Green Roofs - Living Architecture Monitor - Summer 2011 - (Page 24)

PROJECT potential for CSO discharge into the city’s rivers. For the type of green roofs installed on Thin Flats, modeling using the RWS (German) program leads us to expect that the annual volume of runoff, relative to rainfall, will be reduced to about 20 percent of that expected without a green roof. However, this benefit is not distributed equally around the seasons, with reduction to 44 percent in the winter and nine percent in the summer. It is worth noting that green roofs are no different than other Low Impact Development (LID) measures, in that they are more effective in summer than winter. The ability to reduce peak runoff rates and to filter runoff is not as strongly influenced by the season. Based on long-term modeling using actual rainfall data from Philadelphia it is predicted that the runoff rate coefficient (peak runoff rate / peak rainfall rate) will be about 0.30 for the one-year probability rainfall event, and 0.45 for the 10-year probability storm. Philadelphia has made great strides toward incentivizing green roof installations. Commercial property owners can take advantage of a green roof business tax credit equivalent to 25 percent of the cost of the green roof up to USD $100,000, and by definition of the “green roof,” this includes the waterproofing membrane; a significant portion of the total installation cost since the selected membrane is integral to the green roof design. Residential properties, however, do not yet have this incentive. Additionally, the Philadelphia Water Department (PWD) has implemented a “green review” process for both commercial and residential projects which moves the stormwater application to the top of their pile for an expedited review, enabling a faster roadmap to moving through the project permitting process. This can be a significant benefit to the property developer. In July, 2010, the PWD began incrementally transitioning over a four year period to a parcel area-based billing system to capture stormwater related treatment costs from its customers, and in turn, offering fee credits, up to 100 percent, for approved stormwater management practices (SMPs). These credits, however, are only available to non-residential and condominium properties which have commonly owned areas. Thin Flats, composed of four duplexes and one single family home, did not qualify for these fee credits. Although all of the known benefits of incorporating green roofs into a project far out way the challenges, residential development often poses warranty hurdles. As with any backyard garden, plants require ongoing care; the beds must be weeded, watered and proper nutrition applied. In the commercial green roofing world, it is much more viable to implement an ongoing maintenance program with a single building owner to ensure that the green roof is properly maintained over its lifespan, however, in a “ALTHOUGH ALL OF THE KNOWN BENEFITS OF INCORPORATING GREEN ROOFS INTO A PROJECT FAR OUT WAY THE CHALLENGES, RESIDENTIAL DEVELOPMENT OFTEN POSES WARRANTY HURDLES.” for-sale residential project without a homeowners or condominium association, as is the case of Thin Flats, multiple owners with varying degrees of care and interest may be responsible for their green roof gardens. It can be difficult to ensure the long-term success of the green roof system if the owner is unwilling to consistently follow proper maintenance guidelines or to hire a professional to care for their system. The Thin Flats’ green roofs work in unison with a well-insulated and airtight exterior envelop which is the most effective and cost-efficient method for reducing energy consumption. Fresh make-up air is maintained in the homes through the use of Energy Recovery Ventilators (ERV) which effectively capture heat from the houses’ exhaust systems and temper incoming fresh air from the outside. The rooftops incorporate solar thermal arrays which deliver hot water to the homes’ radiant heated floors and domestic hot water. In its practice, Onion Flats continues to explore the typology of the Philadelphia Row house which is historically a long, narrow building and often very dark at its core. Through the use of a glass floor beneath the rooftop skylights, and interior clerestory windows within the inner spaces, natural light showers these rooms eliminating the need for artificial lighting during the daytime hours, further reducing energy consumption. This combination of common-sense passive solar design, a super-insulated and airtight thermal envelope, along with low-tech mechanical systems, the energy consumption in these homes has been reduced by 65 percent of a typical home designed to building code standards. Howard B. Steinberg, AIA, LEED AP, is principal of Onion Flats LLC, Plumbob LLC, JIG Inc., G.R.A.S.S. and BLOX Sustainable Building Systems, a collective of development, architecture, design-build and manufacturing companies based in Philadelphia, Pennsylvania. G.R.A.S.S., an acronym for Green Roofs and Solar Systems, is the construction arm of Onion Flats; it partnered with Philadelphia-based Roofmeadow, a nationally recognized green roof consulting firm headed by Charlie Miller, to incorporate the green roofs for the Thin Flats project. 24 LIVING ARCHITECTURE MONITOR SUMMER 2011 http://www.turfdiag.com http://www.turfdiag.com

Table of Contents for the Digital Edition of Green Roofs - Living Architecture Monitor - Summer 2011

Green Roofs - Living Architecture Monitor - Summer 2011
Contents
Water Worries
Cities Re-Imagined
Design Strategies for Wild Bees
Special Section: Cities Alive 2011 Conference Guide
Thin Flats
Sky Island at the Visionaire
Welcome New Members
New Corporate Members
Professional Calendar
Seven-Year Pitch

Green Roofs - Living Architecture Monitor - Summer 2011