Moisture Behaviour of Hygroscopic and Non-Hygroscopic Insulation Material in Wood Framed Walls under the Influence of Natural Convection

Energy saving requirements has lead towards highly insulated building components. The thickness of insulation in walls is for instance 200mm to 400mm in Central and Northern Europe. Thicker insulation may lead to an increase in the importance of natural convection on energy consumption and on vapour transport. Natural convection within thick wall insulation can cause a high moisture accumulation and a significant mould growth risk at the upper cold corner of the wall. Knowledge of moisture transport within open porous insulation in a wood framed wall due to simultaneous heat air and moisture transport is of importance for the characterization of the behaviour and to avoid risks of condensation and of mould growth. High hygroscopic insulation materials as well as non-hygroscopic insulation materials have a different moisture behaviour under the influence of natural convection. The choice of vapour barrier properties of the wall should be dependent on the different insulation properties to avoid a moisture damage risk due to natural convection. This paper presents a case study about the influence of natural convection on condensation risks and of mold growth risks in a wood framed wall under a yearly climate cycle. The focus are moisture risks within an insulated cavity, on the outside cladding of derived timber products as well as on the neighbouring framework. The insulation materials of the cavity are approved by the building authorities and involve a width spectrum of material properties. The aim of the case study is to show how the design of vapour barriers depends on the insulation properties as well as the thickness of the cavity when natural convection occurs. The investigations were carried out using tests in a double climate chamber under a time-lapsed climate cycle as well as simulations with the WINHAM2D model. WINHAM2D, a 2D heat, air and moisture transport program without liquid transport suitable especially for non-capillary insulation in building envelopes. WINHAM2D is validated by measurements in a similar highly insulated wall structure.