Decisions taken in the early stages of architectural design have an important impact on energy demand and efficiency. In this design phase, architects and engineers need to manipulate common concepts that reveal the influence of building, comfort criteria, and climate on energy consumption. Such a concept is the indoor temperature of the free-running building, which combined with comfort criteria and climate, allows to assess the relative weight of heating, ventilation and air conditioning systems in the energy budget. Based on this concept and on the probabilistic distribution of the outdoor air temperature, a method was developed that indicates the heating and cooling needs, and gives a measure of the energy saved for cooling when ventilation is used. This method also gives the load for which the mechanical systems should be designed to obtain maximum efficiency and shows the feasibility of stack-induced natural ventilation. Weather data needed for this method are available in public domain from satellite investigation. The method can be applied when buildings similar to the existing ones are constructed in a new location, when existing buildings are retrofitted or when completely new buildings are designed.