Real-world validation of climate-based daylight metrics: mission impossible?

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Session 3 Paper 3, Heriot-Watt University Edinburgh, 14-15 April 2016

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One of the criticisms of metrics founded on climate-based daylight modelling (CBDM) is that they are unverifiable in practice. This criticism received some attention following the decision in 2013 by the Education Funding Agency to make CBDM and the useful daylight illuminance (UDI) metric a mandatory requirement for the evaluation of designs submitted for the Priority Schools Building Programme. Some of the difficulties related to the validation of CBDM metrics apply also to daylight factors. However, several other challenges need to be addressed and practical solutions found before any attempt at validation of CBDM metrics can be made. This paper identifies those challenges and describes a framework for the practical evaluation of daylighting performance in real world settings, and thus a basis for the validation of CBDM metrics. The task of validation requires a conflation of state-of-the-art techniques in measurement and modelling. Measurement techniques under consideration include high dynamic range imaging and ‘smart sensors’. A key obstacle to real world validation in, say, classrooms is that it is often not possible to rely on measurements of illuminance taken on the horizontal plane because such locations are rarely free from disturbance during normal use. It becomes necessary therefore to measure illuminance at more reliable locations (e.g. walls) and use these as a proxy for illuminance performance on the horizontal. The relation between wall and desk performance is space-specific and can be determined using CBDM. The first steps towards practical application of this framework are described.