Aperture-Based Daylight Modelling: a new direction for daylight planning

Professor John Mardaljevic BSc PhD, Professor of Building Daylight Modelling, School of Architecture, Building and Civil Engineering, Loughborough University

Paul Bennett talks to daylight modelling pioneer John Mardaljevic about his latest work and its potential to provide a more reliable basis for sunlight/daylight planning than currently used methods.

NOTE: The full version of this article, complete with John's images and diagrams is available in the desktop version of Daylighting Magazine issue 20 at www.daylightingmag.co.uk

Since pioneering what is now widely known as Climate-Based Daylight Modelling (CBDM), John Mardaljevic, Professor of Building Daylight Modelling School of Architecture, Building and Civil Engineering at Loughborough University, has remained at the forefront of research and academic studies in this complex and increasingly widely-referenced field.

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 metric (proposed by John in 2005). This is believed to be the first major upgrade to mandatory daylight requirements since the introduction of the daylight factor more than half a century ago and CBDM remains the predominant basis for research and, increasingly, industry practice worldwide. Since 2018 the requirement was extended to all school buildings funded by the EFA.

Differences of opinion...

In January 2019, in his role of Chair at a meeting of the CIBSE Daylight Group (Chartered Institution of Building Services Engineers), John attended a presentation by UK/EU practitioners in London introducing EN 17037, the first Europe-wide standard to deal exclusively with the design for, and provision of, daylight in buildings. There were stark differences of opinion regarding the daylight part - a small minority were in favour of a national annex essentially keeping BS8206. The majority however favoured the 17037 methodology for daylight, which was reflected in a revised, ‘light touch’ national annex.

In contrast, there was little discussion of the parts of the standard relating to sunlight and planning, perhaps because there was little that was substantively different to what had gone before. However, as a consequence of serving on the panel, John had formed misgivings about the various schema for sunlight and daylight planning, both in the EU standard and guidelines such as BR 209. The rationale for these were conceived ‘in a pre-digital age’ and, in John’s view, produce results which have little connection to realworld performance. The reasons for this are many, though perhaps the most significant is that, for sunlight, the size of the window opening is not accounted for in the traditional approaches.

John concluded that a re-think was necessary, exploring options for taking a back-to-basics approach which would result in a complete reframing of the basis for evaluation. What emerged was a method that was conceptually very simple and yet able to cope with any plausible real-world building complexity located in any scenario/ context.

A simple line-of-sight calculation

The new approach, now called Apertute-Based Daylight Modelling, had three stages of development. “I decided first to look simply at the potential of a given aperture to deliver sunlight. Currently, the guidelines take no account of the size of the aperture”, said John. “The concept of aperturebased modelling starts with a simple line-of-sight calculation between a grid of points on the aperture (window or skylight) and the sun position repeated many times for all the possible sun positions, to arrive at a cumulative annual Sunlight Beam Index (SBI). The SBI, measured in m 2 hrs, is a measure of an aperture’s ‘connectedness’ to all of the annually occurring possible sun positions where sunlight can be incident on the aperture”.

He continued: “The next stage was to devise a complementary measure for skylight, which I called the aperture skylight index (ASI). This is an area measure of the ‘connectedness’ of an aperture to the sky vault in terms of the amount of illumination received from a uniform luminance sky dome, measured in (normalised) units of lumens. As with the sunlight beam index, ASI takes into account the effect of any form of obstruction, e.g. exterior window reveals, overhangs, surrounding buildings, etc. The uniform sky used was normalized so that a 1m 2 horizontal aperture (e.g. rooflight) with no obstructions would receive 2000 lumens – this is the measure of the apertures ‘connectedness’ to the sky. The same 1m 2 aperture vertically (i.e. a window) would receive 1000 lumens because the aperture now sees half the sky compared to the rooflight. Using the lumen in this way appears to be entirely novel.”

However, that wasn’t the end of John’s story:

The Eureka moment

“The ‘Eureka’ moment came early in 2019 when I realised that the numerical value used as the basis for the Aperture Skylight Index (i.e. the lumens received at the aperture) can also be thought of as a measure of the view of the sky from that aperture. So, why not extend this notion to the rest of the scene? In other words, make the ground and any external obstructions self-luminous (to the same degree as the uniform sky) and determine the respective contributions to lumens received at the aperture. For example, the 1m 2 vertical aperture ‘sees’ 1000 lumens from the sky (as before) and 1000 lumens from the ground. With an obstruction in place, the same method gives a measure of the apertures ‘view’ of the obstruction (and, of course, now there are fewer lumens of view to the sky since it is partially obscured). The aperture-based method is informative, remarkably simple, purely-geometrical, scalable to any realistic 3D complexity and essentially impossible to gameplay. In other words, ideally suited as the basis for guidelines or standards “The adoption (in 1955) of the CIE overcast pattern as the basis for guidelines and standards was, I believe, a dreadful blunder.”

John continued: “I was asked why I had not considered using the widely accepted CIE ‘overcast sky’ standard as part of the calculation. However, evidence from monitoring sky brightness patterns has revealed that it represents an extreme type of overcast sky, that rarely occurs in reality. In fact, the adoption (in 1955) of the CIE overcast pattern as the basis for guidelines and standards was, I believe, a dreadful blunder.”

Maps showing examples from John’s ABDM calculations on typical buildings in various different orientations/ configurations are providing an extremely positive insight into the potential for the new methodology to provide consistent and reliable results. In a paper accepted for the CIBSE Technical Symposium (University of Strathclyde, Glasgow, 16-17th April 2020), John makes comparisons with BR 209 using the example of an apartement with balcony facing a large obstruction – a fairly typical urban scenario.

High levels of interest

During 2019, John has carried out exploratory studies using ABDM and has presented his findings to leading academics, designers and planners in London, Rome and New York. Whilst in many respects, ABDM represents a radical shift away from accepted methods, John has reported high levels of interest and acceptance among his recent audiences.

“Among practitioners I have spoken to, there is a general feeling of a ‘disconnect’ between the methods used at the planning stage (usually BR209 in the UK) and the building performance evaluation that invariably follows. The work done for a planning evaluation (3D model etc.) is ‘shelved’ once that stage has been passed, and rarely, if ever, revisited. ABDM offers the potential for a seamless workflow progression from outline planning to detailed building performance evaluation. This includes ‘climatising’ ABDM until the solution becomes full CBDM. Another key attraction for practitioners is that the ABDM measures account for the size of the windows. This means that the sunlight part of the ABDM evaluation can serve as an early indication of the potential for excessive solar gain, and therefore the risk of overheating. Equally, the skylight part gives an indication of the daylighting potential of the windows. Lastly, the view component provides a numerical measure of, say, the visual impact of an external obstruction. Thus the potential now exists to make view a material concern in evaluations to predict daylight loss due to obstructions. It remains to be seen if ABDM could be applied as an alternative to the Waldram method in ‘rights to light’ cases, or perhaps even serve as the basis for a replacement to Waldram. As noted, the ABDM approach only came about from many years serving on the panel that produced EN 17037. In any future revision of that standard (both in the UK and the EU now that they would be dealt with separately), I would propose the consideration of ABDM to replace those parts dealing with sunlight and daylight planning, and perhaps also view.”

In addition to his academic work, John offers a range of Daylight Consultancy Services including Expert Witness in ‘rights to light’ cases.

For further information, visit: http://climate-based-daylighting.com

This year will see the completion of New York’s Central Park Tower – the world’s tallest residential building. The legal agreement for the development of Central Park Tower includes an aperture measure of daylight injury to the studios of the neighbouring Art Students League building predicted by John using CBDM, first in 2005 (outline proposal) and then 2013 (final design). With hindsight, that landmark evaluation could be considered an example of ‘climatised’ ABDM.