September 25th 2018

Climate Based Daylight Modelling – Why This is the Assessment You Should Use

Climate Based Daylight Modelling – Why This is the Assessment You Should Use

Climate based daylight modelling (CBDM) is the prediction of various radiant or luminous quantities using daylight conditions derived from standard meteorological datasets. Climate-based modelling delivers predictions of absolute quantities (e.g. illuminance) that are dependent on the location and the building orientation, in addition to the building's composition and configuration.

In 2013 the UK Education Funding Agency (EFA) made 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 (UDI) metric.

However, CBDM can be used for all building types and not just schools. Rating systems such as LEED v4 and the WELL Building Standard are now driving this approach more and more. For LEED you can run analysis on various design stages such as office core and shell and new construction as well as for various different building types including, Schools, Commercial and Healthcare using a dynamic daylighting approach as part of its EQ Daylight option 1. Achieving daylighting levels to meet the criteria of these rating systems is not always straightforward. These schemes require realising a balance between capturing good daylight whilst maintaining occupant comfort.

What to Consider

The CBDM method for LEED and WELL is considerably different from previous daylight modelling approaches. It includes 2 types of calculation which complement each other towards a satisfactory visual comfort for the design: sDA (Spatial Daylight Autonomy) and ASE (Annual Sunlight Exposure). These complimentary calculations are intended to ensure that a good balance of daylight and occupant comfort can be met. CBDM is a more complex calculation but does not require much more detail than other daylighting approaches. Daylight Factor for instance assumes an overcast sky at a static period of time, which is unrealistic of the overall annual space performance. Whereas CBDM, takes into consideration the whole year and the seasonal variations of sunlight throughout.  The effects of surrounding buildings, local topography, vegetation, etc. are all taken into account to complete the picture.  Indeed this is all before standard building features, shades, window area and glass transmission.

Performance modelling experts often have a misconception they can pass CBDM calculations regardless of this.  However, all of these factors combined could quite quickly become a headache for the architect as more and more parameters enter the mix.  Early intervention in the design development is key to relieving this problem.  By developing an understanding of the site as early as possible, with fixed parameters such as surrounding buildings and topography being assessed to establish their impact, then the risk impact is reduced.  Once the site has been tested, the design can be refined by orientation to maximise the building performance both visually and thermally for the occupants.  For example the façade will need particular solutions to suit its variation in orientation.

This is a common issue with building design. Uniformity all the way round with no customised approach for each of the different orientations as mentioned above. This is one of the weaknesses the CBDM calculation will expose. One façade might handle the conditions brilliantly, whereas another could be almost overexposed based on the amount of sunlight that is pouring over the building.  This would then transfer into real life operation where some zones would perform well and have ideal visual daylight for the occupants but perhaps just a short distance away a number of other occupants would be consistently operating their blinds.

The Knock-On Effect

This in turn would have a knock on effect on the thermal assessment because the building is not operated the way the design model intended, which causes performance gap issues between the model and actual performance. In short, occupants are operating the building’s features differently from the design team’s intend because there was minimal consideration for something as simple as orientation risk. It is a big issue and by performing effective daylight analysis right from the beginning then more accurate reflection of daylight capture is available and therefore what the risks are and how shades would be best included.

Each building has its own unique characteristics and so will have its own unique solar exposure map and captured daylight. There can be no assumption that a new design would perform the same as a previous building. It is difficult for people to perceive the magnitude of daylight a space captures across the year because we sense at one moment. However, as designers and building modellers we can use annual performance analysis CBDM offers us to show where dark and overly lit areas persist. This data can be used for space activity optimisation, looking at where people and equipment should be placed.

How IES Can Help

IES Consulting can help you explore the opportunities for daylight optimisation throughout the design. We can work with you right from concept using the latest in analysis technology to communicate visually and statistically the impact of a potential design strategy. Our daylight optimisation approach works toward an improved environment and enhanced productivity. Project examples include Projet-De-Bureaux in Paris, Sceneo in Bezons, Paris and Schneider Electric - Xpole in Grenoble, France.

To enquire about our Climate Based Daylight Modelling consulting services visit