An empirical correlation for the outside convective air-film coefficient for horizontal roofs

From measurements of surface heat transfer on the roofs of two commercial buildings in Northern California we have developed a correlation that expresses the outside convective air-film coefficient for flat, horizontal roofs as a function of surface-to-air temperature difference, wind speed, wind direction, roof size, and surface roughness. When used in detailed building energy analysis programs, this correlation is expected to give more accurate calculation of roof loads, which are sensitive to outside surface convection. In our analysis, about 90% of the variance of the data was explained by a model that combined standard flat-plate equations for natural and forced convection and that took surface roughness into account. We give expressions for the convective air-film coefficient: (1) at an arbitrary point on a convex-shaped roof, for a given wind direction; (2) averaged over a strip along the wind direction; and (3) averaged over a rectangular roof for a given wind direction.