Daylighting, The Other Renewable Energy
FMI 2008
Energy Track
Orlando Florida
Eric T. Truelove, P.E., LAP, Associate AIA
Daylighting
Using daylight as an alternative to artificial lights.
The benefits of daylighting include:
• Reduced electric bills
• Improved health
• Improved productivity
• Higher sales
Daylighting sounds easy, but it’s not. Effective
daylighting is a science that has been mastered by
only a few designers.
Hours of Daylight
Daylight is readily-available at all latitudes in the
continental United States.
• Summer hours in Orlando: 6:30 AM to 8:30 PM
• Summer hours in Milwaukee: 5:12 AM to 8:35 PM
• Winter hours in Orlando: 7:15 AM to 5:30 PM
• Winter hours in Milwaukee: 7:20 AM to 4:20 PM
Good daylight is available for 6- to 8-hours every day,
depending on the time of year, but does not vary
significantly based on location.
Intensity of Daylight
The sun delivers more light than we need, much more
in fact.
• Corridors and stairwells: 5 to 20 foot-candles
• Offices: 35 to 50 foot-candles
• Classrooms: 50 to 75 foot-candles
• Laboratories: 100 foot-candles
Direct sunlight delivers upwards of 10,000 footcandles along with a fierce amount of unwanted heat.
Cloudy skies can actually deliver more light than clear
skies.
Is This A Good Daylit Room?
Reduced Electric Bills
Depending on the hours of operation, 25 to 50% of a
building’s electric bill is energy for lights.
• A typical 30,000 square-foot retail building having
33 kilowatts of connected light load.
• If the facility operates 70 hours per week, the lights
will consume 120,000 kWh of energy each year at a
cost of around $14,000 (assume 12 cents/kWh).
If you could shut off all of the lights using daylight 6to 8-hours a day, you could save $10,000 a year or
about $0.33 a square foot.
Improved Health
People may be healthier when they have daylight in
their lives.
• Hospital patients perceive less pain and take 22%
less pain medication (Walsch, Rabin, Day,
Williams, Choi, and Kang, - 2005)
• Students grow taller and have fewer cavities
(Heschong Mahone Group - 1999 and Alberta
Education – 1992).
Our eyes are built to accept daylight and our bodies
seem to need it in ways we don’t yet understand.
Improved Productivity
People may be more productive when they have
daylight in their lives.
• Students learn and comprehend 20 to 26% better
and have a 1.6 to 1.9% higher attendance rate
(Heschong Mahone Group - 1999 and Alberta
Education – 1992).
• Greater job satisfaction and lower stress reported
(eb Environmental & Behavior - 1998).
In addition to saving energy, more daylight can
decrease medical costs and provide a productivity
increase.
Higher Sales
People seem to buy more in daylit environments.
• Retail stores with skylights sell 31 to 49% more than
those without (Heschong Mahone Group – 1999)
• When merchandise was moved to daylit areas, sales
increased (Heschong Mahone Group – 2005).
Study results point to daylighting as a potential means
to cut costs and increase sales. The challenge is to do
the daylighting effectively and within your budget.
LEED® Points
Daylighting may help you achieve Leadership in
Energy and Environmental Design points.
• Daylighting Credit IEQ 8.1 (One Point)
• Energy Credit EA 1 (One to Two Points)
• Skylights will not get the views Credit EA 8.2
The most energy-efficient approach is to daylight
using only 5% of the roof surface. If you cover a
greater area with skylights, the energy losses and
gains through the skylight will negate the benefits of
daylighting.
How Do Lights Switch On/Off
You can’t save energy if the lights stay on; therefore,
combine fluorescent fixtures with one of the
following approaches:
• Low budget, doesn’t work – Manual switching
• High end, too costly – Continuous dimming
• Best approach – Stepped dimming (1, 2, or 3)
If you have metal halide fixtures, you will need to
replace them with fluorescent fixtures. Metal halide
does not work well with multiple on/off cycles.
Where To Put Light Sensors
If you plan to value engineer your light sensor
system, then just leave it out completely.
• Low budget, doesn’t work – One sensor on roof
• High end, too costly – One sensor on each fixture
• Best approach – One sensor every 1,500 square feet
Sensors need to be located near floor level, and “see”
light levels on the merchandise. Make sure they do
not “see” skylights or artificial lights. Above all,
consult with a Registered Professional Engineer first.
Skylights
Daylighting will only work in a big-box store if you
use skylights.
• No more than 5% of the roof area should be skylight
area.
• Generally, skylights should be spaced 1.0 to 1.5
times the ceiling height (center to center). For
example, if the ceiling height is 12-feet, the
skylights should be spaced 12- to 18-feet apart.
Don’t cut cost here! Only use reputable, local
contractors with a good skylight system, who offer an
enforceable, blanket roof warranty.
Skylights – Additional
Don’t let your designers talk you into clear skylights.
You will cook in the summer!
• ASHRAE Standard 90.1 recommends a maximum
solar heat gain coefficient (SHGC) of 0.2 to 0.3
depending on the building location.
• This standard can be ordered at the following web
site: www.ashrae.org.
You want diffused light in the spaces, not direct
sunlight. The sun is directly overhead at the warmest
time of the year, so don’t go with clear skylights.
Interior Colors
If your interior designer insists on grey or dark colors
on the ceiling and floor, stop now. Daylighting won’t
work
• People will not tolerate dark or shadowed ceilings.
• Bright white would be nice, but off-white is a good
compromise.
• Surface reflectivity of 0.8 or higher.
A white ceiling mimics the natural sky. If you have
to look up to see if the lights are off, your daylighting
is working.
Additional Enhancements
Many products are available that claim better results.
Review these with a Registered Professional Engineer
who has experience with daylighting and skylights.
• Skylights with sun-tracking ability. The trade off is
better performance versus more complexity and
maintenance.
• Drop ceilings. The trade off is another $1.00/s.f.
Refer your designers to Skylighting Guidelines, Your
Guide to Energy Efficient Design Practices (1998).
Cost Summary
These values are general rule-of-thumb numbers. Get
a specific quote from your general contractor before
making a decision. All values are per square foot (sf)
of floor area.
• Skylights cost $25 to $45/sf of skylight assembly.
At 5% roof area, the overall cost is $1.25 to $2.25/sf
of building floor area.
• Lighting controls add $0.50 to $1.00/sf.
• Total installed cost is about $2.00 to $2.50/sf.
So, what’s the payback?
Payback
Would you believe an immediate payback is
possible?
• Total installed cost is about $2.00 to $2.50/sf. Let’s
go with the higher number of $2.50/sf.
• What if you finance the cost of the skylights (which
you were planning to do anyway) on a 15-year
commercial loan with a 9% APR?
• Annual repayment = $2.50 X 0.1241 = $0.31/sf.
Remember our previous example where we calculated
an annual savings of $0.33/sf just for energy?
Conclusion
• Skylights are an effective means for daylighting.
• They can reduce energy cost, improve employee
health and productivity, and increase sales.
• Skylights can deliver immediate payback.
• Above all, hire professionals who know what they
are doing. Check their references carefully.
• See the Energy Center of Wisconsin - Daylighting
Collaborative Web Site for more information:
www.daylighting.org
Questions?
Eric T. Truelove, P.E., LAP, Associate AIA
Director of Sustainable Design Services
The Renschler Company
Madison, Wisconsin
608-833-2321
erict@renschler.com