This dissertation empirically examines the hygrothermal impact of sheathing selection on super-insulated timberframe wall assemblies. The term "Hygrothermal" pertains to the simultaneous movement of heat and moisture through a material or assembly.
This research was motivated by increasing demands from clients, designers and legislators to achieve much higher levels of thermal insulation in modern day building envelopes. Coupled with this, is Irelands recent and widespread acceptance of timberframe construction as a valid building solution. Considering Irelands somewhat inexperienced knowledge base of this construction, the possible issues relating to super-insulating timberframe assemblies must be adequately analysed and addressed.
This research employed two Passive House Standard, timberframe external walls, both incorporating different insulation depths. Three sheathing types; Oriented Strand Board, SPANO Durelis Populair and GUTEX Multitherm were selected for testing based on their specific hygrothermal properties. Each of these were applied to both aforementioned wall types, creating a total of six test assemblies. Transient hygrothermal simulations were executed to analyse the drying capacity of each assembly when subjected to an extreme Dublin weather climate. A Bio-hygrothermal simulation software was then used to assess the risk of mould growth at critical interfaces within each wall assembly.
The results suggest that when no additional moisture is present, the overall insulation depth has a minimal impact on the drying capacity of an assembly. When a percentage of driving rain penetration is simulated however, the assemblies with lower insulation levels demonstrate a notable increase on drying capability. Additionally, the results demonstrate that there is a strong correlation between sheathing selection and timberframe hygrothermal performance in Ireland. In all simulated test cases, GUTEX Multitherm significantly outperforms the other two sheathing types, particularly when unforeseen moisture ingress is present in the assembly.
This research has shown that although thermal performance is a crucial aspect to consider, it is equally important that hygrothermal performance receives due consideration. An assembly that does not perform well hygrothermally, will have inevitable implications on both thermal performance and long-term durability. With appropriate material selection and assembly sequence, super-insulated timberframe assemblies will reach, and surpass, their life expectancy in a humid climate.