Opinion: Daylighting prescriptions: Keep taking the pills?

Abstract

The role that daylight evaluation plays in the design process has acquired a new impetus as the need to demonstrate compliance with various ‘performance indicators’ becomes ever more pressing. Two of the most used rating systems are BREEAM and LEED which originated in the UK and US respectively, though they are both used world-wide. Until recently, the majority of codes worldwide were founded on the daylight factor. First proposed in the UK in the early 1900s, and formalised into building standards over fifty years ago, the daylight factor is simply the ratio of internal illuminance to unobstructed horizontal illuminance under standard CIE overcast sky conditions (usually expressed as a percentage). One of the key standards documents for daylight in buildings is British Standard 8206-2 ‘‘Lighting for buildings - Part 2: Code of practice for daylighting’’, and many guidelines refer to that. BS 8206-2 recommends that ‘‘the average daylight factor should be at least 2%’’. A slightly more exacting recommendation has featured in LEED: ‘‘75% of all critical visual task occupied space must achieve a Daylight Factor of 2%’’. Since maximum daylight factor values are rarely specified, the prescription of minimum daylight factor values in the majority of guidelines has inevitably led to a perception that ‘more is better’. The Carbon Trust have recommended ‘‘not less than 4%, preferably 6%’’ daylight factor. And one can find statements such as the following in various good practice guidelines for schools: ‘‘Maximise daylight factor to improve student performance (5% or more)’’. Compliance ‘chasing’ is now a major driver in building design and expert daylight designers rarely have any input. Fundamental design parameters that have enormous consequences for overall building performance are being selected on the basis of daylight factor evaluations - a climate and orientation insensitive metric that takes no account of sun. Consequently, the elusive balance between good daylight provision and effective solar control is often not achieved. Among the problems discovered in a detailed review by the Commission for Architecture and the Built Environment (CABE) of forty proposed designs for schools across England were classrooms which are too dark or prone to overheating on sunny afternoons. Extending the basis of the daylight factor approach by incremental means has proved problematic. Recent ‘clear sky’ options in LEED and ASHRAE provide no guidance on normalising the sky output, a significant omission since absolute values are now the