Hygrothermal analysis of prefabricated façades with active components for minimal invasive renovations

To achieve a decarbonised building stock by 2050, the renovation rate must be significantly increased by means of cost-effective, minimally invasive, and scalable solutions. Prefabricated façades with integrated active components such as heat pumps or mechanical ventilation with heat recovery (MVHR) offer a promising approach, particularly in multi-family buildings. These solutions enhance the building envelope performance and accelerate the replacement of fossil-based heating systems. However, there is limited understanding of how integrated components affect the heat transfer in prefabricated façades and influence interior wall surface temperatures. This work presents two case studies involving two types of façade-integrated heat pumps and a MVHR system. Both heat pumps were tested in a laboratory and the temperature and relative humidity around the equipment and within the façade were monitored. A thermal model was calibrated for both cases based on the measurements, focusing on the most critical thermal sections. In case study A with the exhaust-to-supply air heat pump, a significant thermal bridge was found between the window and the heat pump cavity, with heat fluxes reaching 50 W/m2 and surface temperatures up to 2 °C lower than the indoor air. In case study B with the façade-integrated outdoor unit of a mini-split domestic hot water heat pump, an overall heat transfer coefficient for the façade of 0.46 W/(m2 K) was measured. The most critical area was between timber beams and the cavity, where a heat flux of 27.5 W/m2 was observed and the surface temperature was about 1 °C lower than the indoor air temperature. Recommendations are made for proper installation of façade-integrated equipment in prefabricated timber façades to guarantee uncritical long-term operation.