Green Roofs.
Benefits
How
about designing a building that nourishes and restores
living systems? That engages propitiously with the
industrial system in a way that does not destroy nature?
Imagine a structure that is actually fecund, providing
more to the environment than it takes away; that engages
with the sun the way a tree does, with a photosynthetic
connection, moisture transpiration, habitation by
hundreds of species, transformation of microclimate,
distillation of water, and production of complex sugars
and carbohydrates; that sequesters carbon, fixes
nitrogen, and changes with the seasons. Imagine a
building like a tree, a city like a forest.[1]
Stormwater
Load Reduction
Rainwater
retention will vary due to the depth of green roof
growing medium, type of drainage system, and
intensity and duration of the rain event. It is
difficult, then, to cite definitive retention statistics
for all green roofs and all storms. As an example,
a 4" layer of expanded clay growing medium retained
47.29% of a 2.2 inch rain event lasting 30 hours.[2]
To get some idea of the annual stormwater load reduction
potential in Georgia using this data, a 1000
square foot green roof might relieve the municipal storm water system of approximately
14,500
gallons of rainfall annually.[3a-3b] Without the
green roof this rainwater would otherwise be channeled
through the public storm water drainage system, unused
by nature. For municipalities with already taxed
and/or aged stormwater and waste water systems, green
roofs alleviate a crucial burden on the municipal
infrastructure.
Energy
Efficiency
Rooftop
surface temperatures can climb as high as 140 degrees on
a summer day where the ambient daytime temperature was
around 88 degrees,
according to David Beattie, director of the Penn State
Center for Green Roof Research. With a green roof,
daytime rooftop temperatures average 82 degrees.[4]
Thermal insulation properties help buildings retain heat
in winter, too. The green roof contributes to more
comfortable interior temperatures that are stabilized
throughout a 24-hour period. Green roofs present an enormous payoff in terms of
substantially reducing the building heating and cooling
needs that drive energy consumption. Chicago's
City Hall, with a very large green roof, has reduced
heating and cooling system use by 30% over the last 4
years.[5]
Reduced
Heat Island Effect
Heat
island effect is created by artificially higher
temperatures that result from concentrated radiant heat
from building surfaces, rooftops, and asphalt and
concrete paving. These raised temperatures promote
ground-level ozone, depleting air quality in the
immediate environment and rendering humans more
susceptible to heat-related health issues. Both
the US Department of Energy and the EPA promote the use
of green roofs to reduce urban heat island effect.
By
receiving, processing and transpiring rainwater the
green roof cleanses the surrounding air by absorbing
carbon dioxide and producing oxygen. Thus health issues resulting from poor air quality
are also reduced.
Additionally, heat island effect increases summer energy
consumption, using more gas and electricity to cool
buildings, homes and cars. For every 10F
increase in summertime temperature, peak utility loads
in medium and large cities increase by an estimated 1.5
- 2 percent.[6]
When heat
radiance from a typical roofing material evaporates
moisture from the surrounding air, the plant material in
the immediate vicinity is more vulnerable to drought
stress than when temperatures are lower and more even
overall. Green roofs help sustain overall
conditions which reduce normal evaporation rates from
temperature increase. The result for the immediate
vicinity is a more favorable growing environment.
As such, each green roof installed in a given area
provides exponentially higher positive impact on the
macro- and micro-climate; when each plant has the best
possible conditions they flourish, thereby rejuvenating
the immediate environment to their maximum capability.
Wildlife
Habitat
Green
roofs present a favorable habitat to insects, birds and
bees. Where urban and suburban development reduce
the natural habitats available to these creatures, green
roofs help to mitigate survival stress on these
populations. Providing a habitat for pollinators
is critical; approximately 75% of the staple crop plants
the world depends on for sustenance require pollination.[7]
Cost vs. Benefit
The cost
for a green roof will vary widely by application and
size. For a residential extensive green roof, the
initial cost might be two to three times that of a
traditional roofing system. Yet a green roof
protects the roofing membranes from harmful UV rays
which accelerate roof membrane breakdown, typically
extending the life of the roofing membranes by two to
three times. Coupled with the cost savings in
heating, cooling and water utility costs, the benefits
to the local ecosystem often drive the environmentally
responsible property owner to install a green roof.
[1]
McDonough, William and Michael Braungart, MD.
"Eco-Effectiveness: A New Design Strategy."
Sustainable Architecture - White Papers. New York:
Earth Pledge Foundation, 2000, p. 3.
[2]
Penn State Center for Green Roof Research.
"Research: Stormwater Management." 8 Nov. 2004.
Penn State University. 22 Oct. 2005.
<http://hortweb.cas.psu.edu/research/greenroofcenter/research.html>
[3a]
Meeuswissen, Harrie, "Georgia Weather and Climate." 3
Jan 2005. HM USA Travel Guide. 19 Feb. 2005,
chart.
<http://www.hm-usa.com/climate/ga.html>
[3b]
Waterfall, Patricia H., "Harvesting Rainwater for
Landscape Use", University of Arizona Cooperative, 22
Oct. 2005.
<http://ag.arizona.edu/pubs/water/az1052/harvest.html>
[4]
Wall, John. "Time to Mow the Roof." Penn
State Agriculture Magazine. Spring/Summer 2001.
Penn State University. 22 Oct. 2005, paragraph 5.
<http://www.aginfo.psu.edu/PSA/ss2001/mowroof.html>
[5]
Sarnafil Inc. "SarnaProof Project Profile: Chicago
City Hall", 2004.
[6]
EPA: EPA Global Warming Site. "Heat Island Effect." 22
July, 2004, Georgia Green Industry Association. 22 Oct.
2005, paragraph 4. <http://yosemite.epa.gov/oar/globalwarming.nsf/content/
ActionsLocalHeatIslandEffect.html>
[7]
EPF Green Roofs. "Green Roofs: Public and Private
Benefits." 2002. Earth Pledge Foundation.
22 Oct. 2005. <http://www.earthpledge.org/PublicPrivate.html>
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