Sustainable Design for the New Millenium (4)

example, researchers are monitoring the levels of endangered bird, spider, and

other invertebrate species which were found to have come back to the city after

construction of greenroofs on previously disturbed sites.

Economic Benefits

Establishment of a thriving greenroof industry could have innumerable

effects on the economy, including the creation of many new jobs in

manufacturing, construction, and design, as well as in installation and other

services.

Greenroof construction usually entails higher initial costs, but life cycle

analysis reveals that these costs can be offset through extension of the life of the

roof, avoided maintenance and replacement costs, reduction in cooling and

heating costs, increased developable space, reductions in local impact fees, and

the opportunity to take advantage of the amenity of greenspace at roof level.

Other economic benefits may be harder to quantify, but include acoustical

insulation (resulting in noise suppression effects ranging from 8 dB up to 50 dB),

glare reduction, decreased charges for stormwater infrastructure rehabilitation,

and the goodwill and publicity generated from having a high-profile greenroof

project.

Some of the key economic benefits of greenroofs are discussed in more

detail below.

Increased Roof Longevity

Greenroofs in Europe have easily lasted from 40 to 75 years, or even

much longer. Common theory holds that roof life can be at least doubled, and

perhaps tripled or more, with a greenroof. The main reason for this is that the

multiple layers protect the waterproofing membrane and structural elements from

damaging ultraviolet rays, wind, and temperature fluctuation extremes.

In Europe, Japan, and North America, major greenroof providers will issue

at least a 20-year assembly warranty and performance guarantee. In Germany,

direct greenroof subsidies are available in about 30 cities. They range from

$0.51 to $6.20 per square foot, based on avoided maintenance and replacement

costs.

Reduced Energy Consumption and Costs

Thermally insulating greenroofs offer energy savings. Benefits vary by

geographic region and type of system, but it is agreed that they can reduce peak

energy demand by lowering cooling and heating needs, at least for the floor

directly below the greenroof.

Some experts argue that some published energy reports have been

exaggerated. They nevertheless agree it is impossible to issue blanket

statements regarding energy savings for every region of the world, since many

factors contribute to the figures. When estimating energy savings, it is essential

to study each climate individually, using thermodynamic data.

That said, in December 2000, Weston Solutions design consultants

conducted an energy study for the City of Chicago which estimated that it would

be possible to save $100,000,000 in avoided energy costs annually with the

greening of all the city's rooftops. The study's bottom line stated that "[p]eak

demand would be cut by 720 megawatts -- the equivalent energy consumption of

several coal-fired generating stations or one small nuclear power plant.” Weston

also declared that, in general, reductions of up to 50 percent of cooling costs and

25 percent of heating costs could be achieved, at least for the floor directly below

the greenroof.10

A 2003 study commissioned by Seattle's Office of Sustainability and the

Environment states that the Seattle Justice Center is saving as much as

$148,000 each year due to its greenroof.11

Understanding how heat moves through a greenroof can be tricky,

however. Engineer and energy modeler Chris Wark of Green Roof Innovations

explains:

Energy usage is reduced primarily due to the solar heat

management of the foliage and thermal mass of the soil substrate

(not the plants). Plant leaves transfer nearly all excess solar

energy to the surrounding air and absorb the rest, while the soil

mass provides an additional benefit of dampening temperature

fluctuations. Leaf transpiration is one of the ways in which the solar

heat is transferred to the air. If enough water is available,

additional heat can be removed from the plant, but this is a minor

effect with succulents. The fact that leaf temperatures of many

different studied plants tends toward ambient air temperatures

proves this. In most cases, a green roof comes with a heating

penalty if any moisture is at all retained in the soil (and it is).

Chris and Wendy Wark have reported results from a study done on

commercial buildings in Northern California using DOE-212 and a proprietary roof

heat transfer model developed by their parent company, Shade Consulting.

Their study indicated that an uninsulated greenroof could reduce the building

heating/cooling system's demand for most of the year by 30 percent over a

conventional dark roof with R-18 rigid insulation and without a radiation barrier

Increased Developable Space

Major cities that are embracing sustainable design have acknowledged

the economic benefits of greenroofs and are helping to pass the savings they

generate along to owners and builders. For example, city officials may reduce

impervious coverage requirements for developers who incorporate greenroofs

into their site plans.

Depending on local ordinances and applicable BMPs, officials may allow

greenroofs to be installed in lieu of conventional stormwater management

elements. Greenroofs can significantly reduce the required size of unsightly,

space-wasting, and expensive retention ponds or underground galleries, or even

completely eliminate the need for these elements.

In some cities, floor-to-area development ratios can also be increased. In

Portland, Oregon, for example, builders can now increase their floor area ratio

(FAR) when they include a greenroof that covers at least 60 percent of the roof

surface. This FAR bonus grants an additional three square feet of floor area per

square foot of greenroof, to be added to the footprint of the building.

The City of Chicago also increases development square footage, known

as floor area premiums, when developments include public amenities such as

greenroofs.

Reduced Local Impact Fees and Increased Incentives

Reduced stormwater and impervious cover fees, as well as energy credits,

grants, and tax incentives for greenroofs, have been in place in European

countries such as Germany, the Netherlands, Switzerland, and Sweden for

decades. For example, some German municipalities offer stormwater fee

reductions of 50 to 80 percent.

Cities in the United States and Canada are now beginning to offer

incentives as well. Portland, Oregon plans to reduce stormwater utility fees for

buildings with greenroofs by July 2006. The City's Clean River Incentive and

Discount Program promotes placement of ecoroofs atop commercial, industrial,

institutional, multi-family, and single family residential properties.

New York, Seattle, Chicago, Toronto, Vancouver, St. Paul, Atlanta, and

several cities around Washington, D.C., are among those working toward

reducing various fees in exchange for greenroof development.

Greenroofs as Stormwater Mitigation Measures

Greenroofs can sometimes be used as stormwater mitigation measures.

In Portland, Oregon, all building projects that will result in at least 500 square feet

(46 square meters) of impervious surface are required to implement stormwater

pollution reduction and flow control measures. Greenroofs are recognized as

one of the acceptable approaches to meeting this requirement.14

Greenroofs as Heat Island Measures

In a concerted effort to combat the ever-increasing urban heat island

effect in Tokyo, the city's "Tokyo Plan 2000" was implemented on April 1, 2001.

It requires new buildings that are larger than 1,000 square meters (10,000 square

feet), or over one-quarter acre, to green at least 20 percent of their useable roof

space.

Other countries considering these types of measures include South Korea

and Singapore. In the U.S., cities like New York would also greatly benefit from

such measures.

Increased Points in the LEED™ Rating System

The U.S. Green Building Council (USGBC) has developed and oversees

the LEED (Leadership in Energy and Environmental Design) Green Building

Rating System®, a voluntary, consensus-based national standard for developing

high-performance, sustainable buildings. The four levels of certification include

LEED™ Certified, Silver Level, Gold Level, and Platinum Level. Greenroofs

qualify for at least six points in three categories, and more points are possible

under specific conditions.

Many local, state, and federal agencies have adopted sustainable design

stipulations that adhere to LEED™ principles. For example, in 2000, the City of

Seattle adopted its Sustainable Building Policy, which requires many new city

buildings to attain a Silver LEED™ certification rating. The requirement applies

to new and renovated city facilities that are larger than 5,000 square feet.

The General Services Administration (GSA), a federal agency, requires

buildings to be certified through LEED™, and encourages them to achieve a

Silver LEED. EPA requires Silver LEED™ certification for new significant

building construction or acquisition. NASA encourages its designers to strive for

a LEED™ Gold rating, if cost effective.15

Increased Building Marketability

High-rise apartments, office space, and even hotel rooms with the

enhanced natural view afforded by greenroofs can support higher rents or room