Moisture Performance Requirements for Insulation in Exterior Wood-Frame Walls without a Vapour Barrier

An increased interest has been observed, especially among architects, in constructing the building envelope without using a vapour barrier membrane of polyethene (PE) foil. An increasing interest in biogenic building materials has also been expressed, as their use, besides storing embedded carbon, can reduce greenhouse gas emissions, replacing nonrenewable building components. Further, building envelope construction without a vapour barrier reduces expenses and the difficulty of the work process, especially around joints and penetrations. This study aims to determine the most important material properties of biogenic thermal insulation materials that influence the moisture-robustness of exterior wood-frame walls constructed without a vapour barrier. A literature study was performed to examine which material parameters have the most influence on the moisture conditions in an exterior wall without a vapour barrier. Hygrothermal simulations of lightweight exterior walls were performed to investigate the significance of variations in material properties (e.g., equilibrium moisture content and vapour diffusion resistance) and determine their necessary characteristics when used as thermal insulation material in an exterior wall without a vapour barrier in internal humidity class 3 (defined in EN ISO 13788). The moisture-robustness of the construction is assessed based on the risk of mould growth in the layer between the thermal insulation and wind barrier. The study suggests that the moisture capacity of the available common biogenic thermal insulation materials does not significantly affect the overall moisture performance of the wall. Simulations demonstrate that, for the thermal insulation layer in internal humidity class 3, at least one of the following requirements must be met to ensure moisture-robustness in exterior walls without a vapour barrier: (I) high diffusion resistance of the thermal insulation and (II) high moisture capacity of the thermal insulation material at relative humidity between 60% and 90%. Commercial biogenic thermal insulation materials on the market do not meet the latter requirement.