Building age is the greatest determinate of the energy efficiency of Fin- land's building stock. In general, buildings constructed during the rapid urbanization of the 1950's, 60's and early 70's are the least efficient buildings of any decade either before or after. In fact, apartment buildings built at the turn of the 20th century and during the 1940's are, as a class, the most effi- cient in the country. Those built during the 1970's energy crisis are among the least efficient.