assemblies being studied with mineral wool sheathing, leakage rates of 0.1—0.2 L/(s·m) at a 10 Pa pressure difference may be used as performance criteria for moisture convection in a two-storey house with dimensioning moisture excess 4 g/m 3 and a cold outdoor climate. Previous studies on the leakage characteristics show that these leakage rates are easily achievable in practice with careful workmanship.