Ignorance of embodied carbon
Most of the examples of “green building”, I have seen have an element of greenwash that even fools the most eminent experts. The late Professor Sir Peter Hall, Bartlett Professor of Planning and Regeneration at UCL gave the lecture, ECO-TOWNS, Will they be Eco-?, Can they become Towns?, in the 2008 Lecture Series What is Land For?. He first example was BedZed. His summary included:
• UK’s largest eco-village
• 100 homes, community facilities and workspace for 100 people
• Heating requirements: 10% typical home
• 60% recycling aim
• Target fossil fuel car miles: 50% national average
• Food delivery and allotments
I pointed out to Sir Peter that BedZed had a large amount of embodied carbon: 675 kg Co2e per square metre. He was sceptical but after he consulting others by email he conceded.
The late Sir Peter seemed ignorant of the issue of embodied carbon: Other experts seem ignorant too (or simply wish to ignore the issue). The recent report from The Global Commission on the Economy and Climate suggests that the drift to cities can be managed in a way that contains the problem of global warming:
How the world’s largest and fastest growing cities develop will be critical to the future path of the global economy and climate… As pioneering cities across the world are demonstrating, more compact and connected urban development, built around mass public transport, can create cities that are economically dynamic and healthier, and that have lower emissions.
I suspect that the “pioneering cities” are ones with high buildings, made with steel frames or steel reinforced concrete. They have very high embodied carbon – a tonne or two of CO2e for every square metre of floor space. Was the Commission aware of this?
Cradle to end use carbon counting
An accessible article in The Structural Engineer (October 2012) is A comparative embodied carbon assessment of commercial buildings by Dr Michael Sansom and Dr Roger J Pope. They compare buildings from 2 stories to 17 stories with high steel content. They calculate that the 17 story building created 500 kg CO2e per square metre of floor space in its construction but the two storey building created about half as much (per square metre). In general, the higher the building the greater the embodied carbon for each unit of floor space.
The article by Sansom and Pope, in measuring the carbon footprint of steel, uses the closed loop recycling method:
[Steel is] capable of being recycled without loss of quality. This is known as closed loop recycling… Steel is recycled (remelted) into new steel products without any loss of properties or quality (with an efficiency of 99%).
The closed loop recycling method is an extension of Life Cycle Assessment. Sansom and Pope give a carbon rating for steel of just over 1 kg of CO2 per kg of steel.
The widely used ICE database records the embodied carbon in virgin steel as 2.75 kg of CO2 per kilogramme – almost three times as much. Clearly, if Sansom and Pope had used these figures for steel – virgin steel – in their calculations, the embodied carbon in the buildings they considered would be very much higher.
In the scenario suggested by The Global Commission on the Economy and Climate, there is no steady state. Cities are being built not demolished so there is little recycling of the buildings. The figures for virgin steel are the correct ones to use in this scenario rather than the closed loop recycling method because dangerous climate change may come before any cities are demolished. The Commission should have used cradle-to-end-use for carbon counting.
Applying the cradle-to-end-use method suggest that high buildings have embodied carbon in excess of 100 tonnes for a 100m2 of floor space – the size of the larger apartments in BedZed.
In general, the higher the building the greater the crabon footprint for every apartment or office.
Wood, biomass and embodied carbon
Plant a tree and as it grows it stores carbon above ground (and below ground as roots). Use its wood in building and the carbon is stored for the lifetime of the building.
Discussion in the ICE database suggests that the carbon stored in the wood should not be counted. The argument relies on the fact that if timber from renewable sources is used, it diminishes the stock of renewable timber so that other uses will need to use timber from non-renewable sources. (See The ICE approach to recycled materials.)
Carbon positive homes: Every Baufritz home locks away at least 50 tonnes more CO2 than is emitted into the atmosphere during its manufacture and construction.
Even if the argument that taking renewable timber off the market means some more rainforest is destroyed somewhere did have some force, it cannot be levelled at other forms of biomass used in building like hemp in hempcrete construction or straw in straw-bale houses.
Cities with no high buildings?
High-rise buildings became possible with the invention of the elevator (lift) and cheaper, more abundant building materials. The materials used for the structural system of high-rise buildings are reinforced concrete and steel. Most North American style skyscrapers have a steel frame, while residential blocks are usually constructed of concrete.
High buildings don’t seem to be possible without steel frames or concrete reinforced with steel. High buildings with large steel content will have high embodied carbon even use of “lower carbon” cements are feasible. Since the Commission envisage a large expansion of cities, there will be little scope for the use of recycled steel, it will be mostly virgin steel with the much higher carbon footprint that entails.
Buildings that store carbon are possible and the Baufritz claim of sequestering 50 tonnes of CO2 per (2 story) dwelling is possible, compared to the 100 tonnes CO2 that is released as a result of the construction of a 100 m2 high rise apartment.
From the consideration of embodied carbon we cannot be climate friendly and have buildings more than two or three stories tall.
Could this be what the The Global Commission on the Economy and Climate would regard as a city?
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