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Cool Roofs are not Green Roofs

Thursday, January 17, 2013

By: Jörg Breuning

 

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.

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.

Roof Surface Temperatures
(a sunny day in May, air temperature around 72ºF)

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. 

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