external device, since the heat which is generated has already entered the
building, and is more difficult to extract; however the type of shade will
be less vulnerable than that outside, will be easier to maintain and to
clean, so that an overall view must take into account all the factors in
coming to a decision. If of course it is not possible to control the solar gain
sufficiently inside, then other means will have to be adopted.
The most common form of control and one used almost universally in
residential building, is the curtain, and provided these are carefully
designed, perhaps with a reflective lining to reduce the solar gain when
pulled across the opening (whilst at night they can keep out the cold) can
be perfectly satisfactory in our temperate climate... indeed we welcome
the sun on all but the exceptional day.
A more flexible form of control is the venetian blind, which has the
advantage of adjustability in that it can be raised when not required for
sun control, to permit maximum daylight entry.
The demise of this excellent tool has been predicted for many years, but
it survives, offering excellent glare control, can be motorized when used
in large office projects, can be incorporated within panes of glass to
protect it from damage, and specialist versions are available where the tilt
of the blades can be varied to enable the top of the blind to reflect light up
to the ceiling of a room, whilst the lower blades control the sunlight by
reflecting it away from the building. A further advantage of the venetian
blind is that the surface design of the horizontal slats can be varied to
meet the individual requirements of the building.
The obvious advantage of the venetian blind is that it can and should
be raised when not needed for sun control; the problem is that once
lowered it tends to be left in the closed position. A procedure should be
adopted to ensure that their use is optimized and a simple solution might
be for the office cleaners to open the blinds to ensure that each day starts
with them open to admit the maximum daylight. Venetian blinds have a
lot of life in them yet.
Other types of blind are also available, the vertical hung louvre blind
where the louvre slats can be rotated, or moved to one side offer
flexibility, provide privacy, and together with roller blinds and those of
other materials can provide low-cost solutions in the domestic situation.
The heat gain from the sun can be controlled by the type of glass used,
various options being available, First there are the low emissivity
glazings, developments in this field continue, and the thermal properties
of the glass can now be tailored to give good solar control. Their big
advantage is that they admit higher levels of daylight than the original
tinted versions, and can control heat loss.
Prismatic glazing panels have also proved useful; these are limited to
small panels of prismatic glazing which, when attached to high level
rooflights, can allow daylight to enter, but redirects the sunlight on to the
ceiling of the space, or excludes it altogether.
Finally there are the high tech glazings already referred to under the
glazings available for windows. These include the following:
Electrochromic and liquid crystal glazings, which can be made to
darken on application of an electric current
Photochromic glass, which darkens when sunlight falls on it
Thermochromic glass which alters its transmission value on the
introduction of heat.
None of the latter is in the mainstream of development, and it is
unlikely that these will have a major impact for some years.
32 Daylighting: Natural Light in Architecture