Final Report: The Integration and Optimization of Sustainability Benefits from Shading-Type Green Roof TechnologyEPA Grant Number: SU835074
Title: The Integration and Optimization of Sustainability Benefits from Shading-Type Green Roof Technology
Investigators: Loge, Frank , Chacon, Kimberly , German, Alea , Holmes, Amelia , Sherraden, Catherine
Institution: University of California - Davis
EPA Project Officer: Hahn, Intaek
Project Period: August 15, 2011 through August 14, 2012
Project Amount: $11,162
RFA: P3 Awards: A National Student Design Competition for Sustainability Focusing on People, Prosperity and the Planet (2011) RFA Text | Recipients Lists
Research Category: Pollution Prevention/Sustainable Development , P3 Challenge Area - Built Environment , P3 Challenge Area - Energy , P3 Awards , Sustainability
We propose to design and monitor an innovative and affordable green roof technology that uses the shading capacity of plants to reduce solar heat gain to roof surfaces. The vegetated cool roof system consists of a planter box installed along the eaves of a house planted with vines, which are trained on a trellis up the roof. Unlike heavier conventional green roofs, the vegetated cool roof is appropriate for the retrofit of a large population of existing buildings, including the light-frame type widely used for low-rise and residential construction. The proposed technology dramatically reduces a building’s consumption of not just electricity but also water, and points to a major shift away from refrigerated air cooling systems in arid climates.
This building-integrated technology builds on three distinct sustainability benefits. First, it acts as a cool roof by rejecting both incoming solar radiation as well as heat captured by the roof surface thereby reducing a home’s cooling energy use. Second, the vegetated cool roof provides the stormwater benefits of a green roof but uses dramatically less water for irrigation because of the reduced planted area. Third, flowering vines serve as nectar grazing habitat for pollinators who provide critical ecosystem services for our nation’s food crops. These three benefits are achieved with a single technology that can be optimized for any one particular benefit or integrated to reap the benefits of energy efficiency, stormwater management and pollinator habitat combined.
WHAT DISTINGUISHES THE VEGETATED COOL ROOF? Vegetated cool roofs are lightweight when compared to conventional green roofs. The relatively heavy green roof soil, or growing media, is consolidated into planters along the roof eave from which vines trellis onto a cable system secured by anchor points strategically placed on the roof (see Figure 1). This elegant design requires little additional structural support and a fraction of the water than that needed to sustain a conventional green roof. However, the vegetated cool roof still provides stormwater detention and invertebrate habitat, which are known benefits of green roofs. The simplified planter design easily retrofits existing roofs without penetrating the building interior or requiring supplemental structural support.
Key developments during Phase I include:
- Development of an energy simulation quantifying reduced building cooling loads
- Validation of the water system concept via a water resources and irrigation demand daily time series spreadsheet model
- Design development of the vine planter and production of scaled drawings detailing system components, relation to the roof surface and critical points of connection
- Preliminary plant species selection
- Identification of target pollinator species and habitat requirements
- Preliminary cost estimate and benefits analysis
Phase I energy modeling demonstrates that a vegetated cool roof has the potential to reduce energy expenditures in an average household beyond those provided by a typical cool roof. Based on the model developed, the proposed technology reduces the annual electricity use for cooling an existing home by 18% in hot and dry climates such as Fresno, California. Peak demand reduction is estimated at 13%. This finding indicates that this technology provides a substantial opportunity to improve energy efficiency across a large volume of existing older construction. While the research conducted to date has targeted hot and dry climates with high energy use at peak hours, the modeling efforts can easily be expanded to investigate climates across the U.S.
Additional investigations were conducted into additional auxiliary benefits of the vegetated cool roof. Auxiliary benefits that were demonstrated in concept during Phase I include: use of roof as pollen and nectar sources for bumble bees and the elimination of potable water use for irrigation. It was determined that shelter habitat at ground level will need to be provided to effectively attract pollinators due to the ground-dwelling habit of many bee species. A water balance determined that the vine planters could be irrigated with household greywater and seasonal rainwater without the need to store treated water even during the driest summer months in the arid West.
Significant progress towards system design was achieved enabling the technology to provide yet another sustainability benefit: stormwater detention. During Phase I, the vine planter was developed from the originally proposed roof-mounted unit into one that is sunken into the roof in a minimally intrusive fashion. The pocket design enables rain that lands on the roof and vines to flow through the planter system, in which two modes of drainage are provided. The proposed system is therefore robust and functional as both a green roof and a cool roof.