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AMERICAN SOLAR ENERGY SOCIETY 16A
Far beyond energy
savings, daylighting
in buildings benefits
the bottom line
with productivity
and health gains.
Daylit Spac
BY RAMANA KOTI
The entrance lobby of the Lewis and Clark
State Office Building in Jefferson City, Mo.,
allows abundant north light for employees
and visitors alike. Studies show that employ-
ees in daylit buildings like this one are more
productive than employees working in artifi-
cially lit environments.
©2006 ASSASSI. BNIM ARCHITECTS
Our reverence for the sun as the source of life
is as old as human civilization. Even today,
many practitioners of yoga still practice the
Sun Salutation (Surya Namaskara in Sanskrit),
an act of physical exercise and meditation
performed at sunrise. In ancient Hindu liter-
ature, the Sun Salutation is recognized as a means to physical
and mental well-being.
Most people associate daylight with positive feelings. The
news media regularly report scientific studies that acknowledge
the link between the day and night cycle and the human circa-
dian rhythms. These rhythms generally promote activity during
the day and sleep during the night.
Daylighting as a natural, controlled and passive strategy of illu-
minating building interiors predates the profession of building
design. In the recent past, however, daylighting has become an
inseparable part of integrated design, and different building pro-
fessionals value daylighting for different reasons.
In a recent survey on the role of daylighting in sustainable
design by National Research Council Canada and Lawrence
Berkeley National Laboratory, researchers asked designers and
engineers which of five benefit categories — architectural,
building energy consumption, cost, lighting energy savings,
and load management — they most closely associated with
daylighting (see chart, below). The architectural category
included “health and productivity concerns and the interplay
of natural light and building form.”
Based on the responses of the 120 professionals who partici-
pated, the architectural and building energy-consumption-ben-
efit categories were the most relevant. This trend is further sup-
ported by the fact that architects and engineering consultants are
turning to building information modeling (BIM) that integrates
daylighting design and energy performance to lay the foundation
for a good design during the early stages of a project.
Daylighting Studies
Early daylighting studies focused exclusively on the energy
savings. For U.S. office buildings until the late 1970s, daylighting
was generally believed to result in a 15 percent to 20 percent
savings in energy consumption over a non-daylit building. With
increasingly efficient light fixtures, daylighting no longer has as
much impact on energy savings. However, during the past 15
years, studies have identified significant psychological and phys-
iological benefits of daylighting that translate into tangible
benefits for building occupants and owners.
While productivity benefits are hard to quantify, there are
some encouraging studies that make a strong case for daylight-
ing. Romm and Browning’s studies in the 1990s documented
eight buildings with various energy-efficiency measures, some of
which involved incorporating daylighting through roof skylights,
sloped ceiling for indirect lighting, lightshelves and atria, among
other strategies. Companies occupying daylit buildings reported
an increase in productivity of 5 percent to 15 percent and a
reduction in absenteeism of 15 percent to 40 percent. The authors
concluded that the increased productivity resulting from improved
daylighting measures would pay for those measures in 1 to 4.5
years. In another study done in 1998, Professor Harvey Bryan, a
daylighting expert at Arizona State University, demonstrated a case
in which a 0.5 percent increase in productivity could pay for the
building’s energy costs, while a 6.6 percent increase could pay for
the entire building!
Recent research by the Heschong Mahone Group for the Cal-
ifornia Energy Commission also indicates that daylighting boosts
productivity in a variety of settings. In one test of mental func-
tion and attention for office and call center workers, a 20 foot-can-
dle increase in daylight levels resulted in a 13 percent perform-
ance improvement. Call center workers with the best views
processed calls 7 percent to 13 percent faster than those with no
views. Office workers with the best possible view performed 10 per-
cent to 25 percent better on mental function and memory recall
tests than workers with no views.
Not surprisingly, glare affected performance adversely. The
study also found a uniformly positive and statistically significant
correlation between the presence of daylighting and student test
scores in three school districts. The “daylighting effect” was
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es,Productive Places
A recent survey indicates that designers and engineers consider
architectural and building energy-consumption issues to be the most
relevant benefits of daylighting in sustainable design.
C.F. REINHART & A. GALASIU, 2006
ing on scale models and performed daylight-illuminance analy-
sis using the computer simulation program called Lumen Micro
to determine the configuration of external shades and internal
light shelves. The resulting design integrates external shades on
the south façade into the precast structure of the building. The
design is visually pleasing and maximizes the use of daylighting.
In an online survey conducted after they moved in, the build-
ing occupants were asked to characterize their overall level of
comfort. Categories in the survey included overall building,
lighting, acoustics, air quality and temperature. Eighty percent
of the respondents indicated they were comfortable or neutral
with overall lighting in the building, while 89 percent of the
respondents were comfortable or neutral with the amount of day-
light in the building. However, 43 percent noted discomfort
caused by glare, especially from the low winter sun. The build-
ing owners and BNIM are conducting further tests to determine
how to mitigate the glare.
LEED and Daylighting
Daylighting has always been an important part of the U.S.
Green Building Council’s (USGBC’s) Leadership in Energy and
Environmental Design (LEED) rating system. Up to 2 points may
be earned for daylighting and views. The USGBC has made some
changes to the metrics to qualify for points. While LEED version
2.1 and 2.2 relied on the same daylight factor calculation to demon-
strate compliance for the credit Indoor Environmental Quality
(EQ) 8.1, the term “daylight factor” was renamed “glazing factor”
in 2.2. A couple of additional options are available in version 2.2,
including demonstrating the availability of 25 foot-candles of day-
light in 75 percent of regularly occupied areas at the workplane level
(30 inches, or 76 centimeters, above the floor) at noon on the
equinox using computer simulation or demonstration of the same
through measurements on site (on a 10-foot, or 3-meter, grid).
The LEED Reference Guide, in the credit EQ 8.1, illustrates vari-
ous daylighting strategies and cautions against unwanted glare. The
guide suggests ways in which designers can avoid glare. Despite its
usefulness, the intent of the LEED credit is to quantitatively assess
daylight availability at an instant in the year, for a building in which
daylighting design intent may or may not be based on robust
scientific principles. Because such a framework for assessing a
building’s performance is often interpreted by design teams as a
design guide, the metrics used by it assume critical importance.
The Lewis and Clark building achieved both the EQ credits per-
taining to daylight and views. In the credit EQ 8.1, the metric for
demonstrating daylight availability, the building achieved a 2 per-
cent daylight factor in 75 percent of the regularly occupied spaces,
which exceeds the LEED requirements. The building was award-
ed a LEED Platinum certification last year, making it only one of
21 facilities that have earned USGBC’s highest level of recognition
for new construction to date.
Other Metrics and Tools
Daylight factor, one of the metrics used by LEED, is the most
widely used metric for daylighting-performance analysis and pre-
diction. Daylight factor originated as a minimum legal lighting
requirement and is based on the worst-case scenario — a uniform-
ly overcast sky. It did away with the complications of having to
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AMERICAN SOLAR ENERGY SOCIETY 18A
DAYLIT SPACES
Daylighting Glossary
Daylight Factor: The ratio of the internal illuminance at a
point in a building to the unshaded, external horizontal
illuminance under a CIE overcast sky. (CIE is the Commission
Internationale de l’Eclairage, which has developed a series of
mathematical models of ideal luminous distributions under
different sky conditions.)
Daylight Autonomy: For a sensor point, it is the percentage
of occupied times of the year when the minimum illuminance
requirement at the sensor is met by daylight alone.
Useful Daylight Illuminances: Aims to determine when daylight
levels are useful for the occupant, in terms of being neither too
dark nor too bright (between 100 lux and 2000 lux).
Continuous Daylight Autonomy: Similar to Daylight Autonomy
except partial credit is attributed to time steps when daylight
illuminance lies below the minimum illuminance level.
Daylight Saturation Percentage: The daylight saturation
percentage for 40 foot-candles (DSP40 ) to 400 foot-candles
(DSP400) is the percent of hours and the percent of classroom
floor area between 8 a.m. and 3 p.m., Monday through Friday,
from Aug. 15 through June 15 when daylight provides at
least 40 foot-candles or more of illumination at a work plane
located 30 inches (76 centimeters) above the floor. Achieving
a DSP of 400 is an indicator of over-lighting and glare, and
is therefore penalized.
attributed to quantitative and qualitative aspects such as improved
visibility, better distribution of light, better color rendition,
absence of flicker and highlights on three-dimensional objects.
Daylighting Design
Creating visually stimulating spaces through the interplay of
natural light and building form has historically been an impor-
tant objective in architectural design. An architectural design for
Phoenix’s climate, for example, would be inherently different from
a design for Chicago’s climate. The concept of “critical regional-
ism,” a response to “placelessness” in architecture, offers an inter-
esting insight into the relationship between space and light.
(Placenessness refers to architecture that seems to have no rela-
tionship to its location — it could be anywhere.) It argues that the
character of architecture in a region emerges from the way design-
ers work with building shapes and the arrangement of windows
to deal with light and climate control. It advocates the use of con-
trolled daylight that, for example, causes the exhibition volume
in an art gallery to change with time, season and humidity, as
opposed to the exclusive use of artificial light.
When BNIM Architects began work on the design of the Lewis
and Clark State Office Building for the Missouri Department of
Natural Resources in Jefferson City, the expression of the design
evolved from similar objectives. From conception, optimal depth
of the available floorspace, orientation, solar control and access
to daylighting were integral to the sustainable strategies under
consideration. BNIM conducted solar studies on site, did solar test-
deal with dynamically changing sky conditions. Daylight-factor
calculation for a space does not take orientation into considera-
tion and disregards shading and glare-control strategies, which are
not concerns in an overcast-sky condition. The premise was that
if the design is good for the worst-case scenario, it must be good
for all other conditions. Daylight factors for different design con-
ditions, such as geometry and surface properties, vary and can help
in narrowing down the best design solution. The calculation is
simple, intuitive and easy to communicate among design teams.
Not all experts agree that daylight factor is the best way to
measure daylighting performance. Christoph Reinhart, John
Mardaljevic and Zack Rogers, daylighting specialists at the Nation-
al Research Council Canada, De Montfort University and Archi-
tectural Energy Corp., respectively, argue that the daylight factor
approach does not result in realistic performance expectations.
They suggest metrics that account for varying climate conditions
throughout the year. Such metrics also consider the occupancy
profiles of a building.
Annual daylight prediction through computer simulation has
become more accurate, down to the time step of an hour or less.
Higher accuracy of such daylight prediction has ceased to mean
longer simulation duration. Computer simulation also accommo-
dates complex but critical concepts such as user behavior in con-
trolling blinds. The ability to predict daylight accurately on an
hourly basis with stress on climate-based design has given rise to
metrics such as daylight autonomy, useful daylight illuminances
and continuous daylight autonomy (see “Daylighting Glossary,”
facing page). California’s Collaborative for High Performance
Schools, which offers guidance for designing high-performance
schools, suggests another metric called daylight saturation percent-
age. A designer can optimize a particular design based on these
metrics or compare different design alternatives to arrive at the
most suitable one.
Radiance, a UNIX-based daylighting- and lighting-simulation
software, is another highly regarded tool that facilitates accurate
modeling. The use of climate-based daylighting metrics is evident
in one case study by Matt Franks, a lighting consultant with
Arup Lighting in New York. The case study illustrates the use of
Radiance-based DAYSIM software to optimize the predicted illu-
minance levels on a typical art hanging point in a museum
gallery on an annual hourly basis.
One other tool worthy of note is The Green Guide for Health
Care, a joint project of the Center for Maximum Potential Build-
ing Systems and Health Care Without Harm. This self-certifying
sustainable design toolkit for the healthcare sector draws upon
research and resources in the sector to maximize the benefits of
increased daylighting, connectivity with the outside world and
lighting design that reinforces circadian rhythms for caregivers,
patients and families. It deals with operations both in existing
facilities and in new construction and is structured to work along-
side the LEED rating system.
As designers and engineers continue to develop creative
designs, reliable metrics and user-friendly computer simulation
tools, good daylighting design has become more accessible than
ever. The shift toward increased use of clean, sustainable, inex-
haustible solar energy indicates our willingness to come full cir-
cle and tap a resource that has been there all along.
Ramana Koti is a sustainable building analyst with BNIM Architects
and a LEED-accredited professional. Contact him at rkoti@bnim.com.
12TH ASES NATIONAL SOLAR TOUR
WWW.NATIONALSOLARTOUR.ORG 19A
Daylighting Design Resources
IESNA Lighting Handbook, Illuminating Engineering Society of
North America: www.iesna.org
Daylight in Buildings, Lawrence Berkeley National Laboratory
(LBNL): http://gaia.lbl.gov/iea21/
Energy Design Resources Design Brief:
www.energydesignresources.com/docs/db-01-daylighting.pdf
Collaborative for High Performance Schools Best Practices Manual:
www.chps.net/manual/documents/BPM_2006_Edition/
CHPS_II_2006_Lighting_and_Daylighting.pdf
Dynamic Daylight Performance Metrics for Sustainable Building
Design, National Research Council of Canada:
http://irc.nrc-cnrc.gc.ca/pubs/fulltext/nrcc48669/
“Building Information Modeling and Green Design,” Environ-
mental Building News, May 2007: www.buildinggreen.com/
auth/article.cfm?fileName=160501a.xml
Matt Frank’s and others’ studies on the Radiance Presentation
Archive from the National Research Council Canada
http://irc.nrc-cnrc.gc.ca/ie/light/RadianceWorkshop2005/
DRAWN BY JEREMY CORDELL, BNIM ARCHITECTS
Two views of the south façade of the Lewis and Clark State Office
Building, which uses the precast structure of the building to control
solar gain and allow daylight penetration.
The design of the Lewis and Clark State Office Building for the
Missouri Department of Natural Resources in Jefferson City inte-
grates external shades on the south façade into the precast structure
of the building.
©2005 SINCLAIR, COURTESY OF BNIM ARCHITECTS