Onondaga County sought to evaluate the energy and water retention performance of green or vegetative roofing systems relative to other conventional and energy-efficient roofing systems. A major roof replacement project on multiple buildings at the Jamesville Correctional Facility offered the opportunity for a side-by-side test to evaluate different roofing systems. Monitoring equipment and instrumentation were installed to measure the performance of the different systems. The test considered four different roofing systems:
1. A conventional roof with 4 inches of foam insulation and a black Ethylene Propylene
Diene Monomer (EPDM) membrane
2. A roof with 4 inches of foam insulation with a white Thermoplastic Polyolefin (TPO)
3. A vegetative roof with 4 inches of foam insulation
4. A highly-insulated roof with 8 inches of foam insulation and a TPO roof membrane.
Onondaga County extended the design contract with Ashley McGraw Architects to complete this testing. CDH Energy was hired to develop and implement a monitoring approach to quantify compare the performance of the four roofing systems. Temperature sensors and other instrumentation were installed in the roof assembly during construction in the Summer and Fall of 2009. Continuous data collection at 15-minute intervals has continued since October 2009 to
obtain performance data to assess performance of the different roofing systems for all seasons of the year.
The measured results showed that the TPO and vegetative roof systems had much lower roof temperatures than the conventional EPDM surface. The reduction in solar absorption reduced solar gains in the summer but also increased heat losses during the heating season. Compared to the EPDM membrane, the TPO roof had 30% higher heating losses and the vegetative roof had 23% higher losses. The TPO roof with extra insulation did have lower heating losses than the EPDM roof.
Overall the TPO roof was cost neutral compared to the EPDM roof when both heating and cooling losses are considered. The vegetative roof had net cost savings of $7 per year per 1000 sq ft of roof area. The vegetative roof retained a significant amount of the rainfall across the year. On an annual basis only of about 20% of the measured rainfall was sent into the storm drain system.
CDH Energy Corp.
Source: Onondaga County Department of Correction
Publication Date: October 2011