Building envelope and heat capacity: re-discovering the thermal mass for winter energy saving

In order to simplify the procedure for evaluating winter building energy performances, most of existing standard and rules make reference to the building energy balance under steady state conditions. In fact also the recent implementationof European Performance Building Directive(EPBD) in Italy is related to this simplified approach.Based on the above approach, the effect of thermal mass in building envelope is drastically reduced even if, as a matter of fact, the real heat transfer process in buildings construction hardly depends on materials thermal capacity.Moreover, it is common belief that the positive effect of thermal mass is associated only to building performance in summer season in warm climatic conditions.However, the well-established scientific literature (Olgyay 1963, Givoni, 1967, Szokolay 1980, ASHRAE 2001), as well as traditional architectural solutions, supports the importance of the thermal capacity also in winter conditions which is not considered in common building practices.In the frame of a research project on Massive BuildingEnvelope Thermal Performances, developed in Politecnico di Milano, a set of building energy dynamic simulations has been carried out. The effects of light, medium and heavy envelope constructions, having the same U-values, and the effects of the positioning of the insulation layer in stratified walls, have been evaluated referring to different building typologies, orientations, heating and cooling systems operational modalities in Italian climatic context.As a main result, the thermal capacity reveals its decisivecontribution for energy savings, both in winter and in summer conditions.