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When properly designed and effectively integrated
with the electric lighting system, daylighting can
offer significant energy savings by offsetting a
portion of the electric lighting load. A related
benefit is the reduction in cooling capacity and use
by lowering a significant component of internal
gains. In addition to energy savings, daylighting
generally improves occupant satisfaction and
comfort. Recent studies are implying improvements in
productivity and health in daylighted schools and
offices. Windows also provide visual relief, a
contact with nature, time orientation, the
possibility of ventilation, and emergency egress.
This section includes the
following:
The Daylighting Zone
High daylighting potential is found particularly in
those spaces that are predominately daytime
occupied. Site solar analysis should assess the
access to daylighting by considering what is "seen"
from the various potential window orientations. What
proportion of the sky is seen from typical task
locations in the room? What are the exterior
obstructions and glare sources? Is your building
design going to shade a neighboring building or
landscape feature that is dependent on daylight or
solar access?
It is important to
establish which spaces will most benefit from
daylighting and which spaces have little or no need for
daylighting. Within the spaces that can use daylight,
place the most critical visual tasks in positions
near the window. Try to group tasks by similar
lighting requirements and occupancy patterns. Avoid
placing the window in the direct line of sight of
the occupant as this can cause extreme contrast and
glare. It is best to orient the occupant at 90
degrees from the window. Where privacy is not a
major concern, consider interior glazing (known as
relights or borrow lights) that allow light from one
space to be shared with another. This can be
achieved with transom lights, vision glass, or
translucent panels if privacy is required.
The floor plan
configuration should maximize the perimeter daylight
zone. This may result in a building with a higher
skin-to-volume ratio than a typical compact building
design. A standard window can produce useful
illumination to a depth of about 1.5 times the
height of the window. With daylighting lightshelves or other
reflector systems this can be increased to 2.0 times
or more. As a general rule-of-thumb, the higher the
window is placed on the wall, the deeper the
daylight penetration.
Window Design
Considerations
The daylighting that arrives at a work surface comes
from three sources:
1. The exterior reflected component. This includes
ground surfaces, pavement, adjacent buildings, wide
windowsills, and objects. Remember that excessive
ground reflectance will result in glare.
2. The direct sun/sky
component. Typically the direct sun component is
blocked from occupied spaces because of heat gain,
glare, and UV degradation issues. The sky dome then
becomes an important contribution to daylighting the
space.
3. The internal reflected
component. Once the daylighting enters the room, the
surrounding wall, ceiling, and floor surfaces are
important light reflectors. Using high reflectance
surfaces will better bounce the daylight around the
room and it will reduce extreme brightness contrast.
Window frame materials should be light-colored to
reduce contrast with the view and have a non-specular
finish to eliminate glare spots. The window jambs
and sills can be beneficial light reflectors. Deep
jambs should be splayed (angled toward the interior)
to reduce the contrast around the perimeter of the
window.
Remember that the most
important interior light-reflecting surface is the
ceiling. High reflectance paints and ceiling tiles
are now available with .90 or higher reflectance
values. Tilting the ceiling plane toward the
daylight source increases the daylight that is
reflected from this surface. In small rooms the rear
wall is the next important surface because it is
directly facing the window. This surface should also
be a high reflectance matte finish. The sidewalls
followed by the floor have less impact on the
reflected daylight in the space.
Major room furnishings such
as office cubicles or partitions can have a
significant impact on reflected light so select
light-colored materials.
Since light essentially has
no scale for architectural purposes, the proportions
of the room are more important than the dimensions.
A room that has a higher ceiling compared to the
room depth will have deeper penetration of daylight
whether from sidelighting (windows) or toplighting
(skylights and clerestories). Raising the window
head height will also result in deeper penetration
and more even illumination in the room. Punched
window openings, such as small, square windows
separated by wall area, result in uneven
illumination and harsh contrast between the window
and adjacent wall surfaces. A more even distribution
is achieved with horizontal strip windows.
Effective Aperture
One method of assessing the relationship between
visible light and the size of the window is the
effective aperture method. The effective aperture
(EA) is defined as the product of the visible
transmittance and the window-to-wall ratio. The
window-to-wall ratio (WWR) is the proportion of
window area compared to the total wall area where
the window is located. For example, if a window
covers 25 square feet in a 100 square-foot wall then
the WWR is 25/100 or 0.25. A good starting target
for EA is in the range of 0.20 to 0.30. For a given
EA number, a higher WWR (larger window) results in a
lower visible transmittance.
Example: WWR = .5 (half the
wall in glazing),
VT = .6, EA = 0.3
Or WWR = .75, VT = .4 for same EA of 0.3
Typically lowering the
visible transmittance will also lower the shading
coefficient but you must verify this with glazing
manufacturer data since this is not always the case.
Light Shelves
Since luminance ratios or
brightness is a major consideration in view windows,
it is often wise to separate the view aperture from
the daylight aperture. This allows a higher visible
transmittance glazing in the daylight aperture if it
is out of normal sight lines. Since the ceiling is
the most important light-reflecting surface, using
this surface to bounce daylight deep into the room
can be highly effective. Both of these strategies
are utilized in light shelf designs. A light shelf
is a horizontal light-reflecting overhang placed
above eye-level with a transom window placed above
it. This design, which is most effective on southern
orientations, improves daylight penetration, creates
shading near the window, and helps reduce window
glare. Exterior shelves are more effective shading
devices than interior shelves. A combination of
exterior and interior will work best in providing an
even illumination gradient.
Toplighting Strategies
Large single level floor areas and the top floors of
multi-story buildings can benefit from toplighting.
The general types of toplighting include skylights,
clerestories, monitors, and sawtooth roofs.
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Skylights
Horizontal skylights can be an energy problem
because they tend to receive maximum solar gain
at the peak of the day. The daylight
contribution also peaks at midday and falls off
severely in the morning and afternoon. There are
high performance skylight designs that
incorporate reflectors or prismatic lenses that
reduce the peak daylight and heat gain while
increasing early and late afternoon daylight
contributions. Another option is lightpipes
where a high reflectance duct channels the light
from a skylight down to a diffusing lens in the
room. These may be advantageous in deep roof
constructions.
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Clerestory Window
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Here
stepped clerestory windows
provide daylight for the
interior workspace.
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A clerestory window is vertical glazing located
high on an interior wall. South-facing
clerestories can be effectively shaded from
direct sunlight by a properly designed
horizontal overhang. In this design the interior
north wall can be sloped to better reflect the
light down into the room. Use light-colored
overhangs and adjacent roof surfaces to improve
the reflected component. If exterior shading is
not possible, consider interior vertical baffles
to better diffuse the light. A south-facing
clerestory will produce higher daylight
illumination than a north-facing clerestory.
East and west facing clerestories have the same
problems as east and west windows: difficult
shading and potentially high heat gains.
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Roof Monitor
A roof monitor consists of a flat roof section
raised above the adjacent roof with vertical
glazing on all sides. This design often results
in excessive glazing area, which results in
higher heat losses and gains than a clerestory
design. The multiple orientations of the glazing
can also create shading problems.
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Sawtooth Roof
A sawtooth roof is an old design often seen in
industrial buildings. Typically one sloped
surface is opaque and the other is glazed. A
contemporary sawtooth roof may have solar
collectors or photovoltaic cells on the
south-facing slope and daylight glazing on the
north-facing slope.
Unprotected glazing on the south-facing sawtooth
surface may result in high heat gains. In these
applications an insulated diffusing panel may be
a good choice.
Daylighting Controls
A building designed for daylighting but without an
integrated electric lighting system will be a net
energy loser because of the increased thermal loads.
Only when the electric lighting load is reduced will
there be more than offsetting savings in electrical
and cooling loads. The benefits from daylighting are
maximized when both occupancy and lighting sensors
are used to control the electric lighting system.
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Occupancy sensors
detect when a space is occupied by using passive
infrared, ultrasonic, or a combination of the
two technologies. Once the heat or movement of
the occupant is no longer detected, and after a
preset delay time, the sensor will emit a signal
to extinguish the lights. Occupancy sensors used
alone are good for low or intermittent use areas
such as storage rooms, restrooms, and even
corridors.
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Light level sensors
have a photoelectric "eye" that measures the
illumination in a room. Threshold on and off
values can be set to respond to specific
lighting conditions. These sensors can operate
on/off switching of various luminaires or lamps
within luminaires and they can also operate a
continuous dimming system. Continuous dimming
system will obviously cost more than switching
systems but they have greater user satisfaction
because the change in lighting levels is not as
noticeable.
Fluorescent lighting
systems are the most common daylight control lamp
source because of the availability of step switching
and dimming systems. HID sources are typically not a
good choice for daylight switching because of the
extended strike and re-strike times. There are now
two-step HID sources available that may be useful in
some stepswitching applications where the "off" mode
is not desired during a typical day. A daylighting
design will use both occupancy and light sensors.
With these two control strategies the lights will
come on only when the room is occupied and only if
there is insufficient daylight. In most designs a
manual over-ride is provided for user convenience.
Design Coordination
When using daylighting, the electrical lighting and
interior design require special consideration.
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Electric Lighting
Design Coordination
The coordination of the electrical lighting
system with the daylighting design is critical
for the success of the system. The layout and
circuiting of the lighting should correspond to
the daylight aperture. In a typical sidelighting
design with windows along one wall it is best to
place the luminaires in rows parallel to the
window wall and circuited so that the row
nearest the windows will be the first to dim or
switch off followed by successive rows. Visit
the Building Components section for more on
lighting technologies.
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Interior Design
Coordination
In order to maintain the designed performance of
the daylighting system, the person responsible
for interior finishes and furnishing must be
aware of the desired reflectance values. Dark
interior finishes can compromise an otherwise
great daylighting design.
Modeling Daylighting
Physical models are a very effective way to analyze
daylighting performance. Even simple models can
begin to inform the designer of how daylight will
behave in the building. It is important that the
daylight apertures be accurately modeled and that
the materials used to construct the model have the
designed reflectance values. The model can then be
tested on the actual site or under artificial sky
conditions in a daylighting laboratory. A sundial
for 36 degrees north latitude attached to the model
base allows the designer to simulate various dates
and times of the year. Computer analysis is another
method of testing a daylighting solution. Several
lighting programs such as
Lumen-Micro,
Radiance,
and Lightscape
have daylighting calculations. Typically a
three-dimensional digital model is constructed using
computer-aided design software that is then imported
into the lighting software. The programs then
require the operator to define all surface
characteristics, sky conditions, location, and date
and time. Many of these programs can produce
photo-realistic renderings of the proposed design.
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