During the renovation of an existing building keeping the original window and installing an added convex window is beneficial for the reduction of heating energy demand, furthermore this solution is free of the disposal and/or recycling of demolition waste. The energy need for heating can be further reduced, if the natural ventilation of the premise is through the buffer zone of the added convex window. In this case the fresh air is pre-heated in the buffer zone, increasing the utilised passive solar gain of the building. Measurements were carried out on experimental huts for analysis of this impact. Dynamic simulations of the huts have been carried out as well. In this paper the results will be presented, which proves the energetic effect.