Cool Roofs are not Green Roofs


By Jörg Breuning, The Green Roof Patroller
Inaugural Contributing Editor Column
March 4, 2013

In Physics class we learned basics about thermodynamics and about thermodynamic equilibrium.  In simple words the content of a black coffee mug “adjusts” faster to the surrounding temperature than in a white coffee mug assuming the material and material thickness of the mugs are the same.

 Source: Labor für Bausanierung & Energieeffizenz, Berlin

Exposing these mugs to the sun light the black mug will convert the electronic energy (sun light) into heat faster and no light or shade cools it faster assuming the ambient temperature is lower.  The physics behind converting light into heat is called radiation-less transition. White (or bright) colors reflect the (sun) light more effective than black.

Any macroscopic or microscopic body on our earth is exposed to these physics properties including all particles in the air of which can be plentiful in polluted cities.  While the sun light enters in to the earth's atmosphere, all bodies (particles in the air) receive portions of the sun's electronic energy and convert them in to heat.

For example, on a high mountain we experience much more sunlight, or electronic energy, because the air is "thinner" or has less particles in which the light has to travel through.  On a beach the light intention is also very high and this is caused by the reflection of the light off the the water and sand.  Sunburns on the beach are of high risk because of the consistent wind, we don't experience the transition from light into heat on our skin as fast.  No living organisms can reflect the light 100% effectively, but they can actively reduce the consequences (heat) by using techniques such as evapotranspiration.

 Photo: Jörg Breuning

Reflective roofs can't reflect 100% of the light either, but they are good to a certain degree (see roof temps above).  However while reflecting the light back in to the atmosphere, this reflected light will meet all the particles (bodies) again and substantially increase their temperature.  It is well proven that less than 5% of sunlight is reflected back into space once it's in our atmosphere.

White roofs relocate the transition point of light into heat from the roof into the atmosphere or surrounding bodies (e.g. buildings).  They simply don't eliminate the problem of increasing temperatures (heat island effect) caused by human settlements and they don't decrease the environmental foot print of structures.  In addition, the white surface has a slow thermodynamic equilibrium.  It takes a long time to to heat up in the winter (when heat is necessary) and cools down slower during summer nights.

The kingdom of plants were the only organisms over millions of year who developed a super-efficient mechanism to reduce the heat transitioned from the sun light.  It is called evapotranspiration - powerful, miniature AC units powered simply by solar energy.  These super little AC units are found naturally on Earth, but can be practically placed anywhere.  By using this naturally occurring phenomenon and installing green roofs, the outcome will go further beyond any man-made machine or material.

In many countries where insulation requirements are more intense than in North America, it is questionable whether reflecting roof material makes sense to start with.  From a global environmental perspective and understanding the basic science of physics, reflective roofs are just relocating the heat issue caused by human development.  It is similar to building a chimney higher at a coal power plant so not to pollute the air, when in fact the wind will just carry the pollution toward a different city.

Sometimes I wish all LEED™ professionals would have more common sense and follow these laws of nature and their own environmental gut feeling - it is simply better living under a tree canopy rather than under a polar glacier. 

Jörg Breuning

Publisher's Note:  We're very happy to have Jörg on board as our brand new contributing editor!  We saw this blog post of his from January and felt this type of article would be a great addition to our movers and shakers here.   Jörg is extremely well known in our industry - a bit of a braggart as an expert, yes, (well, he has a right to be since he's been in the business for over 30 years), a little forward, but very honest and opinionated (very German!).  He's written for us before - you may recall his Guest Feature Article "Modern Green Roof Technology Literally Shipped from Germany!" from November, 2008.  A project, by the way, which won the 2009 Awards of Excellence Special Recognition from Green Roofs for Healthy Cities.

See Jörg in action this Thursday at the Greenroofs & Walls of the World™ Virtual Summit 2013, held by, in association with the World Green Infrastructure Network (WGIN), on March 7, 2013.  Don't miss:

"Green Roofs & Solar: Double Your Benefits!" by Jörg Breuning and Ryan Miller with live Q & A.  The pair provide a view into how combining the two technologies of solar photovoltaics plus vegetative roofing equals profits for the environment and the building owner.

Green Roofs & Solar

Solar and Greenroofs!

Not registered yet?  Do so here.  As a reader of, use discount code GCOM to only pay $39 (only $29 if you're a student/faculty or government employee).

Jörg Breuning, principal of Green Roof Service LLC (Green Roof Technology)

Jörg was born in the green roof capital of world. He began working with green roofs in Stuttgart, Germany in 1980. Since then, he has installed and maintained tens of millions of square feet of green roofs throughout the world.  Remember, everyone, that English is not his mother tongue!

During his career Jörg has earned multiple degrees and certifications in landscaping, horticultural, and green roofing. In January 1983, Jörg Breuning was one of the first people in the world to earn a green roof professional accreditation from Optima, now Optigrün AG, who were the first green roof manufacturer to offer an in depth educational system for the installation and maintenance of their products. In order to be eligible for the accreditation, a candidate had to have completed 3-years of apprenticeship with a certified landscaping company. In 1985 he earned his national certification and horticultural technician degree – a program of work and study that required 5-years of practice in an approved landscaping company and 4 semesters of University study.

In 2003 Jörg transferred his German Company (founded in 1985) to the United States after being the key consultant for Chicago’s City Hall green roof. As an experienced horticultural expert, he has been able to educate many of North America’s green roof pioneers, providing them with fundamental green roof knowledge to start and grow their own businesses.

His company Green Roof Service LLC (Green Roof Technology) has introduced numerous innovations to the North American green roof industry. Many of these innovations have been adopted by companies such as Roofmeadow, Emory Knoll Farm, GAF, CETCO, Rooflite, Green Roof Solutions, Celebrity Cruises Miami and Optigreen.

Jörg’s articles will strongly focus on topics that ensure common sense is always practiced, ensuring that the horse stays in front of the cart, and provide valuable insights into making green roofs as popular and efficient as they are in Europe.

Contact Jörg at

Past Green Roof Patroller 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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