Envelope first / Inside later: Aperture sunlight and skylight indices

This paper describes a back-to-basics rethinking for quantifying the sunlight and skylight potential of building apertures. The recently formulated sunlight beam index (SBI) approach has been conflated with a complementary metric called the aperture skylight index (ASI). The sunlight beam index is a measure of an aperture’s ‘connectedness’ to all of the annually occurring possible sun positions where sunlight can be incident on the aperture. In a complementary fashion, the aperture skylight index is a measure of an aperture’s ‘connectedness’ to the hemispherical sky vault. In the absence of any localised shading/obstructions, the sunlight beam index depends on the location, orientation (i.e. azimuth angle), aspect (i.e. zenith angle) and size of the aperture. The SBI is the cumulative measure of the cross-sectional area of sunbeam that can pass through a window aperture. In contrast, the ASI depends only on the size and aspect of the aperture in addition to shading/obstructions – it has no dimensions of time and it is not dependent on the azimuth orientation of the aperture relative to the hemisphere of sky. Obstruction of the aperture’s ‘connectedness’ to either the sky or the entirety of possible sun positions can be due to: features integral to the facade (e.g. external window reveal); shading devices; or, any external structures. The effect of obstructions, whatever their origin or complexity, is automatically accounted for in the computation of the indices – the only requirement is that they are included in the 3D model of the scene. The approach provides designers and product specifiers with an intuitively simple “aperture rating system” to evaluate and compare the in-situ performance of building apertures in combination with any shading system. Application of the rating system is demonstrated for a residential dwelling.