The determination of the surface conductance of metal deck roofing

Abstract

The heat flow through a surface exposed both to the air and to solar radiation is discussed. The heat load on a building under these conditions is commonly calculated by using a “sol—air temperature”, being the hypothetical air temperature which would cause the same heat flow as the actual design conditions. The sol—air temperature can be calculated if the values of solar radiation, absorptivity and surface conductance are known. It is shown that the sol—air temperature and surface conductance vary with (1) the nature of the surface, and (2) the intensity of the incident radiation, and are therefore not capable of being independently established. In this paper, the concept of a “black sol—air temperature” is introduced, being the equilibrium temperature reached by a matt black body under the specified exposure conditions, and insulated on all other faces. The surface conductance is then redefined and is shown to be a function of the absorptivity and of the black sol—air temperature. A series of experiments on aluminium, galvanised iron, and white-painted sheetmetal decks is described.