The performance of heat transfer through double pane window has been investigated using experimental and numerical methods. The range of the gap distance between glasses are 0.5-10 cm. The convection heat transfer plays the dominant role for the case of the wide gap distance together with the large Rayleigh number and thereby, reduces the enhanced thermal resistance due to the increased air gap, while the conduction heat transfer does the major role for the case of small gap distance. In order to enhance the thermal insulation of the double pane window, the heat transfer of triple pane window, which is constructed to put one more glass at the middle of the double pane, is investigated to check the reduction of the convection heat transfer together with the effect of the radiation shield due to the presence of the additional glass. Further, a spacer is installed at the middle height of the double pane, and the effect of which on the suppression of the convective heat transfer is analyzed carefully, using experimental and numerical methods. For the case of the spacer-installation, the amount of energy saving is considered about 10%, but the energy saving increases a lot to 30-50% for the case of triple pane window, due to the substantial radiation shield effect of the presence of the additional glass.