Analysis of a New Index for the Thermal Performance of Horizontal Opaque Building Components in Summer

Abstract

The summer behavior of an opaque building component subjected to the solar cycle depends on the combination of its thermal insulation, inertia, and solar reflectance. To rate the component dynamic behavior while an air conditioning system ensures a steady indoor temperature, a ‘solar transmittance index’ (STI) has been proposed. This is a component-based index calculated from a ‘solar transmittance factor’ (STF). STI takes into account the radiative properties at the outer surface and the thermophysical properties and layer structure of the materials beneath. It correlates the peak heat flux and temperature at the inner surface, relevant to cooling energy and thermal comfort, to the peak solar irradiance. Similar to the well-known ‘solar reflectance index’, STI is determined comparing the STF with two reference values, corresponding to a performance relatively low and very high, respectively. Thanks to its simplicity, the approach may allow defining easy to apply requirements to prevent building overheating, improve indoor comfort, reduce cooling energy demand, and mitigate some fallouts of the urban heat island effect. In this work, focused on roofs above occupied attics, peak heat flux and ceiling temperature are calculated by numerical simulation and compared with STF values for a wide range of roof types.

Bioinspired daytime radiative cooling performance: a new experimental protocol for scaling up passive cooling through the atmospheric windows

“…In this regard, the Solar Reflectance Index (SRI), developed by Akbari et al [4][5][6][7][8], offers a standardized metric for assessing the thermal behavior of surfaces under solar radiation. This metric has been widely adopted in rating systems and regulations, enabling fair product comparison and assessment of building performance.…”