To start, the jury is still deliberating as claims are quite different from well-respected green roof professionals and universities as to whether green roofs can save energy costs for building owners. In North America, buildings account for about half of all energy consumed annually. I have been in the green roof industry for a meager 10 years and have my own opinions about the energy benefit, so for this blog, I spent some time trying to find data which convinced me I was wrong. Believe it or not, I found an article which stated that during winter “root activity” created heat. I have my doubts about many claims, including this one.
I’m not convinced of major benefits, but with minor benefits I am. If you are considering energy reduction as a primary reason to install a green roof, then place your money in increased insulation. It will be money better spent. There are many benefits of green roofs, however possibly the smallest benefit actualized may be energy reduction. This topic needs a lot more studying, over time to be more conclusive. There are hundreds of factors to compare, including embodied energy, I’ll leave that for others.
In the summer a healthy green roof will always be cooler than a conventional roofing membrane
Due to seasonal variations, such as soil depths, canopy and coverage area, precipitation, humidity and wind speed all which influence heat/cooling transfer, establishing an R-value (a measure of the resistance of an insulating or building material to heat flow) is also unrealistic.
I do believe if one is retrofitting an existing or older building with minimal insulation, there will be some benefits to installing a green roof. In the summer a healthy green roof will always be cooler than a conventional roofing membrane. I would expect the value of a green roof installed on a commercial building to be somewhere between $.01-.02/sf annually. So, for a 10,000-sf roof, the annual savings would be approximately $150., hardly the primary reason to install a green roof.
Concluding, install green roofs primarily in urban settings to reduce the storm water, create habitat, to reduce heat island effect and extend the life of the membrane of your building. Next week’s blog will address the life of the roof membrane and long term financial aspects.
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