Climate-based daylight modelling and its discontents

In 2013 the UK Education Funding Agency (EFA) made climate-based daylight modelling (CBDM) a mandatory requirement for the evaluation of designs submitted for the Priority Schools Building Programme (PSBP). School designs submitted to the PSBP must achieve certain ‘target’ criteria for the useful daylight illuminance metric. This is believed to be the first major upgrade to mandatory daylight requirements since the introduction of the daylight factor more than half a century ago. In the US, a climate-based daylight metric approved by the IESNA has appeared in the latest version of LEED. Perceived as long overdue in some quarters, in others the EFA decision was seen as controversial and is not without its critics. Whilst it may appear that the case for CBDM has effectively been made, and that wider adoption in standards and guide- lines is likely, it is important not to ignore or dismiss out-of-hand the critics of CBDM. Nor should it be overlooked that CBDM and the metrics derived using it are both still evolving. This paper: reviews the recent developments; the reactions to them; and, forecasts what might be expected in the near future. Attention is given to the formulation of the PSBP requirements for daylight and how the various stakeholders have responded to this major new development in building codes.