With per-square-foot prices for vegetative roofs easily reaching double digits, it’s important that contractors strengthen their selling propositions when marketing these systems. 

With per-square-foot prices for vegetative roofs easily reaching double digits, it’s important that contractors strengthen their selling propositions when marketing these systems.

Fortunately, a recent study conducted by Oak Ridge National Laboratory (ORNL) and SPRI shows that vegetative roofs can help reduce heat gain and heat losses, resulting in significant energy savings in mixed climates.

The reduction in heat gain in cooling dominated periods - and heat losses in heating dominated periods - translates into lower heating and cooling demands for the conditioned space. However, the study notes that the energy savings offered by vegetative roofs are climate dependent and affected by the efficiencies of heating/cooling equipment. ORNL’s testing took place in the mixed climate of East Tennessee.

The study also hinted at some possible roof performance advantages of vegetative systems. ORNL reported lower membrane temperatures and temperature fluctuations under the vegetative roofing system than in the control black-EPDM and white-TPO roofing systems.

The ORNL research project was initiated in June 2008 by SPRI - the association representing sheet membrane and component suppliers to the commercial roofing industry - to quantify the thermal performance of various vegetative roof systems relative to black and white roofs.

“As part of the effort to reduce energy use in commercial buildings, energy codes are greatly increasing the required thermal efficiency of roofing systems,” says SPRI Technical Director Mike Ennis. “It is important to understand the energy savings associated with vegetative roofs so that they can be properly credited in these codes.”

A summary of the ORNL/SPRI study written by Andre Desjarlais, Abdi Zaltash and Jerald Atchley of ORNL and Mike Ennis of SPRI was released earlier this year at the RCI Annual Convention.

Demand Increases for Vegetative Roofs

Vegetative roof systems have enjoyed significant growth in the last decade in the United States. The numerous benefits that green roofs provide have helped to fuel their resurgence in industrial and urban settings. For these reasons, it was important that SPRI and the roofing industry quantify the thermal performance of various vegetative roof systems relative to black and white roofs.

In addition to energy savings, vegetative roofs offer many environmental and economical benefits. These include:

• storm-water retention.

• reducing the urban heat island effect.

• increasing the longevity of the roofing membrane.

• adding plants that enhance biodiversity and filter air contaminants.

• beautifying the surroundings by incorporating green space.

With that said, the goal of SPRI and ORNL in this study was to accurately measure the annual cooling and heating loads per unit area of three vegetative roofs. The study also included side-by-side comparisons with black and white roofing systems, as well as a test section with just the growing media without plants.

Stay tuned for a full report on ORNL’s “Thermal Performance of Vegetative Roof Systems” in an upcoming issue of Roofing Contractor magazine.