Cooler Than Cool Roofs: How Heat Doesn't Move Through a Green Roof

Green Roof Energy Series Intro
By Chris Wark, Energy Editor
April 5, 2010

Inaugural Green Roof Energy Column

Eight years ago, my wife, Wendy, and I attended the Chicago’s Greening Symposium as part of our exploration into the world of roof gardening.  At the time, I was researching fuel cell systems for Caterpillar and thermodynamics was fresh on my mind.  So during the Q & A of a green roofing presentation at the Symposium, I had to ask what the energy benefits are of vegetating a roof.  The answer was quick and almost terse: “There aren’t any.”  My brain responded just as quickly: “That can’t be right.  I feel a mission coming on.”  Thus began a year-long endeavor to apply my mechanical engineering education and experience to unravel the mysteries of how heat moves through a vegetated roofing system.

The result was the creation of a computer program that describes, in detail, the heat transfer through any type of roof system, energy savings (or costs) associated with it, and the subsequent financial impact.  At the time, validation was a challenge due to the dearth of instrumented green roof demonstration projects.  In fact, the only one published by 2002 was the study done by the National Research Council Canada.  Fortunately, this computer program, eventually dubbed Q-Calc, relied entirely on well-established engineering calculations and data, and as it turned out, was easily validated by some key plant biology research.

Shortly after attending Chicago’s Greening Symposium in 2002, Wendy and I established SHADE Consulting (later renamed Green Roof Innovations).  For the next 3 years we would provide a variety of green roof consulting services and develop 3 different modular green roof concepts, one of which has been picked up by a company in China.  With SHADE/GRI, I had the opportunity to provide energy consulting for some prominent green roofing projects, including pilot programs for the City of Chicago, the City of Portland (OR), The City of New York, and projects for Earth Pledge, including the Silvercup Studios green roof.

In 2005, I moved on to do more whole-building and other specialty energy analysis, but green roofs still hold a special place in my heart.  So I decided to spread the love and put together a primer following my discoveries of how heat moves through a green roof.

“Cooler than Cool Roofs: How Heat Doesn’t Move Through a Green Roof” is a 7- part series explaining the key aspects of green roof heat transfer issues and the best ways to take advantage of a green roof’s energy benefits.  The topics that will be covered are:

1. The Essentials: Heat Transfer by Layer
2. Keeping Drought-Resistant Plants Cool
3. Evapotranspiration
4. When Weight is Good
5. Assuming Insulation
6. Green Roofs vs. Cool Roofs
7. The Green Roof Energy Secret and How To Use It

Some of this discussion applies to more than just vegetation systems.  In fact, you may find it useful for any roofing system or exterior wall.  All of the discussion is based on long-established textbook physics, very conventional building engineering, and a little bit of recent plant research.

I hope you enjoy this series and are able to get a few “Ah hah!” moments out of it, or at least gain a better appreciation for the complex nature of something as simple as a green roof.

Chris and Wendy Wark

Chris Wark
Senior Energy Analyst
Viridian Energy & Environmental

Publisher's Note:  We've known Chris and Wendy for seven years now, and I'm proud to say we've remained friends.  We met at the first Greening Rooftops for Sustainable Communities Conference in Chicago, IL in 2003.  They both impressed us with their vitality, intelligence and passion for greenroofs.  FYI, read Wendy's Guest August 2004 Feature Article "Growing Green Roofs in the United States."

Christopher Wark has over 20 years of multidisciplinary engineering experience providing mechanical, thermodynamic, and electronics support and services to manufacturers, universities and national labs. For the past 8 years, he has focused his efforts on the development and promotion of technical solutions in architecture and construction. Chris is currently a Senior Energy Analyst for Viridian Energy & Environmental, which was previously a division of Steven Winter Associates. At Viridian, he conducts whole-building energy analysis, air flow modeling, and provides green roof design consulting.

Before joining Viridian, Chris provided energy analysis and LEED consulting services for several companies, including subsidiaries of the Integral Group and previous to that served as Technical Sales Manager for Mentor Graphics Mechanical Analysis Division (formerly Flomerics Inc.), offering energy and air flow analysis solutions for architectural engineers. In 2002, Chris established SHADE Consulting/Green Roof Innovations with his wife Wendy.
With SHADE/GRI, Chris developed and marketed several innovative modular eco-roof systems, a roof system heat transfer and cost computer program, and conducted green roof system heat transfer analyses for 5 major cities and organizations. He continues to develop modular planting systems in partnership with Guiyang Chuangjia High-Tech Accelerator Co. LTD in Guiyang, China where their first project will be installed this spring.

Chris has also been involved in other thermodynamic related work, including advanced engine research and fuel cell system development at Caterpillar Inc. and laser development at Lawrence Livermore National Lab and 2 private research laser development companies. He has presented at numerous conferences, has several articles published on a wide variety of engineering topics, and has had the privilege of working directly with several universities, including Stanford’s Center for Integrated Facility Engineering (CIFE) program.

Chris holds Bachelor’s and Master’s of Science degrees in Mechanical Engineering (with a minor in Materials Science) from Washington State University. His graduate work focused on thermodynamics, fluid dynamics and combustion.

Contact Chris at:

Green Roof Energy Series Articles

The opinions expressed by our Guest Feature writers and editors may not necessarily reflect the beliefs of, and are offered to our readers to simply present individual views and experiences and open a dialogue of further discussion, debate and research.  Enjoy, and if you have a particular comment, please contact the author or send us an email to:


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