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North facing, triple-glazed roof monitors
light the large, top floor art galleries. The windows are
mounted at 53 degrees from horizontal and have a transmittance
of 58%. Internal blinds are used to control and diffuse the
daylight levels, whilst external blinds block low level
sunlight. Fig. 9.3 shows one of the galleries. Note the curved
ceilings to reduce the shadows on the walls.
Fig. 9.4 shows the vertical daylight
factors on the north and south walls, with and without the
internal blinds. The transmittance of the blind is 35%, giving
a factor of 3 reduction in daylighting levels, when required.
Fig. 9.5 shows the horizontal daylight factors.
Uses several layers of light dispersing
polycarbonate and laminated glass underneath a top lit
attic, see figs. 9.6 and 9.7. Fluorescent luminaires are
located below attic glazing to balance daylight levels, see
fig. 9.8.
9.2 Internal Blinds
These can either be inside a room or within
the glazing units. The simplest methods are curtains, roller
blinds or Venetian blinds fig. 9.2.1. 72
They are controlled manually and are
reasonably successful in their operation.73 Venetian blinds (with the exception
of midpane varieties) are notoriously difficult to keep clean
and maintain, and bright white blinds can cause glare problems.
They do allow reasonably accurate control of daylighting.
Internal blinds can be used to keep
sunlight off people and objects, though the room will still
heat up due to solar gain and are cheaper to install than
external shades. They can be used for privacy control and
avoiding the black hole effect of a bare window at night.
9.3 External Shading
External blinds are more successful at
controlling unwanted solar gain as they prevent solar radiation
from reaching the windows. It is more expensive than internal
shading, due to size and the need for robustness. Interpane
blinds will keep the sun from reaching the inner pane(s) of
glass and similarly reduce solar radiation and conduction to
the interior.
However, permanently fixed shading reduces
skylight all year, requiring correspondingly larger glazing
areas for the same internal daylight levels. Adjustable shades,
retractable blinds and awnings see fig. 9.2.3, avoid these
problems but have to be strong enough to withstand operation
and wind loading. Fully automated systems are not yet reliable
enough for commercial use.
Overhangs, balconies and beams can also be
used for external shading to restrict sunlight at certain times
of the year and day, though they will reduce the daylight
factor.
A recent innovative approach to external
shading is to use trees and vines.75 Deciduous trees are particularly suitable
as they allow the low level winter sun in when they drop their
leaves but will provide shade when in leaf.
Blinds and shutters containing insulation
can be used to reduce the heat loss through windows at night,
as well as providing shade during the daytime.
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