Energy performance characterisation of vented opaque envelope through simplified methodologies

Opaque vented façades are innovative and widely-used technological systems adopted both in new constructions and in building renovations. According to European Directive 2010/31/EU (EPBD recast) on the energy performance of buildings, each Member State should give priority to passive cooling techniques in order to enhance building performance during the summer period. For this purpose opaque vented envelope could be an appropriate technological solution to reduce the summer peak loads and the energy consumption. Although the EPBD recast has submitted the development of calculation methods for the energy performance evaluation to the European Committee of Standardisation (CEN), there is a lack in European Standards on the calculation of non conventional building envelope performance, including vented façades. The object of the present work is the thermal performance characterization of vented vertical opaque enclosures in real conditions of use, through simplified parameters. Starting from EN ISO 6946 and EN ISO 13786, new equivalent thermal parameters are defined, such as the equivalent steady state thermal transmittance, the equivalent periodical thermal transmittance and the time shift. Equivalent parameters are obtained by evaluating surface inside face conduction in the opaque components, under stabilized periodic external conditions, for the summer design day. An equivalent outside temperature is used, which considers both the convective and the radiative thermal exchanges (solar and infrared waves), for different boundary conditions (orientation). The tool used for calculations is based on the conduction transfer function – CTF – method, as implemented in the thermal dynamic simulation program Energy Plus. Through a sensitivity analysis, different opaque enclosures are analyzed, varying the design parameters such as the thickness, the height and the length of the vented cavity.