In this study, the effect of window-to-wall ratio of a residential building is evaluated for different internal volumes of a by lumped capacitance approach. The computations are carried out for different window-to-wall ratios varying from 0% to 100%, internal volumes (namely, 45m3, 75m3 and 105m3) and insulation material thicknesses varying from 10mm to 30mm. The influence of the considered parameters on the indoor temperature of the room having a double-pane window is evaluated together with time lag and decrement factor. Computed results show that the window-towall ratio has a very significant effect on the considered parameters. Time lag increases and decrement factor decreases with the increase in insulation layer and indoor volume. However, the most significant parameter is window-to-wall ratio. As the window-to-wall ratio increases, time lag decreases and decrement factor increases considerably. The minimum decrement factor is obtained as 0.05 for 105 m3 indoor volume and 0% window-to-wall ratio whereas the maximum decrement factor is observed 0.98 for the 100% window-to-wall ratio. The effect of insulation layer thickness on the considered parameters is more profound particularly for low window-to-wall ratio values.