A novel approach to evaluate the thermal calibre of capacitive building envelopes using data envelopment analysis

Periodic thermal response of a building envelope is a function of the thermal boundary conditions, time and an interplay between the thermal resistance and thermal capacity of the constituent building materials. Thermal performance efficiency of building envelope is hence context sensitive. This study presents a novel approach for ranking the thermal performance efficiency of building envelopes to support building envelope selection in the early design stages. The study introduces thermal calibre as a performance metric which expresses the relative performance of a building envelope in a given boundary condition to that of a best -in -class performance. A theoretical simulation- Data Envelopment Analysis approach is used for ranking the efficiency, where minimizing the heat gains and maximizing utilization of the heat capacity were used as objective functions. The approach is demonstrated by considering 125 building envelopes applied to a residential building in a hot -dry climatic region. The thermal calibre of the envelopes vary from 0.65 to 1. Thermal calibre effectively captures the impact of thermo-physical property, location of thermal mass in the envelope system, as well as the variations in thermal boundary conditions due to orientation of envelope, seasonal and diurnal variations. Effect of thermal transmittance, external & internal areal heat capacity of the envelopes and fenestration size on thermal calibre is presented. A decrease in U -value is associated with a high thermal calibre. External areal heat capacity of 100-119.99 KJ/m2K and internal areal heat capacity of 70-79.99 KJ/m2K yields maximum thermal calibre. Based on the data envelopment analysis, the envelopes are classified into four performance efficiency classes. This approach yields a set of pareto solutions which can be used for building envelope selection during design stage.