There is always a heat balance at the exterior surface of a building: the rate of heat gain is equal to the rate of heat loss. The main components of the heat transfer to or from the surface are: conduction into or out of the wall, convection between the exterior surface and the ambient air, and radiation exchange with the surroundings as illustrated in Fig. 1. Each of these components depends on the surface temperature and as the conditions change the surface temperature varies in such a way that the surface heat balance is maintained. Thus, all of the components of the heat flux at the surface are interdependent and the calculation of any one component requires the simultaneous calculation of all the others. The most uncertain information in such a calculation is the convective heat transfer coefficient which relates the convective heat transfer to the difference between surface temperature and ambient air temperature.

The convection coefficient depends on the velocity of the air relative to the surface, and on the thickness of the air layer where the temperature is influenced by the surface. It is difficult to simulate these factors on a scaled-down model, so it is preferable to determine the coefficient by experimenting on various exterior surfaces of an actual building. This paper presents the results of such an experiment carried out on a 6-story building in Tokyo.