An attempt at the verification of simple designing rules of chosen building components is made in this paper, taking into account not only heating energy demand but also avoidance or at least reduction of cooling demand. Large south-oriented windows supply a large amount of solar energy and thus minimize the heating demand. Oversized windows and the resulting large energy gains, which are not accumulated in building’s thermal capacity and used immediately for heating, do not reduce energy demand but create unbearable conditions or a big cooling load in living spaces. A green flat roof is today a very popular solution. Its insulating and dynamic properties have been evaluated.