Evaluating the hygrothermal performance and associated benefits and risks of replacement infill panels for historic timber-framed buildings in the UK

Historic timber-framed buildings, although a small part of the UK's historic building stock, contribute to UK national cultural identity. However, their thermal performance is typically inferior to their masonry counterparts, and their defining exposed frames limit retrofit options. Where historic infill is missing or damaged, there exists the opportunity to infill with thermal insulation. However, this may increase moisture accumulation leading to biological decay. The research in this paper, funded by Historic England, has monitored the hygrothermal performance of eight experimental infill panels in Cardiff, since 2019. Four infills materials were monitored, wattle-and-daub, a wood fibre/wood wool composite, expanded cork board, and hempcrete, within a reclaimed oak frame. Two finishes were applied, NHL 3.5 and sand render, and a non-hydraulic lime with hemp shiv aggregate. Moisture content and temperature were monitored at nine positions within each panel. Over five years, significant moisture fluctuations were recorded. Initially no interstitial condensation was identified, with wetting and drying cycles corresponding with wind-driven rain events. However, in the last few years, incidences of interstitial condensation were identified in the wood fibre/wood wool composite and the wattle-and-daub. Additionally, extended periods of high moisture content were recorded at some perimeter junctions. Overall, those panels finished in the less moisture permeable NHL 3.5 show higher moisture contents and longer drying times. Comparative WUFI® Pro simulations are now underway using measured climate data and material properties. The final results will inform best practice guidance as we aim towards a sustainable future for these iconic buildings.