Commission, the UN commission for development and the environment, has recommended a great effort on more efficient energy use, so drastic that the industrial countries would half their use of energy. Already today, it is possible to reduce the energy use in buildings with the help of known and economical technology. The technical committee ISO/TC 163, Thermal insulation, of the International Organization for Standardization, ISO, has produced some of the tools we need in order to determine the energy flows in buildings.