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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 Daylight Zone
High daylight potential is found particularly in those spaces that
are predominately daytime occupied. Site solar analysis should assess
the access to daylight 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
daylight and which spaces have little or no need for daylight. 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 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 daylight that arrives at a work surface comes from three sources:
The exterior reflected component. This includes ground surfaces,
pavement, adjacent buildings, wide windowsills, and objects. Remember
that excessive ground reflectance will result in glare. 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. The internal reflected component. Once the daylight 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
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Lighting
Shelves |
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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.
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. - Clerestory Window
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Clerestory
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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.
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.
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.
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.
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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