It is hard to imagine the modern market of enclosing structures without the double-skin facades (DSF). Double-skin facades have established themselves as a multifunctional system capable to improve the energy efficiency of buildings. The air gap under the facing is responsible for allocating the moisture from the construction. In the design of modern buildings, special solutions for enclosing structures can be applied. For example, the combined facades which includes DSF and a glass curtain wall. As a result, the air gap with variable width is formed. The purpose of this work is hydraulic calculation of the structure of the vertical air gap with variable width. It was proved that loss-reducing effect in air gap with variable width fails when building has large height and insufficient length of cantilevers. It was also proved that in the small gap free-convective flow is impossible. The results of this work may be found as practical application in the design of similar buildings and analysis of free-convection flow in the air gap of DSF.