Wooden beam ends in combination with interior insulation: An experimental study on the impact of convective moisture transport

Abstract

The risk free applicability of interior insulation in the presence of wooden beam ends is still a point of discussion. Two phenomena enlarge the risk on wood decay when interior insulation is applied: (1) a higher moisture load in the masonry due to a reduced drying potential and (2) a lower temperature in the masonry resulting in a higher risk on condensation if exfiltration of moist indoor air occurs. This paper studies - based on a hot box-cold box experiment - the impact of convective moisture transport and this for two vapour tight and one capillary active vapour open interior insulation system applied to a masonry wall with an embedded wooden beam. A comparison is made to a non-insulated wall. No wind-driven rain is included, making a pure analysis of convective moisture transport possible. For non-sealed beam junctions, a significant increase in relative humidity due to moist indoor air reaching the colder beam surface was clearly visible. The impact of air exfiltration was most pronounced for the capillary active system and was influenced by the way of sealing the airgap between the beam end and the wall. It is recommended to avoid air exfiltration as well as air circulation near the beam end. For mineral wool, the importance of an airtight sealing was less pronounced. An overpressure was found to influence the hygrothermal conditions near the beam ends of the poorly sealed test setups. Additionally, the convective air flow near the beam ends was analysed based on the measured temperatures.