Attic floors are a favoured solution in refurbishing/rehabilitating residential buildings as well as for new buildings in Romania, under the conditions of climate changes and especially for adapting to the global warming phenomenon. Optimizing the thermal protection during the hot season can be made by a layer of ventilated air disposed directly under the roof covering. In order to analyse the ventilation process, and to establish design recommendations the authors have used a physical small scale model, in laboratory conditions, according the Grashof criterion. The paper is presenting a profitable utilization of similitude conditions leading to opportunities of analysing the indoor ventilation in passive system for a better energy efficiency.