Lessons learned from climate based daylight modelling (CBDM)

Since the Education Funding Agency (EFA) announced several years ago that new and refurbished school projects would henceforth be assessed for natural daylight using climate-based methods, the world of natural daylight design has been turned upside-down. New software has been released by a handful of keen specialist providers (and one or two household names), decades-old rules of thumb have been thrown on the bonfire of progress, and daylight related hate crimes are on the rise.

Well, I may be exaggerating somewhat, but it certainly seems to have come as a surprise to the design community, while at the same time, having gone almost unnoticed by many. On the other side of the pond, the LEED standard has quietly adopted a comprehensive climate-based approach to natural light in its assessment protocols, and the US market has equally quietly adapted to this new paradigm. So it seems that climate-based daylight is here to stay, and consensus is building gradually as to its advantages over the daylight factor and other methods.

So, what of school designs for the UK? Until BREEAM catches up with LEED (surely inevitable in the end), the only buildings in the UK which mandatorily require CBDM are new and refurbished schools funded by the EFA. The EFA has provided model schemes on which providers can base their designs, though these have at the least been architecturally controversial. But what of our old rules of thumb? How does a real design team muddle through this new situation without even a basic idea of how CBDM will measure their design?

To answer this question, and to test some recently released CBDM software, we have assessed four possible ‘standard’ classroom designs to see which can meet the new EFA criteria for good natural light. This was part of a larger study, in order to develop design rules of thumb. Each classroom was the same standard geometry, and had low-e double glazing of 0.8 visible transmittance. Internal finishes were the fairly standard reflectance values of 20-50-80% for floor-walls-ceiling.

Depending on how many classrooms of recently built schools you have visited in the summer, you may or may not be surprised to find that the more sophisticated CBDM approach to natural light suggests that we may have been over-glazing some of our classrooms.
The ‘standard classroom’ achieves a helpful 4.2% average daylight factor – in line with industry best practice for a single-sided space – but using CBDM (The UDI-e and UDI-a metrics in particular – we don’t consider Daylight Autonomy here) reveals that the room is over-lit for 20% of occupied hours, i.e. too bright, so ‘blinds down, lights on’. Worse still, when we add rear skylights to this space, the over-lit proportion rises to 40% of the occupied year. This means that our beautifully lit exemplar of natural light probably has the blinds down for at least 50% of occupied hours, as users do not tend to raise and lower manual blinds optimally. All this is in addition to the roughly 10% of hours where there is insufficient external light available, so the lights are on anyway (see table below for more information).

Less surprising is the North-facing classroom, which achieves the same 4.2% average daylight factor, but now is over-lit only 3% of the time when assessed with CBDM using the UDI metric. Perhaps we should face all our classrooms North from now on…

… But since space planning will always require a variety of teaching space orientations, there is another option: less glazing to South-facing classrooms. Based on our standard classroom test, optimum annual daylight was achieved with just 25% of the South-façade glazed, and modest clerestory North-lights. Alternatives include brise soleil, light shelves, overhangs and other methods of shading, which are assessed in more detail in our ‘daylight mind map’, shown as a sample below – please contact us if you would like more information on this tool, or associated training and consultancy.

So, in summary, CBDM quickly shows us what instinct and experience has said for many years, and with much greater precision: glazing strategy for teaching spaces needs to be orientation sensitive, with particular care taken not to over-glaze South-facing spaces. As an added benefit, this is the same conclusion reached when considering summer overheating of naturally ventilated spaces, so we may find ourselves achieving both good natural light, and good temperature control, with the same design strategy. This is beginning to sound holistic…

What a pleasure to finally have a comprehensive tool at our disposal for sensible natural daylight design – let’s make sure it gets used on every project.

Please Click Here to view the Daylight Mind Map.

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