5Calculations

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 penetra-
tion 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 publicationGood
Practice Guide No. 245, Desktop guide to Daylighting for Architects