7.0 Main Daylight Design Areas
7.1 Daylight
Daylit spaces are recognised to be more attractive
and more efficient to work and live in, if well designed.50 They reduce
the likelihood of developing psychological and physiological diseases,
such as SAD, headaches, fatigue and vitamin D deficiency.51 There are
specific reasons why the use of daylight is desirable. These
include:-52
• Quality of
light.
• Importance
of daylight as a design element.
• Resource
responsibility
• Energy /
environmental conservation targets which need to be met can often be
satisfied with the correct use of daylighting as primary or secondary
lighting.
• Energy Cost
Savings
• Peak demand
tariff cost savings.
• Enhanced
opportunity to develop integrated structural and mechanical systems
such as natural ventilation.
The requirement for daylighting depends on the
building’s function, hours of operation, user, view requirements,
ventilation requirements and privacy.
7.2 Definition of Daylight
Daylight is classed as the natural light from an
overcast sky.53 The CIE Overcast Sky is the standard used in
Northern Europe for all daylighting calculations, with the exception of
special cases where sunlight needs to be taken into consideration such
as in the design of innovative daylighting systems.
Sunlight is the direct sunbeam, unobscured by
clouds but taking atmospheric distortion into account.
The standard CIE Overcast Sky is three times as
bright overhead than it is at the horizon, and of the same brightness
in all compass directions. It is assumed that the minimum yearly
average outdoor illumination is 5000 lux for 85% of a normal working
day, representing a dull day. The maximum design illuminance is 100,000
lux. This represents a sunny day and would be used in assessing glare
and sunlight utilisation.
It is considered that if a building is designed for
daylighting under these very overcast conditions then, for times that
the outdoor illuminance is brighter, it would have a natural
lighting performance significantly better than estimated.
7.3 Daylight Factor
To be able to predict and model the availability of
daylight in a building, a set of standards have been developed.
However, daylighting is not an accurate science. It
varies every minute of every day and only general predictions can be
made. The sun’s path can be accurately used, as data is available
for sun paths all around the world. However, daylight depends on cloud
cover, which can never be predicted accurately.
The daylight factor is the CIE recommended method
to determine the performance of a daylighting system. The daylight
factor is used, as it does not use exact illuminance levels; rather a
ratio between indoor and outdoor illuminance, which is dependent on the
window design and location. It is useful for estimating the amount of
glazing needed to daylight a space.54,
55
Definition
The daylight factor does not have a specific value
of illuminance. It is defined as the ratio of interior illuminance on a
horizontal surface to the exterior illuminance on a horizontal surface
from an overcast sky. The greater the amount of light entering through
a window, relative to the outside illuminance, the higher the daylight
factor will be. See glossary for further definitions of illuminance.
The illuminance in a daylit interior will vary, depending on the sky
luminance, but the daylight factor remains the same.
Daylight Factor Less than 2%
A room twith a daylight factor of less than 2% will:
• look gloomy
under daylight alone.
• often need
full electric lighting during daytime
• be
dominated by the appearance of electric lights.
Daylight Factor 2% - 5%
A daylight factor of 2% - 5% is usually the optimum
range of daylighting for overall energy use:
• rooms have
a predominantly daylit appearance but supplementary electric lighting
is needed away from windows and during dull weather.
Daylight factor of 5% or more
• the room is
strongly daylit.
• daytime
electric lighting rarely needed.
• major
thermal problems from large windows.56,
57
Daylight Factor for Sports Halls
The suggested daylight factor for sports halls is
5%, with the minimum value being 3.5%.58 This only
takes into consideration the lighting levels. It does not involve
optimising the glazing to achieve correct solar gain/heat loss/glare
ratios.
7.4 Sunlight
Sunlight is different to daylight. It is the direct
light from the sun, rather than the diffuse skylight which classifies
daylight. Sunlight has several effects on a building, its contents and
its occupants. In the domestic situation sunlight is usually welcomed
into the interior. It enlivens the colours and stimulates the mind.59
Non-domestic situations will also value some sunshine, depending on the
activity in the space. For example, atria and other circulation spaces
will appear bright and cheerful in sunshine.
In general, positive aspects include:-
• Passive
solar heating;
• Natural,
dynamic lighting;
• Increased
lighting levels;
• Improving
peoples’ well-being;
• Saving
heating and lighting energy;
Not all of the impacts are beneficial, especially
in connection with non-domestic buildings, including:
• Sports
halls;
• Offices
with computers;
• Drawing
offices;
• Museums
containing light sensitive artefacts;
• Hospital
operating theatres;
• Factories.
Some of the adverse effects include:
• Overheating;
• Bleaching
of colours and artefacts;
• Ultra-violet
damage of plastics and other materials;
• Control is
difficult to achieve
• Glare.
In the case of sports halls direct sunlight should
be excluded due principally to glare although all the other factors can
be relevant. To address the problems sunlight needs to be well
controlled, whilst aiming to achieve a high daylight factor.
