Life Cycle Assessment, Part 3: Relative proportion of embodied and operational carbon
Originally posted 28th October 2020
2.5 - 3 minute read
This post is a somewhat belated follow up to the last three posts, which looked at embodied carbon of building materials, introduced whole-building life cycle assessment (LCA), and finally explained LCA further with a worked example.
A key point from those blogs was that it was essential to look at the emissions and energy use of a building over it’s entire life span, including the emissions from manufacture of materials, construction of the building, operation of the building, and demolition of the building and disposal of materials at end of life. Failure to include all stages can lead rapidly to false accounting, which might make certain options appear to be hugely low carbon until you factor in the emissions caused by their disposal.
Missing from the LCA example in those blogs was a breakdown of the relative proportions of embodied and operational CO2e emissions (CO2e emissions are all greenhouse gas (GHG) emissions converted to an amount of CO2 that would have an equivalent global warming potential (GWP)).
The chart below shows separately the embodied, operational, and cumulative (embodied + operational) emissions of a sample passivhaus over a 60 year lifespan (for more details of the modelled building and full methodology, see https://www.sustainablebuildconsultancy.com/blog/lca2), with heating and hot water provided by an air-source heat pump, and alternatively by a gas boiler, for two construction options.
Option 1: PUR wall insulation, lime/cement screed and foamed glass aggregate floor.
Option 2: Cellulose fibre wall insulation, reinforced concrete and XPS floor.
The figures for heat pump demonstrate that when using low-carbon heat sources in a passivhaus, the embodied carbon emissions (blue bar) are by far the most significant part of the buildings life cycle carbon impact, with both construction options. Ignore embodied carbon at your peril.
On the other hand, when heating with gas (fossil fuel) the balance tips. With option 2 (the construction with the lowest embodied carbon) the operational emissions far outstrip the embodied carbon. Ignore operational emissions at your peril.
Clearly the lowest life cycle carbon comes from the option with low embodied and low operational carbon, and this is where we all need to be aiming, urgently.
As ever, there are some important assumptions in these figures. The negative emissions figures for the electric-fuelled heat pump are based on UK national projections which include a hugely increased proportion of renewable energy but also an element of carbon capture. There is much debate about how realistic a genuinely carbon negative electricity grid is, but the amount of renewable generation is increasing rapidly. The grid is decarbonising, to the point that it is reasonable to assume that the heat pump operational emissions over 60 years would still be significantly smaller than the embodied carbon of the building, even if they do not actually become negative.
As before, I am indebted to Tim Martell and the AECB’s PHRibbon tool, which makes these calculations and extraction of data from them very much simpler than they would otherwise be.
There is still more to write on this topic. Next blog will probably look at the accounting of life cycle carbon in plant-based building materials, which is largely ignored in the current RICS and RIBA 2030 Climate Challenge guidance - an oversight that needs addressing, although much work also needs to be done to produce and disseminate good quality data.
