Managing moisture - the key to healthy internal wall insulation retrofits of solid walls

This study investigates the appropriateness of internally insulating solid walls to the Passivhaus standard. A number of variables are assessed using numerical hygrothermal simulation (under EN 15026) to check (1) associated risk of mould growth on original substrate and (2) if they result in a greater likelihood of timber decay at built-in joist ends. While air and vapour control layers (AVCLs) perform well regarding mould risk in low absorption walls, the results question their appropriateness where joist ends are built-in: when AVCLs are used, moisture levels at joist ends can potentially rise above critical thresholds, even in low absorption walls. The risk is exacerbated in climates with severe exposure to driving rain.