From the point of view of monument preservation, but also for reasons of resource conservation, the focus when renovating historical windows is on preserving the construction. At the same time, however, the windows must also meet the demands of today's living comfort and energy efficiency. The preservation of historical buildings and energy efficiency are not necessarily contradictory - solutions ranging from the careful repair of frames and glass to constructive energy optimisation with varying degrees of intervention in the substance, to complete replacement with replication can raise the energy standard of an existing window to that of a modern window, while at the same time upgrading it to meet the requirements of a listed building.

When renovating historic windows to improve their energy efficiency, it is often helpful to evaluate the renovation measure thermally. The two calculation tools were developed within the EU FESR PLANFenster project to evaluate the thermal transmittance and the energy saving potential.