Fenestration systems are important elements of building facades. Although there are fenestrations placed at different angles in facades, not only vertical and roof glazing, there is little information about calculating procedures of thermal resistance and thermal transmittance for reversed glazing in which heat flows in a downward direction. The aim of this paper is to propose some amendments to EN 673-Glass in building - Determination of thermal transmittance (U value) - Calculation method [1] as well as to ISO 15099 - Thermal performance of Windows - Detailed calculations [2] to improve calculation accuracy for glazing under downward heat flow direction. In the paper CFD modeling of glazing thermal transmittance is presented. CFD calculated data were then validated by measurements performed in a calorimetric chamber in a test stand prepared for thermal transmittance measurements under different glazing angles resulting in a downward heat flow direction. Measurement data are then compared to simulation results. After achieving satisfactory agreement, some additional simulation results are presented and some amendments to glazing thermal resistance calculation procedure given in EN 673 and ISO 15099 standards are proposed. (C) 2016 Elsevier B.V. All rights reserved.