Green Roofs: Calculating the Benefits

by Alison Kibe

The known benefits of green roofs are nothing new; they can reduce building heating and cooling costs, aid in the remediation of the “heat island effect” often observed in cities, and reduce storm run-off. With this in mind, and perhaps putting aside risks and costs associated with green roofs, should I install one? Green Roofs for Healthy Cities, a non-profit industry association for green roof companies, strives to make that an easier question to answer via its Green Roof Energy Calculator (GREC) (greenbuilding.pdx.edu). Developed by University of Toronto and Portland State University and funded by The US Green Building Council, GREC estimates annual cost and savings estimates for office and residential buildings with just a few clicks of the mouse.

A paper by David Sailor and Brad Bass detailing the development process of the GREC cites the need for better tools for architects, urban developers, and building owners who may have limited or no experience with green roofs. For those familiar with building energy simulation software, GREC can also help architects and developers understand storm runoff and urban heat impacts while considering limitations to soil depth and the roof area of the building. The calculator takes roof square footage, geographical location, soil depth, and plant coverage into account and gives the option to adjust the provided utility rates.

With the availability of such tools, are we likely to see the adoption of more green roofs? The answer is probably not. Like many other green technologies, there are still improvements to be made before green roofs become economically practical. An article written for Scientific American about New York City’s program that offers tax abatements to those who install green roofs has not gone quite as planned. The plant of choice for green roof projects was Sedum – a hardy plant that is able to survive the harsh conditions found atop tall buildings. However, it has been found that Sedum actually absorbs heat rather than reflecting it during warm months. Sedum is also not as efficient at absorbing water as other plants so minimizes potential benefits in that area.

A study by Jaffal et al. also shows that we are still learning how green roofs can be most effective. They found that green roofs in hot environments create the greatest benefits, but can cause a higher heating demand at certain points in the year. Insulation, another means of reducing a building’s heating and cooling cost, was also found to impact the effectiveness of green roofs, with greater insulation often reducing the heating and cooling impact of the green roofs.

The complex nature of how green roofs operate highlights the need for a tool like GREC for anyone considering a green roof that is not an expert in the field. The model used in the calculator is simplified, but for what it seeks to accomplish could be a useful tool when trying to design a more energy efficient building.

Jaffal, I., Ouldboukhitine, S., Belarbi, R., 2011. A comprehensive study of the impact of green roofs on building energy performance. Renewable Energy 43, 157-164.

 

Kraft, Amy. “Why Manhattan’s Green Roofs Don’t Work – and How to Fix Them.” Scientific American. May 17, 2013. http://www.scientificamerican.com/article/why-manhattans-green-roofs-dont-work-how-to-fix-them/

 

Sailor, D., Bass, B., 2014. Development and features of the Green Roof Energy Calculator (GREC). Journal of Living Architecture 3, 35-58.

 

The Green Roof Energy Calculator http://greenbuilding.pdx.edu/GR_CALC_v2/savings_v2.php

 

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