For calculating the flow of heat through walls of buildings the steady state formula based upon average temperatures has been found inadequate. A solution for the periodic heat flow through such walls is given, which has been simplified so that it depends upon two dimensionless quantitiesfand &bgr;1, which are functions of the physical properties of the walls and the periodicity. Curves are given which make it possible to read off the solution directly as soon asfand &bgr;1 are known. An experimental check is given.