Orientation-dominated mould risk and hygrothermal remediation in bio-based internally insulated masonry walls: In-situ monitoring and calibrated simulation in an oceanic climate

Insulating a heritage masonry wall from the inside is often the only option when the façade must be preserved — and it is also the most likely to fail, because it traps moisture where the insulation meets the wall. Which bio-based systems survive this, and on which façades, is still poorly understood in oceanic climates. To find out, four bio-based systems — hemp–lime concrete, cellulose wadding, sheep wool and flax wool — were installed on the four differently oriented façades of a historic brick building in northern France and monitored for a year at the insulation–masonry interface, and mould risk was assessed with the extended VTT model. The three fibrous walls had near-identical U-values (0.099–0.107 W·m⁻²·K⁻¹) and thermal masses (567–575 kJ·m⁻²·K⁻¹), and almost identical interface vapour pressures. Differences in their mould response can therefore be attributed mainly to orientation rather than to the insulation material. Their mould indices ranged from below 1 to above 3: colder north-facing interfaces held the relative humidity high enough for mould, while sunnier façades stayed dry. The north-west flax-wool wall spent 256 days above 90% relative humidity and reached a mould index above 3; the south-east hemp–lime wall averaged 69% and stayed below 0.3. This failing wall was reproduced in a calibrated heat-and-moisture model and six corrective measures were tested over three simulated years. The most effective was counter-intuitive: removing the smart vapour retarder — a layer meant to control moisture — was the only measure that brought the mould index back to zero. Thinner insulation helped by warming the interface, while a vapour barrier, thicker insulation, or a hemp–lime replacement all made things worse. The result is orientation-specific guidance for using bio-based internal insulation safely in oceanic climates.