Low energy buildings should satisfied two main tasks: to assure a low energy use and provide an excellent residence comfort. The building envelopment elements, especially the transparent one, have major influence on both tasks. The paper presents research results of heat transfer and fluid flows through double pane window with tight, opaque insulation screen. The insulation screen was installed in such a way, that a semi open air gap was formed. An air gap is connected with the building interior through an opening on the bottom and with exterior through siphon at the top. This "S" shape air gap enables the reduction of the heat loses in the winter time by heat recovery through ventilating air and it reduces the heat loads during the summer as shading and solar chimney element.