Energy retrofit of historic buildings under wall mould growth constraints: A case study in Suzhou, China

In the hot, humid climate of southern China, energy retrofit of historic buildings is constrained by poor envelope thermal performance and a high mould risk. Although internal insulation can reduce energy demand, it alters wall hygrothermal conditions and may increase the risk of interstitial mould. This study develops an integrated simulation-based approach to optimize energy retrofit strategies under mould control constraints, using the Anderson Building at the former Soochow University site as a case study. In-situ thermal transmittance measurements were conducted to assess envelope performance, supported by infrared thermography. Building energy performance was simulated with DesignBuilder and validated against measured data, while wall hygrothermal behaviour and mould risk were evaluated with WUFI and WUFI-Bio. Economic feasibility was assessed through dynamic payback period analysis. Results show that internal insulation alone reduces mould risk at the interior surface but leads to severe mould growth at the masonry-insulation interface. In contrast, combining insulation with hydrophobization effectively controls mould at both locations for all orientations. Among retrofit measures, a 1.0 ACH reduction in infiltration achieved the highest energy saving (9.82%), while a 0.5 ACH reduction yielded the shortest dynamic payback period (2.53 years), outperforming wall insulation and window retrofit. Under mould safety constraints, integrated measures can achieve up to 17.66% annual energy savings with economic feasibility. The proposed approach provides a practical decision-making framework for balancing energy efficiency, mould safety, and economic feasibility in the retrofit of historic buildings in hot and humid climates.