Air flows between prefabricated insulation modules and the existing façade: A numerical analysis of the adaption layer

Prefabricated lightweight envelope modules have the potential to renovate existing buildings in a fast way, by simply installing the modules against the existing façade. In order to accommodate all imperfections, the new elements are equipped with an adaption layer, typically executed in a compressible insulation material. Even though this seems to be a simple solution, air flows in this layer might adversely influence the thermal performance of the envelope module. The impact on the overall thermal performance of e.g. compression rate of the adaption layer, leveling of imperfections and airtightness of the new and old building envelope, is not clear. In this paper, two configurations of the adaption layer (as a strip located at the imperfections or as a continuous layer) on three types of existing façades (imperfections of ±50 mm and ±200 mm and a façade 'out-of-plumb') were evaluated. Next to that, the impact of the compression rate of the adaption layer and of the airtightness of the new and old building envelope was assessed. To model the compression rate, the airflow resistance Rair (Pa.s/m²) of mineral wool samples was measured (EN 29053) and used as input in the model. From these simulations, the adaption strip seems to have potential if the airtightness of the joints between the prefabricated modules is guaranteed. In case the joints are not airtight, the compression rate of the material in the adaption strip can improve the thermal performance, but the effect is limited. Nonetheless, with air leakages from outside it is advised to fit the entire surface of the existing façade with the adaption layer. Future research will further explore the potential of an adaption strip at façades with large imperfections, through simulations in dynamic weather conditions and in-situ measurements.