Interior thermal insulation has been widely used to improve energy efficiency and indoor comfort of historic buildings. However, traditional vapour-tight insulation materials strongly influence the hygrothermal behaviour of facades, which may lead to moisture-related problems. Capillary-active insulation aims to enhance moisture permeance, ensuring the hygrothermal response of the masonry wall is more aligned with that of an uninsulated wall. While some capillary-active systems have proven their efficiency, global climate change and extreme weather conditions introduce additional uncertainties. This study investigates this specific issue. The hygrothermal responses of three massive masonry walls — one with vapour-tight insulation, one with capillary-active insulation, and one uninsulated — are monitored and simulated under real climate conditions in Leuven, Belgium. The simulations, conducted in DELPHIN 6 with monitored climate datasets as boundary conditions, are validated using measured temperature and relative humidity. The validated model is subsequently used with one current and two future climate datasets to compare the impacts of vapour-tight and capillary-active insulation on the hygrothermal behaviours of the masonry walls under climate change. The former exhibited more serious moisture-related risks, offering only marginally better energy performance, while the latter showed a similar hygrothermal behaviour to the uninsulated wall. Exterior climates significantly influence hygrothermal responses: both insulation systems inevitably increase mould growth and frost damage risks, though moisture failures are reduced in warmer outdoor environments. This paper highlights the nuanced balance between energy efficiency and moisture safety for researchers and practitioners.

Mold growth poses a high risk to a large number of existing buildings and their users. Air leakage through the air cavities of the building walls, herein gaps between walls and air conditioner pipes penetrating the walls, may increase the risks of interstitial condensation, mold growth and other moisture-related problems. In order to quantify the mold growth risks due to air leakage through air cavity, an office room in a historical masonry building in Nanjing, China, was selected, and its indoor environment has been studied. Fungi colonization can be seen on the surface of air conditioner pipes in the interior side near air cavity of the wall. Hygrothermometers and thermocouples logged interior and exterior temperature and relative humidity from June 2018 to January 2020. The measured data show that in summer the outdoor humidity remained much higher than that of the room, while the temperature near the air cavity stays lower than those of the other parts in the room. Hot and humid outdoor air may condense on the cold wall surface near an air cavity. A two-dimensional hygrothermal simulation was made. Air leakage through the air cavities of walls proved to be a crucial factor for mold growth.