Correspondence/Rebuttal pubs.acs.org/est
Response to the Comment on “Material Nature versus Structural Nurture: The Embodied Carbon of Fundamental Structural Elements”
I
am gratified that my article1 has encouraged a rapid, incisive response from such a distinguished international ensemble; I shall endeavor to respond with similar insight.
climate impact than other common building materials.” They have previously asserted that timber-framed buildings have a negative carbon footprint5 and that wood should be allocated negative embodied carbon, or net “carbon credit”, of 3.9 tCO2 per ton.7 This implies (to those unwilling to unravel the carbon accounting) that using wood sequesters carbon. I concede that my allusion to this was somewhat cryptic, and am happy to decrypt it. The credit was derived by assuming that 70% of forestry waste, 100% of sawmill coproducts, and 100% of wood in demolition waste were recovered to replace fossil fuel for heat and/or power generation.2−7 Considering only 26% of timber production is sustainably managed,8 it seems unlikely that the prevalence of such diligent preuse waste recovery is any higher. The assumption of 100% energy recovery from postuse demolition wood seems even more optimistic. Of the wood recovered from UK demolition waste in 2008 that was not sent for reprocessing, only 21% was incinerated; 15% went to landfill, and the balance went to “unknown” disposal.9 A great deal more wood is informally incinerated at the demolition site to save disposal costs. Biofuel waste-to-energy projects are often abandoned owing to fierce local opposition (e.g., ref 10). Even if we accept all these optimistic assumptions, why should the carbon credit generated by recovery of wood as fuel be necessarily allocated to wood as a material? Recovery of demolished wood is not an intrinsic technical, commercial, or economic concern of timber manufacture (in contrast to, e.g., steel recycling). If the carbon credit associated with incinerating pre- or postuse waste wood has already been allocated to imbuing the timber and/or structure with a negative carbon footprint, the wood as fuel is no longer carbon neutral; the credit cannot be “double counted”. This removes at a stroke the major incentive for energy producers to recover wood waste to replace fossil fuels. The carbon credit associated with incineration of demolition wood should be considered separately from the material itself; it is outside the system boundary for my study. Thus, while Sathre et al. may feel that their system boundary is more sophisticated than mine, it invokes a cascade of optimistic assumptions that are germane only for a very narrow subset of structures; housing built with timber from supersustainably managed forests in jurisdictions where recovery of demolition wood for energy recovery is compulsory. The system boundary used in my study is not wrong, but merely an alternative, pragmatic approach applicable to a much wider range of structural scales. As I implied in my article,1 there is no need to invoke negative embodied carbon in order to present timber as the low-carbon choice for small structures. Figure 1 shows ECbeam for light-duty 5 m beams. Below a residual section capacity of ∼100 kN m, timber is clearly superior.
Figure 1. ECbeam for light-duty 5 m beams. Legend − as ref 1 but TB = softwood (C24) timber beams. Dashed lines represent effective ECbeam for overengineered concrete beams.
Sathre et al.2 suggest that my study uses an inadequate functional unit, as buildings are complex systems. The title of my article was clear; the study of fundamental structural elements, not buildings. When we study other complex systems, we derive insight by studying their components and subsystems. Do doctors dismiss the findings of cell biologists, or lung, liver, and lymphatic system specialists? Furthermore, buildings are a subset of the built environment. The residential buildings prevalent in the studies to which Sathre et al. refer comprise a still smaller subset. Their own analyses (e.g., 3−6) are based on two alternate designs for a single “1190 m2” apartment building. The floor plan for this building6 suggests no single beam therein is longer than ∼5 m; the maximum residual section capacity demanded would be ∼40 kN m. This is
