By focusing on the effect of inclination angle on the thermal transmittance of glazing systems, this paper investigates the flow structure and heat transfer of air-filled uncoated multi glazed windows since the design of inclined windows are mostly considered as vertical ones in the construction sector. Revealing the influences of tilt angle on the energy saving performance of these type of window units is one of the main objectives of this study in which the examined parameters are not limited to inclination angle, which is considered from theta = 0 to 90 degrees, but also includes air layer thickness ranging from L = 6 to 18 mm and number of panes (namely, double, triple and quadruple pane window units, without considering the window frames and edge seals). The outcomes of the computations are presented by streamline and isotherm contours which give a solid opinion about the flow and heat transfer inside the gap of the glazed windows, and also U-values which provide a quantitative understanding. The results showed that the air layer thickness optimized for vertical glazing units can be considered for inclined installation on condition that theta >= 45 degrees. However, at lower inclination angles the air layer thickness should be optimized considering the inclination angle and number of panes as increasing the layer thickness above L > 9 mm may increase the thermal transmittance of glazing unit. The risk of increasing U-value with larger air layer applied to inclined surface reduces by using more panes. (C) 2020 Elsevier B.V. All rights reserved.