The designer will probably also carry out a daylight factor analysis because it’s a snip to do, looks techie and they can charge the client for it – even if they take minimal notice of it themselves – John Mardaljevic
has unfortunately led to a conflation, in many minds, of actual daylighting performance with what the DF tells us. It is, of course, a proxy for daylight, but how good or bad a proxy depends on those important parameters that the DF approach cannot account for, such as prevailing climate (meaning the totality of sky and sun conditions) and building/ site orientation. The expert daylight designer does, of course, appreciate these intrinsic deficiencies. If sufficiently experienced, the designer can roughly guesstimate the likely daylighting performance of the space and so recommend suitable façade treatments to temper the luminous environment. Thus, the expert intuits what (in technocrat
speak) is called the spatio-temporal dynamics of natural illumination. We, of course, shouldn’t be surprised to learn that the designer recommends different treatments for the north, south and east/west elevations. Nor that the advice would change if the building were relocated from, say, Stockholm to Madrid. After all, ‘climate-adapted design’ is a notion that relates closely to vernacular architecture. The designer will probably also carry out a DF analysis because it’s a snip to do, looks techie and they can charge the client for it – even if they take minimal notice of it themselves. If, however, the client demands that the
daylight credit from a particular guideline document (BREEAM, LEED and so on) must be achieved, then the success of the design will hinge, to a large degree, on the nature of the target sought – invariably some measure
16 CIBSE Journal December 2012
based on the daylight factor. In that case, the best the designer can do is try to make good the failings that might – and often do – result from compliance chasing. The client may even decide that the expert is not required since the façade treatment will be ‘optimised’ by someone using a software tool: tweaking until the compliance target is reached. If the standards are proving to be insufficient to ensure good daylighting design, then we should look to improving them rather than ignoring or ditching them altogether. Climate-based daylight modelling (CBDM) is the prediction of luminous quantities founded on standardised meteorological files specific to the locale for the building under evaluation. CBDM delivers predictions of, say, internal illuminance on an hourly (or shorter) basis for a full year, accounting for the contribution from varying sun and sky conditions. Thus, it models how daylight is experienced: holistically – the illumination effect of sun and sky together. CBDM is more than a decade old and has been used effectively on a number of projects, large and small, from the New York Times Building to residential dwellings. Metrics founded on CBDM include useful
daylight illuminance (UDI) and daylight autonomy (DA). While as yet there are no target values for these metrics – they are currently under formulation/debate – designers have always remarked to me how much easier it is to understand the daylighting performance of a space from, say, UDI plots than trying to guess how a DF relates to actual daylight. Although it is work yet to be done, I’d wager a good supper that metrics founded on CBDM will be able to distinguish between what are generally agreed to be good, bad and mediocre daylighting designs. The intention is not to engineer out the
expert designer. Architect Lisa Heschong made a memorable comparison between design and gastronomy: ‘The standard should ensure that a minimum ‘nutritional’ value is achieved, while the chef (designer) imparts their own flair using the available “ingredients”.’ Not all buildings will get the design input of
the New York Times, and not all meals will be a la carte, but better standards can ensure that even our most commonplace buildings get a ‘good helping’ of daylight – well-balanced, avoiding both too much and too little.
l JOHN MARDALJEVICH is professor in building daylight modelling at the School of Civil and Building Engineering, Loughborough University. Go to
www.climate-based-daylighting.com for material by Mardaljevich on climate-based daylight modelling and a critique of daylight and compliance.
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