Retrofitting historic timber-framed buildings in the UK : Monitoring the risk of increased moisture content

The risk of unintended consequences arising from the energy retrofit of buildings, especially concerning moisture movement through the building fabric, is well recognised. As highlighted by Publicly Available Standards (PAS) 2030 and 2035, this is particularly relevant to buildings of traditional construction, defined as those ‘consisting of solid brick or stone external walls, or pre-1919 timber-framed external walls with any infill’. This paper focuses on this last construction typology, presenting ongoing research, funded by Historic England, monitoring the hygrothermal performance of four pairs of mock-up replacement infill panels for timber-framed walls. The chosen materials, informed by current guidance, are wattle and daub, expanded cork, wood fibre/wood wool, and hempcrete. Each pair of panels features two finishes: one with a natural hydraulic lime (NHL) 3.5-based render, and the other with a non-hydraulic lime/hemp mix. The panels are installed as the northern façade of a test cell, exposed to Cardiff’s climate, with controlled internal conditions during the heating season. Since December 2019, moisture content (%) has been monitored by measuring electrical resistance every 30 minutes in a total of 60 positions. Previously published results from the initial 18 months of monitoring reported no evidence of interstitial condensation, with wetting and drying cycles directly corresponding only to wind-driven rain events. Following four years of monitoring, however, there now appears to be evidence to suggest that interstitial condensation is occurring within both the traditional wattle and daub and the composite wood fibre/wood wool infill panels. Although predicted by simulation and previous trials, this condensation might have previously been obscured by wind-driven rain. Its emergence in both traditional and retrofitted infill materials underscores the complex nature of moisture behaviour in this construction typology. This development is under ongoing review, and the monitoring of case study buildings is planned. The results will eventually inform best practice guidance for the energy retrofit of historic timber-framed buildings in the UK.