Heat transfer through anchoring elements in a rear-ventilated rainscreen insulation system for façade retrofit

Rear-ventilated rainscreen façade insulation systems are becoming a popular retrofit solution in Southern European climates, due to their good thermal performance in both winter and summer conditions. In these assemblies, the substructure of the new cladding is anchored to the original building, thus puncturing the thermal insulation. This study assesses the impact of such thermal bridges on the overall thermal performance of the external wall. The renovation of a conventional Spanish wall construction with a ventilated façade is used as a case study, featuring an anchoring system with L-shaped aluminium brackets. Two scenarios have been assessed: in the first one these brackets are fixed onto both the reinforced concrete slabs and the existing brickwork of the building, while in the second one they are anchored solely to the structural slabs. Simplified one-dimensional calculations are compared with two-and three-dimensional numerical models. Results indicate that anchoring elements can account for a substantial increase in heat flow. If the additional heat transfer through the anchoring elements is not taken into account, the energy savings delivered by the renovation could be considerably lower than expected by calculations, resulting in an increase of energy consumption over predicted values.