Cool vs. Green Roofs:
How They Top Out
Reflect on the sunny possibilities of these two environmental
Cool (or reflective) roofs and green (or vegetative) roofing
systems are two of the most effective ways to increase a building's
energy efficiency. While similar in some aspects, they are not
interchangeable. The two have distinct attributes that should be
evaluated and compared prior to installation.
"Cool roof is a term encompassing several different product
types," says Jessica Clark, marketing manager at Cool Roof Rating Council,
Oakland, Calif. "Today there are cool options in all types of
roofing materials, from coated metals and coatings, to capsheets
and single-plies, to shingles and tiles."
A cool roof can be aluminum, hot-dipped galvanized, or
Galvalume. "Cool metal roofing is available in a wide range of
profiles and colors, and is able to produce many different looks
for a commercial or residential structure," says Mark A. Thimons,
P.E., LEED AP BD&C, executive director, Cool Metal Roofing
You can even have a black roof that qualifies as cool.
"Reflective pigment technology can increase the solar reflectance
of a metal roof (in wavelengths outside of the visible range)
without changing its appearance or color," Thimons adds.
A cool roof reflects and emits the sun's heat back to the sky
instead of transferring it to the building below. Coolness is
measured by two properties: solar reflectance and thermal
These cool properties can deliver significant energy savings. In
the United States, annual cost reduction is typically between 7 and
20 cents per square feet, according to an energy savings model
developed the U.S. Department of Energy. Cool roofs significantly
reduce energy costs related to cooling a building. Energy
represents 30 percent of a typical office building's costs and is a
property's single largest operating expense.
Cool roofs lower ambient temperatures helping to reduce the
urban heat island effect associated with densely populated cities
and suburbs. They lower internal building temperatures improving
Cool metal roofing has been designed to meet the criteria of
Energy Star, the United States Green Building Council's LEED
program, California's Title 24, ASHRAE 189.1, and numerous state
and municipal programs. "The ability of a cool roof to reduce
energy consumption has been widely documented," says Laura Lanza,
marketing chair, National Coil
Coating Association, Cleveland. "The EPA estimates that a cool
Energy Star labeled roof on a building can save up to 40 percent in
annual energy costs."
Green roofs started in Babylon in the 7th century B.C. The first
was the Hanging Gardens of Babylon, a majestic structure built by
King Nebuchadnezzar II for his wife, Amyitis. Today, a green roof
is a partially or completely covered roof with vegetation like
shrubs, trees, durable grasses and sedum plants in a growing
medium, over a waterproofing membrane.
Green roof systems may be modular, with drainage layers, filter
cloth, growing media and plants already prepared in movable,
interlocking grids, or each component of the system may be
installed separately. They may also include additional layers such
as a root barrier and irrigation system. Green roofs convert carbon
dioxide to oxygen, provide insulation, create a natural wildlife
habitat, process airborne toxins, offer aesthetic qualities and
like cool roofs, mitigate the heat island effect.
A vegetative covering protects the waterproof membrane from
daily temperature fluctuations and the ultraviolet radiation of the
sun that breaks down conventional roofing systems. Similar to a
green roof, a vegetative roof's lower surface temperature reduces
the heat island effect in urban areas. Their increased insulation
values stabilize indoor air temperatures and humidity, and reduce
the heating and cooling costs for a building.
There are two types of green roofs: intensive roofs, which are
thicker and can support a wider variety of plants but are heavier
and require more maintenance, and extensive roofs, which are
covered in a light layer of vegetation and are lighter than an
intensive green roof.
Green roofs are most effective on flat-roof surfaces (maximum
slope of 1:12) using the largest continuous roof area. Slightly
higher slopes up to 2:12 are feasible but may require additional
Green roof vegetation selection is critical and must be
considered in initial planning. It must be chosen with regard to
the amount of care it needs, its mature size and roof wind
conditions. Vegetation that is oversized may suffer in shallow
green roof soils and become uprooted in high winds if there is no
shelter. Plants must be able to withstand the harsh climates and
weather extremes found on roofs.
Cool metal roofing is virtually maintenance free and can last 30
to 40 years. "Coatings are considered 'self-cleaning' in that they
resist dirt pickup," says Lanza. "Some paint manufacturers
recommend an annual sweet (tap) water rinse in highly corrosive
environments, but unless your building is in a heavy industrial or
seacoast environment, no maintenance is required."
Thimons agrees, saying cool metal roofs require minimal
maintenance and cleaning. "Research at the Oak Ridge National
Laboratory demonstrates that metal roofing retains its solar
reflectance over time better than other roofing products because it
resists the growth of organic matter and sheds dirt more readily
than other materials. It maintains its surface properties and
On the contrary, a green roof is a highly engineered living
system needing a routine and consistently executed maintenance
protocol including watering, weeding, fertilizing, replanting, and
replacing dead or dying vegetation. An ineffective maintenance
program will result in a wild and overgrown look that lessens the
green roof's success. Weeds, discolored or scorched plants, small
plants and plant loss are signs that a green roof is not being
correctly maintained. Fungal diseases and insect problems must be
checked on a regular basis.
According to Thimons, there is typically no additional expense
for installing cool metal roofing when compared with non-cool metal
roofing. And, "it pays for itself over time with its energy
efficiency and durability benefits," he says.
However, the initial expense of green roofs is higher than cool
roofs. This is because they require more extensive design, careful
structural analysis, and multiple layers and systems. Some types of
green roofs have more demanding structural standards, especially in
seismic regions of the world.
"For initial green roof construction, you are going to have
multiple layers and a lot more cost involved with insulation, as
compared to metal roofing," says Brian Partyka, president of Drexel Metals, Philadelphia. "It
has to do with the depth of the roof and how many inches you're
going to create with the vegetative roof; there are a lot of
variables here." John Ferraro, general manager at Asphalt Roofing
Manufacturers Association, Washington, D.C.,says the same principle
applies to asphaltic roofing products. "Construction and
installation costs for an asphaltic cool roof can vary greatly
depending upon the desired cap sheet or surfacing material. A green
roof on the same building could generally prove more cost intensive
than a cool roof due to the labor, materials and drainage issues
but, of course, each case is different."
It is important to remember that because a green roof protects
the waterproofing membrane from the elements, particularly UV
light, the membrane's life expectancy is doubled or even tripled,
leading to recovered initial cost differentials.
One of green roof's best attributes and one of its biggest
pluses over cool roofs is its ability to absorb, collect and reuse
rainwater. Most buildings are designed to shed rain; they're built
with hard, impenetrable roofing surfaces. Rainwater bounces off
them and collects as runoff, picking up impurities like infectious
bacteria from animal waste as well as harmful pesticides and
fertilizers on the way to municipal storm sewers. All of this
eventually empties into local waterways.
Green roofs decrease the total amount of runoff and slow the
runoff rate from the roof. They can retain up to 75 percent of
rainwater, gradually releasing it back into the atmosphere via
condensation and evapotranspiration (a natural process that cools
the air as water evaporates from plant leaves), while retaining
pollutants in their soil.
Correct drainage from a green roof's vegetation is critical. If
excess water is not properly directed off the roof, or if the
system is incorrectly designed and becomes clogged, this adds far
more weight to the green roof than it is supposed to hold. If a
green roof is incorrectly assessed for the weight it can bear, or
if that weight limit is not heeded, the roof can be damaged or even
collapse from the combined weight of plants, soil-holding water,
drainage systems and other green roof construction materials.
Also, the excess water can become stagnant or drown the plant
roots. Leaks in a green roof arise from poor design or inadequate
root barriers. Root barriers are specific materials designed to
inhibit the slow but inexorable dig of roots, which can break apart
even sturdy roof materials in search of nutrients. Leaks also occur
from holes or improper installation of the waterproof lining layer
of a green roof. Installers must ensure that no punctures occur,
because tiny holes are difficult to find and repair.
Routine monitoring of a green roof's underlayment is no
different than maintaining any comparable roofing system. When
leaks are detected under green roofs, a procedure exists for
cutting out and removing a section of green roof and reinstalling
it after repair.
Unlike the growing cool roof industry, the benefits of green
roof technologies are poorly understood and the market remains
immature in the United States. However, in Europe, green roofs have
become very well established. This is the direct result of
government legislative and financial support at both the state and
municipal level. This support has led to a multimillion dollar
market for green roof products and services in Germany, France,
Austria and Switzerland. In Germany, it is estimated that 14
percent of all buildings are covered with vegetation, a number that
is increasing as the German green roof industry continues to grow
10 to 15 percent per year.
Will green roofs ever catch on in the United States like they
have in Europe? According to Michigan State University's Department
of Horticulture's Green Roof Research Program, several barriers to
widespread green roof acceptance exist in North America. These
include a lack of awareness regarding green roofs, potentially
higher installation costs, limited quantifiable data pertaining to
the benefits they provide, no technical information on how to build
them, and a lack of government incentives or tax breaks. But these
problems are not insurmountable and are being addressed.
Meanwhile, in Chicago, codes offering incentives are available
to builders who put green roofs on their buildings. The Chicago
City Hall green roof is one of the first and most well-known
examples of green roofs in the United States. It was planted as an
experiment to determine the effects a green roof would have on the
microclimate of a roof. According to the Chicago Department of
Environment, on hot days, temperatures atop it are typically 25 to
80 degrees Fahrenheit cooler than an adjacent county office
building with a black top roof.
The best of both
When installed correctly, cool roofs and green roofs have
excellent insulating properties. But, "cool roofs and green roofs
are different technologies used for different applications," says
Clark. "Both have their own environmental and energy efficiency
benefits. Cool roofs and green roofs are complementary
technologies, and can be combined by using cool pavers on a green
roof patio space."
Ferraro agrees, saying both cool roofs and green roofs can
increase a building's energy efficiency and recommends adopting a
"whole building envelope approach" when trying to reduce building
energy consumption and achieve maximum efficiency results.