Our work with listed properties, especially the restoration and renovation and therefor also the energy-saving building in the range of the preservation of monuments and historical buildings, lead our team to continuously develop exterior wall elements with very low thermal transmittance values. This is done to the point of passive house standardized exterior walls especially for listed buildings. In the process an energy-saving strategy comes into existence where all exterior walls are used as heat exchanger walls. Those walls are using the transmission heat, that would normally drift from the inside to the outside, in the exterior wall element to preheat the fresh air and induce it to the inside of the building.