The recent publication of CIBSE's Research Insight Report 'Daylight Calculation Methods in BS EN 17037' represents an important step towards the regularisation of daylight performance testing.
The British Standard for daylighting, BS 8206-2, was replaced by the more complex BS EN 17037 in 2019, which introduced climate-based daylight modelling for more accurate daylight predictions. Associated which this change in methodology is a shift towards more computationally intensive methods of daylight prediction. Recognising that the results of computational daylight prediction are only as reliable as the inputs which are used to define the underlying simulations, CIBSE set out with this report to the increase the repeatability of daylight testing by steering practitioners towards a narrower set of recommended inputs.
The research project involved extensive modelling and analysis using various computational tools, including DIVA-for-Rhino, ClimateStudio, and Honeybee[+], to calculate daylight factors and daylight autonomy for different room types and orientations. Initial studies focused on determining optimal computational parameters, ground plane size, calculation grid size, and weather datasets. The main analysis used the four-component method in DIVA-for-Rhino, with additional calculations performed using progressive path tracing and the two-phase method to compare results.
The findings revealed discrepancies in daylight provision estimates between the different computational methods, with progressive path tracing generally overestimating and the two-phase method underestimating compared to the four-component method. Despite these differences, the study showed that meeting the standard's recommendations was more feasible using the daylight illuminance-based method, which accounts for site-specific weather data and orientation, rather than the orientation-independent daylight factor method.
The study also compared the ease of meeting the daylighting recommendations of the new standard against the superseded BS 8206-2 and the UK National Annex to BS EN 17037. It was found that the targets set by the UK National Annex were more attainable, with the daylight illuminance-based method being the preferred approach for both office and residential spaces. The research suggested that the daylight illuminance-based method better reflects real site characteristics and should be the method of choice for daylight provision calculations.
Lastly, the research provided recommendations for practitioners on the use of computational parameters in Radiance, the inclusion of a ground plane in models, the size of calculation grids, and the selection of weather files. It emphasized the need for high-quality computational settings to ensure accurate results and suggested that these settings may need adjustment based on the complexity of the building models. The study recommended using fine grids, appropriate wall offsets, and weather files closely matching the site's location to enhance the precision of daylighting assessments.
GV8 would encourage all daylight consultants working at present to take note of these new recommendations as they will help to ensure that the predictions presented in daylight reports can be confidently relied on.
The report can be accessed here.
