EN ISO 13790 is the international standard used for the calculation of the thermal energy requirement of buildings. The evaluation of solar gains through the glazed surfaces is done with a simplified procedure that determines the transmitted solar energy through the transparent surfaces as the product of the normal incidence solar gain coefficient g(gl),(n) and a correction factor F-W, that considers the directional aspects of the beam solar radiation. In the absence of adequate national guidelines, the correction coefficient F-W in EN ISO 13790 for non-scattering glazing is set constant and equal to 0.90. In this paper, the results obtained by the TRNSYS computer code demonstrate that a generalized use of a constant value of F-W = 0.90 provides significant deviations. Therefore, a correlation to evaluate a variable F-W coefficient, in function of the incidence angle of solar radiation and of the number of glass panes, is reported. The accuracy of the obtained results was verified comparing the calculated values with those provided by the TRNSYS software. Finally, with reference to two buildings located in two localities with different weather data, the thermal energy needs by applying the EN ISO 13790 procedure with F-W = 0.90 and with variable F-W have been compared. (C) 2015 Elsevier BM. All rights reserved.