The impact of convective vapour transport on the hygrothermal risk of the internal insulation of post-war lightweight prefab housing

Interior insulation is being applied in dwellings constructed with the ‘Airey’ system, a post-war prefab construction system which has recently been gaining heritage significance recognition. It's well-known that applying internal insulation in historic buildings may result in interstitial condensation, surface condensation, mould growth and decay. A review of literature on hygrothermal performance assessment of internal insulation of historic buildings is presented. The results show a strong focus on buildings constructed with solid masonry walls; an evaluation of construction types similar to the Airey system remains absent. The methods that have commonly been applied – being a 1D or 2D simulation of the envelope, focussing on vapour transport through diffusion – were developed to address the hygrothermal behaviour of solid masonry walls. These may not be suitable to identify hygrothermal risks in the Airey system, which is lightweight and has many air cavities. A method for hygrothermal performance assessment that is tailored to these specific characteristics of the Airey system is proposed, combining a whole building HAM-model with a coupled 3D heat transfer model of the construction details. The calibration of the HAM-model to on-site measurements demonstrates the importance of modelling convective vapour transport, specifically as it relates to air leakage through the vapour barrier. The results highlight the importance of accounting for convection in hygrothermal assessments and indicate that simulating a vapour barrier as being installed with perfect airtightness may lead to unrealistic outcomes. Moreover, the results demonstrate the need to further develop the understanding of hygrothermal risks associated with applying internal insulation to non-solid walls, and to adapt the methodology of hygrothermal performance assessments in order to do so.