So much for my New Year's resolution to stop writing about concrete planters in the sky.
In my New Year's resolutions for writing in 2018 I specifically called out the work of Stefano Boeri for calling his buildings green, because of all the extra concrete that goes into holding up those trees. I tried to do a bit of math:
I resolved not to report on tree-covered buildings unless I saw the math, showing that the trees absorbed more CO2 than was emitted by the extra concrete. But like all my other resolutions, I have failed, because there are other benefits of putting trees on buildings including that it's biophilic and makes us happier.
Your average growing tree will suck up 13 pounds of CO2 per year. Making a pound of cement generates a pound of CO2, and concrete is about 10 percent cement, so 10 pounds of concrete generates a pound of CO2, not including the CO2 generated digging up and transporting all the sand and aggregate that makes up the other 90 percent of concrete. A cubic foot of concrete weighs 150 pounds. So if it takes just an extra cubic foot of concrete to support the weight of that tree, it will take 11.5 years for the tree to pay off the carbon debt of the concrete that is supporting it. But I suspect that it is a whole lot more, that just the planter itself has a couple of cubic feet of concrete in it, let alone the structure under it.
And Boeri's latest building is being rented to people who perhaps have not had a lot of happiness; it is a social housing project in Eindhoven. According to Boeri's press release,
"The high-rise building of Eindhoven confirms that it is possible to combine the great challenges of climate change with those of housing shortages. Urban forestry is not only necessary to improve the environment of the world’s cities but also an opportunity to improve the living conditions of less fortunate city dwellers,” declares Stefano Boeri.
It also may not be as big a concrete and carbon load as previous buildings; they seem to be optimizing and streamlining it all. The planters also look significantly smaller than the ones on the original Vertical Forest.
"The Trudo Vertical Forest sets new living standards. Each apartment will have a surface area of under 50 sq m and the exclusive benefit of 1 tree, 20 shrubs and over 4 sq m of terrace. Thanks to the use of prefabrication, the rationalization of technical solutions for the facade, and the consequent optimization of resources, this will be the first Vertical Forest prototype destined for social housing,” states Francesca Cesa Bianchi, Project Director of Stefano Boeri Architetti.
They do claim it will absorb a lot of CO2, it being "an authentic eco-system with over 70 different plant species able to counteract atmospheric pollution, thanks to the capacity of trees to absorb over 50 tons of carbon dioxide every year." I am not going to try and convert that into tons of concrete equivalent; only to note that there is no free lunch in this biz, that every pound of concrete has a carbon bill to pay.