Sustainability in numbers
One of the debates in sustainable building focuses on the materials used in construction: “how sustainable are they?”, “what effect does their production have on the environment?” and many similar questions. The answers are not always obvious or easy to find.
For our project, these questions need to be addressed within the context of a retrofit project where most of the original mass of the building is conserved and where space is at an absolute premium. This can mean that the use of more sustainable, but less effective, materials is not always feasible.
It is clear from many of the photos we’ve posted that there is a lot of steel going into this building; 5.65 tonnes to be exact. Steel production is infamous for high energy use and environmental impact, yet the use of steel in this retrofit will allow us to greatly reduce the CO2 emissions of the house… In order to get an idea of the scale of these numbers it has been necessary to resort to that old engineer’s trick - mathematics.
Our PHPP model suggests that the CO2 produced by heating, cooling, hot water and electricity after the retrofit will be around 4.1t/a (tonnes per year) in this 235m2 house. We have not been able to obtain historic figures for energy use, but a quick PHPP analysis on the “baseline” case suggests that the total CO2 emissions before the retrofit to keep the house at a comfortable temperature may have been around 18t/a. (Warning: PHPP becomes less accurate when modelling high energy houses, but these figures are sufficient to give an idea of scale.)
Now for the steel: numbers taken from a 2007 International Energy Agency report “Tracking Industrial Energy Use and CO2 Emissions” suggest that the CO2 emitted in producing all the steel in the house from scratch should be around 10t. Producing the steel from recycled steel (like most of the steel in the UK) means emissions of only 2.8t. In other words, the energy in the steel is repaid in less than 3 months!
This is a limited analysis that does not include the steel beam fabrication or the transport to site, nor does it examine all of the dozens of other materials used in the building. However, working from the assumption that the production of crude steel is likely to be the worst culprit in terms of embodied energy, it starts to give an idea of just how significant the energy and CO2 emissions savings from these retrofit works will be.
