Calculations

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Early methods of calculation ... model studies ... computer analysis ... artificial lighting ... examples of daylight studies

The very real importance of the daylighting strategy in modern buildings makes it a major consideration in design, and whilst an architect's innate understanding of the rules which have applied in the case of buildings since mediaeval times, the complications of modern structures and their interrelationships makes an understanding of daylighting desirable, if not essential. However it has to be said - and this is the experience gained from many of the case studies at the back of this book - the resulting daylighting strategy has evolved as much from the architect's past experience as from any detailed analysis and calculation.

Early daylight studies were limited to assessing the daylight penetration in sidelit rooms where a simple rule of thumb method was often used; since it was known that the head height of the window in a room influenced the depth to which the light would penetrate, a start could be made in determining the quantity of daylight where the light penetration was twice the head height of the window. An example of this might be in a stately home with windows reaching a ceiling height close to 7 m, the useful daylight would penetrate some 14 m into the space, and this is indeed our experience of such buildings. Likewise in a modern building with a much lower ceiling height of 2.5 m, the useful daylight penetration might be as little as 5 m or in a room with windows on both sides, a penetration of 10 m.

This is not to say that the quantity of daylight available at a distance of 5 m from a single window wall would provide a daylight factor considered adequate for office work overall, but it might be enough to provide a 5 per cent daylight factor close to the window with a 2 per cent daylight factor at the rear of the space, providing an overall sense of the space being daylit; and this would permit significant savings of energy if a system of 'daylight linking' artificial light was to be incorporated.

This simple rule of thumb, has to be hedged about with a large number of questions; such as the percentage of glazed area the window represents to the wall, the nature of the glazing, what external obstructions diminish the view of the sky outside the window and so on.

A useful start for an architect is provided by the publication Good Practice Guide No. 245, Desktop guide to Daylighting for Architects produced for the DETR by Professor Peter Tregenza in their Best Practice Programme on energy efficiency. This goes through a number of 'rules of thumb' and relates daylighting design to the different stages of the RIBA plan of work. Architects can learn a lot from the commonsense approach adopted in this little book, in establishing their initial design proposals before checking these out with the methods of computation available.

Rule of thumb. Daylight penetration

The type and configuration of the window or windows, the nature of the glazing and their surroundings will make a significant difference, and whether the window is obscured by surrounding buildings, and how much of the floor area has an unobscured view of the sky. All these and other considerations need to be taken into account if an accurate calculation of the overall daylight picture is to be made.

From the architect's point of view, the physical modelling of a space has advantages. Simple design models are a part of his design tools, and have the advantage that they can be modified easily to accept changes to the section and layout. Alterations to the interior reflectances can be made and the interior effect photographed to show a client. Placing the model below an artificial sky and using a grid of photocells; such models at scales of 1/20 to 1/50 can be used to calculate the average daylight factor, and if found to be unsatisfactory the model can be modified to make the necessary changes to comply with the daylighting strategy, as depicted by the brief. A simple alternative which may be used as a rough check, is to place the model under an area of unobstructed sky.

It is not intended to go into the detail of the methods of calculation which are available; architects' offices will no doubt have the required computer software to deal with the straightforward problems of the calculation of the average daylight factor (DF) for buildings with traditional side windows. It is most important to establish the correct daylight strategy for a building in its environment, its orientation, and its neigbouring buildings, together with a 'client brief' that recognizes the advantages of the natural source.

The calculations for daylighting in large commercial buildings is intimately bound up with the requirements of artificial lighting, and by the means adopted for 'daylight linking'. When calculating the levels of daylight (DF) available during the day, this influences the level of the artificial light which must be available, since considerable savings of energy can be made by the sophisticated control systems now available.

Rather than duplicating the information on the methods of calculation which are available elsewhere, there follows a design report which considers both the daylighting and the artificial lighting of a modern office building, to indicate both methods of computer calculation and physical modelling.

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