Energy Cost Savings through Green Roofs – a Myth, by Jцrg Breuning

To insulate or not? It seems like common sense. Graphic Source: baulinks

In my experience, (almost 35 years of designing and installing green roofs), it is proven that a green roof can extend the lifespan of the roofing by at least twice. The green roof acts like an additional all season two layer system on top of the roofing where healthy plants are the first layer and protects against direct environmental impact. The green roof growing media (green roof soil) is the second breathable layer.

There is only one difference; the breathable layer (growing media) on a green roof is also the basis for the wellbeing of the plants and must be able to store water and air at the same time for a healthy growth. If this layer doesn’t fully support the plants (and only the plants), the entire coat does not function and the plants will indicate this by suffering or change of plant varieties.

Above we learned that a wet coat in winter causes problems because water is not a good insulator, and so we have to consider heat loss in winter when it comes to a green roof. We also understand that dry green roof soil in summer will store heat (in the aggregates) which in turn increases the cooling needs.

A green roof (and green walls that grow on growing substrates on the wall with consistent irrigation) are only thermal masses with hardly any insulating value. Considering and fully understanding this matter of fact should make building owners cautious when somebody tells them that green roofs are good insulators. This is just not the case, or only if the building envelope is insulated correctly from the beginning. Fixing heating and cooling loss only through green roofs and/or green walls is simply impossible or the short-term profit thinking of individuals with no long-term experience in advanced building physics.

I believe that current research done in this field saying green roofs are good insulators is eyewash. Adding a little real insulation below the original coat (waterproofing or walls) would be much more effective (cost-wise and physically) than any vegetated layer combined with growing media or growing substrate alone.

However, the thermal mass “green roof” certainly has lifespan-extending properties for the waterproofing (and – again- I can confirm with projects that are much older than 35 years). This is the key to start thinking in long terms (50 years+) in the building industry and is most sustainable approach. Longevity is hardly considered in LEED™ and with emphasis on longevity, many LEED™ awarded buildings might fail for a certification because they can’t be upgraded easily when the costs of energy increase. I am not talking about how wasteful the footprints of many of these “innovative” building design are.

Photo DM Products. Penn State’s futuristic Millennium Science Complex earns LEED™ Gold for this space wasting empty overhang. The “water head” of the campus (or of their bureaucracy?). Not even plants can grow below – how can people survive?

In the last 35 years energy costs have gone up by 8-10 times (!) and we all know that they will increase accordingly over the next 3-5 decades (the life span of green roof). With this fact in mind, selling a green roof for insulation purposes will unavoidably end up in a costly disaster for the building owner when removing a fully functioning green roof in less than 25 years from now just to add more insulation, meet future requirements, or to keep your heating and cooling costs low.

Green roofs do extend the life span of selected and important building components dramatically; this is simply the highest environmental friendly approach in the building industry and the most cost efficient way to promote green roofs to reduce costs for the owner over decades. However, if scientists and green roof professionals do not understand this unique property of green roofs and don’t design underplaying components accordingly, the building owner won’t be a happy building owner in 20-25 years from now and for sure he or she won’t reinstall the green roof because it just added new costs without benefits as originally promoted. He might not understand why he should throw away a green roof just when the vegetation has been well established — only because individuals led him in the wrong direction two decades earlier?

Conclusion:

• As a building owner, be careful when people try to sell you a green roof as a good insulator without mentioning that you have to add insulation to the building in first place for future energy needs.

• Building owners have to understand that any available research about the insulation value of green roofs reflect only a current snapshot and potential savings in a very short time (less than half life time of a green roof) and in my opinion are worthless when the intention is to build for half a century.

• There is no energy study done over 50+ years comparing a green roof (plus additional insulation) and a conventional roof that will be re-roofed in 20 years from now and insulation added at the same time (typical re-roofing practice).

Trust only experts that recommend installing additional insulation under the green roof because then they expect that your investment will last a human lifetime, and will be profitable as well as also affordable during this time. Designing the roof (or wall) for five decades or more requires a lot responsibility and expertise of the designers – if they value their customers.

By the way, utilizing “Passive House” design principles (ongoing research and development in Germany since 1990) goes beyond LEED™ and is less costly — or if you find no space on the wall to hang-up a little plaque of the certification. Energy consumption of a Passive House is between 5-10 lower than an average LEED™ building, and the actual footprint to the environment is 2-5 times smaller. In 2010 there were over 25,000 Passive Houses in Germany and around 13 in the USA. Passive Houses have green roofs. (Source: Wikipedia. Learn more about Passive Houses. )

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