The great energy-saving potential for renovation are public buildings constructed between 1930 and 1940. The energy index of modern movement public buildings is much higher in comparison to today’s standard for new houses. Current building regulations seek to improve the energy performance of all buildings, including existing ones, when altered and upgraded. Better energy efficiency can be achieved only by physical change to the building fabric. Differences in applying renovation measures result from the level of building conservation. Improvements to the building envelope, especially the introduction of thermal insulation measures, can be particularly difficult for architecturally important buildings and some alterations are often impossible to carry out without impact to the historic fabric.