Final Report: Implementation of Green Roof Sustainability in Arid Conditions

EPA Grant Number: SU833951
Title: Implementation of Green Roof Sustainability in Arid Conditions
Investigators: Jenkins, Bryan M. , Bhat, Ashwini , Botros, Christopher , Fong, Karen , Patel, Vishal , Valladares, Carmen , Watts, Andre
Institution: University of California - Davis
EPA Project Officer: Nolt-Helms, Cynthia
Phase: I
Project Period: August 15, 2008 through August 14, 2009
Project Amount: $8,915
RFA: P3 Awards: A National Student Design Competition for Sustainability Focusing on People, Prosperity and the Planet (2008) RFA Text |  Recipients Lists
Research Category: P3 Challenge Area - Built Environment , P3 Challenge Area - Energy , Pollution Prevention/Sustainable Development , P3 Awards , Sustainability

Objective:

Green roof technology has already been implemented in certain parts of the world including Europe, Canada, and Sweden where the ability to buffer heat transfer through a building has been proven. Research also indicates that green roofs can mitigate problems associated with storm water runoff, the urban heat island effect, wildlife habitat, and air and water quality (Kosareo, 2006). Few studies have been done in dry, arid climates where the effects of heat on buildings are extreme. Some of the major concerns for building owners to implement these roofs are the lack of information regarding the construction and maintenance of green roofs. This research will attempt to design, build, and present a self-sustaining green roof capable of maintaining a lower indoor temperature and lower water runoff than a conventional roof that complies with the requirements proposed by the Environmental Protection Agency (EPA). The roofs will be tested for their ability to maintain low temperatures throughout their layers and retain water runoff. These results will allow us determine if the extensive green roof is a viable option for improving overall building energy efficiency.

Summary/Accomplishments (Outputs/Outcomes):

We successfully designed and fabricated accurately scaled prototypes of a green roof and a conventional white roof and began testing in simulated conditions of 115-70°F with relative humidity of 13%. The design parameters were based on analytical models created through verified equations programmed into MATLAB (The MathWorks, Natick, MA). These equations were based on composite thermal resistance and evapotranspiration by the plants. Our green roof model resulted in a 14°C (25.45°F) difference in temperature for the air below the green roof compared to the air below the white roof. This is a heat gain reduction of 25% by the green roof compared to a 4% reduction in heat gain by the white roof. We also achieved moisture retention of 1.1 pounds of water per square foot of roof. This water will potentially be lost through evapotranspiration by the plants. The total weight of the green roof is 40 pounds per square foot due to our multiple layers, the water being retained, and the volume of growing medium. The green roof was effective under short term testing in reducing heat transfer through the roof while also surviving the harsh arid climate suggesting this green roof model is both energy efficient and durable.

Conclusions:

One of the most efficient methods for reducing the environmental impact of commercial buildings is to reduce the amount of energy used in heating and cooling. Due to evapotranspiration in addition to increased thermal resistance, the surface and subsurface temperatures of a green roof can be significantly cooler. The layer of biomass atop the green roof provides a direct reduction in the amount of solar radiation transmitted through the roof. By reducing the heat transfer and having a heat sink in the form of evapotranspiration a building equipped with a green roof is better able to regulate its internal temperature. Testing has shown that a green roof is able to reduce dead air temperature by 14° C compared to a conventional white roof. A lower heat transfer leads to a direct decrease in cooling cost thus reducing the amount of energy demand. This reduction of usage of energy contributes to a greener planet by reducing harmful carbon emissions.

Proposed Phase II Objectives and Strategies:

The objectives of Phase II are as follows: to implement a soil tension device to automate roof irrigation under the extreme conditions of arid environments; to use drought adapted plants such as buffalograss as well as other vegetation for the green roof instead of sod; to reduce the weight of the green roof by 20%, and to build a larger scale model, preferably up to about 400 square feet. We want to increase the size of the green roof to reduce edge effects and better simulate full scale applications.

Supplemental Keywords:

Sustainable development, green building, economics, vegetated roof, energy efficiency, heat transfer,