Opinion: Climate-based daylighting modelling in practice

A recent project sponsored by the BRE Trust and CIBSE SLL has examined the utility of new metrics for daylight design, based around climate-based daylight modelling (CBDM). CBDM uses concepts like useful daylight illuminance (UDI) to categorise daylight provision. UDI is the proportion of the year for which daylight illuminance falls within a ‘useful’ range. One of the most interesting parts of the project was a workshop organised with the Education Funding Agency (EFA). The EFA has incorporated CBDMrequirements in its specification for new school buildings. In the workshop, consultants involved in modelling and advising on new school designs shared some of their experiences. Generally their response was positive. UDI was seen as a valuable metric, because it encouraged them to think about shading as an integral part of the daylight design process. In school classrooms, it led to designs with appropriate shading, good uniformity, and where possible high-level glazing and either rooflights or windows in more than one wall. However, our study also showed that there were issues to be addressed if CBDM is to be used more widely. First, the definition of UDI can vary. The target illuminance above which daylight is deemed sufficient, and the maximum illuminance, above which it is seen as excessive, vary between authors and standards. Further, UDI may refer to the illuminance at a particular point within the space or an average value over the space. Also, yearly operating hours can vary according to the type of building. The EFA’s requirements are realistic because they are based on the school day; a similar criterion based on office or daylight hours might be impossible to achieve. Thus, CBDM recommendations which look similarmay result in quite different levels of daylight provision in different applications. Second, the detailed calculation methods used to generate UDI can affect the results. The choice of grid size, the use of an offset excluding points close to the walls, the time step for calculation, the weather data file and assumptions about the way shading is operated can all affect the calculated UDI. Third, other researchers have questioned the basis for the approach, particularly the idea that high illuminances automatically are not useful, and there is still uncertainty about what the useful range should be. The type of space needs to be taken into account. In particular, UDI is a less appropriate metric in domestic type spaces where high illuminances could be welcome. Finally, CBDM is much more complex than simpler methods based around average daylight factor. Some of the consultants saw this as a bonus, because it meant that developers had to involve a daylight specialist if they were to meet the requirements. But there is a danger that daylight design could be limited to certain specialist building types, or those with an unusually demanding, or enlightened, client. Simpler methods still have their place, because they allow for a basic level of daylight design input to a much wider range of buildings. A final report for the project will be published on CIBSE’s Knowledge Portal.