That’s a question we’ve been wrestling with as well. I generally don’t understand how existing embodied carbon of an asset could be on the books of an owner for carbon reporting. It seems that it would show up only when they do something to emit or sequester carbon, effecting a net change in atmospheric carbon. But I do think that buildings can have a carbon value…see long explanation below. This is just one idea, I’m sure others have others.
The first part is looking at a transactional LCA. The idea is that a 60-year WBLCA omits demolition and some site carbon that are part of our design agency, and WBLCA also includes end of life of structural materials 60 years down the road over which we have very little agency. (My opinion is that if we haven’t figured out how to take care of our buildings to extend their life or completely reuse structural materials in 60 years we have failed to take action on climate change and the world is on fire. Likewise, it’s nonsensical to think we are going to replace today’s carpet with new carpet in 30 years that will have the same GWP as today’s; if that is our carbon assumption we should just give up on climate change action now). We have great agency over what is happening now, within our contract, for demo, reuse, construction, and estimated operations. You can see my LinkedIn post on what could be called ‘Transactional LCA,’ which is part of the Path To Zero Carbon series. https://www.linkedin.com/posts/kjell-anderson-faia-leed-fellow-aa5a7010_what-is-our-carbon-agency-during-the-design-activity-7080587569079463936-fp4_?utm_source=share&utm_medium=member_desktop
My guess is you need to blend the transactional LCA with the WBLCA in some way. But certainly we should be considering the existing stuff and it’s demolition and salvage.
Second, we need to value existing materials from a carbon perspective (and many others, but this idea is just carbon). One concept would be to take the remaining service life of a material and then apportion embodied carbon to a transactional LCA: essentially multiply initial embodied carbon by the remaining services life. If the design team chooses to landfill a material, the wasted remaining carbon is part of the calculation. If the material is salvaged and reused elsewhere, the remaining carbon is retained and isn’t part of the calculation. This is covered in Post 10 https://lmnarchitects.com/lmn-research/10-existing-building-reuse with some explanation and how this would work with existing buildings/spaces to value retaining existing materials and value. What we found with one TI was that current WBLCA practice would suggest that landfilling existing wood doors to buy new ones was a net carbon benefit to the world. This modification of calculation would suggest it is not.
I believe these two concepts combined could work across ownerships and time. If owner A builds new and demolishes the old building at year 0, then their carbon accounting includes transactional materials (existing C and D, new A and B1-B3). They complete a minor renovation in 10 years and account for the demo of any existing C and D, new A and B1-B3 in their minor renovation (which covers B4-B5). Normal operations include B6-B7 energy and water use emissions. At year 20, they sell the building to owner B. Owner B now begins carbon accounting for B6-B7. They also complete a major renovation and account for demo of existing C and D plus new A and B1-B3. The carbon value is basically the remaining value of the elements of the building. It is not an emission or negative emission, just a point from which emissions can be calculated.
One challenge with this approach – as with all current LCA – is that the circular economy is not well captured. Materials that are inherently reusable or repairable are not valued more highly. This method better accounts for contributions to the circular economy, but not well. It also doesn’t work as well as 60-year accounting for including materials that have a theoretically long life and need less replacement. But what we’ve found is that materials and even buildings are often demolished before they actually wear out…they are often removed for other reasons. It does appear to work well for ownership models and actual agency that owners have over building construction and the carbon handoffs that occur that was the initial question in this thread.
For biogenic materials, one could simply stuff a project with wood and then have a zero carbon project theoretically. My thought is that is unlikely, and buildings should get some credit for storing biogenic CO2. There are several methods for accounting for this. As above, if a building incorporates biogenic materials that have a lifespan of 15 years, I would hope that we have better ways of dealing with their end of life than simply letting them rot in an open field. Even landfilling biogenic materials sequesters a reasonable amount of carbon over the long term.
I would love to hear other ideas or feedback on this idea. It’s also very possible that I haven’t explained it very well.