Energy efficient building envelopes. The role of the periodic thermal transmittance and the internal areal heat capacity to reach a high level of indoor comfort.

The necessity to decrease the building energy demand for heating and for cooling and to reach a high level of indoor thermal comfort in summertime is the subject of many current scientific researches. In particular, a recent study, carried out by Università Politecnica delle Marche (UNVPM), has shown that considering the values of superficial mass (Ms) and periodic transmittance (Ymn) of external walls is not sufficient in order to achieve energy savings and a high level indoor comfort in the summer period. For this reason experimental reference values for the parameter of internal areal heat capacity (k1) were introduced. The aim of this study is to understand the interdependencies between some thermal parameters (U, Ms, φ, Fa, Ymn and k1) of massive and lightweight external walls with respect to their summer energy performance in use. The study analyses in a first step eight external walls with good values of U and Ymn. Then the same selected walls have been modified in order to reach an average k1- value. The energy demand for heating and for cooling, necessary to ensure a defined level of indoor thermal comfort, has been verified by means of thermodynamic simulations applied to a virtual test room localized in two different Italian cities: Milano and Catania. The results of this research are: to show the relationship between the Ymn and k1 values of the selected external walls; to demonstrate that for both types of external walls, massive and lightweight, it is possible to obtain good k1 values, with easy and often economic building envelope design solutions; to describe some design solutions for external walls to improve their thermal performance and the level of indoor thermal comfort; and to quantify the energy demand for heating and cooling of the improved walls in comparison to the selected walls.