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Rule of thumb for daylighting of rooms with external obstructions.

Article Excerpt
Introduction

The dictionary definition for 'rule of thumb' is a set of principles based upon experience which seems to work (Procter, 1995). This could, at worst, be regarded as an unreliable source of knowledge as it is not based on scientific reasoning as argued by Stevens (1988). Even if they do provide appropriate solutions, rules of thumb do not generally clarify the assumptions that they are based upon and daylighting is no exception to this as highlighted by Robbins, (1986). However, despite such shortcomings, daylighting rules of thumb in architecture have been continually developed, e.g., by the eminent lighting scientist Hopkinson, utilized and enshrined in building standards and legislation as well as in design guides. The more reliable rules have been based upon scientific experiments with effort taken to clarify most of the assumptions behind them and present any limitations (Hopkinson & Kay, 1969). For example, most daylighting rules of thumb address only ideal conditions such as an open site where the impact from surrounding buildings or structures are negligible. Therefore, daylighting rules of thumb which include the effect of external obstructions are quite scarce but in real urban contexts such obstructions are very likely.

One of the earliest rules of thumb which dealt with external obstructions was proposed by Vitruvius (c.80-20 BC) in his Book VI of De Architectura, where he stipulated that the amount of daylight that can reach a point indoor is equivalent to the amount of sky area visible from the spot. He proposed a rule of thumb to roughly determine the amount of potential light (Figure 1):

"On the side from which the light should be obtained, let a line be stretched from the top of the wall that seems to obstruct the light to the point at which it ought to be introduced, and if a considerable space of open sky can be seen when one looks up above the line, there will be no obstruction to the light in that situation" (Rowland, 1999, p.262)

This ancient rule can be seen as the basis, or 'archetype' of modern scientific daylighting tools such as the Waldram Diagram and the Pilkington Dot Diagram, all of which adapt the idea that the amount of attainable interior light is proportional to the area of visible sky ('Archetype' in this sense is borrowed from Goethe's 18th century transcendental zoology (Steadman, 1979) as a theoretical and universal fundamental principle with the implication that the variety of daylighting tools, or methods, are variations and transformations of it). This approach is also behind the 'no-sky line' requirement for skylight assessment of 'ancient lights' legislation (Bell & Burt, 1995) and the average daylight factor formula by Littlefair which addresses the influence of external obstruction on sidelit interiors (Figure 2):

D[F.sub.avg] = [[tau].sub.w] [A.sub.g][[theta].sub.c]/[A.sub.s] (1 - [R.sup.2])% (1)

where D[F.sub.avg] = average daylight factor

[A.sub.g] = window glazing area ([m.sup.2])

[[tau].sub.w] = transmission of window glazing

[[theta].sub.c] = sky angle measured at the center of the window in degrees

[A.sub.s] = total area of the room surfaces: ceiling, floor, walls and windows ([m.sup.2])

R = the average reflectance-for fairly light coloured rooms such as in the case studies, a value of 0.5 is normal.

[FIGURE 1...

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