Hey, Lauren! Three approaches I’d love to throw in the mix–one ‘by the books’ LCA approach, a practical solution for the professional context, and a description of the way this is treated in existing wbLCA tools. Each has its merits, I think.
There are three approaches to this question outlined in ISO 14044. The standard recommends using them together rather than separately since looking at one in isolation can lead to major omissions (structural adhesives, for instance, have a very low mass but very high relative impacts). ISO 14044 identifies (quoting the standard):
Mass: an appropriate decision, when using mass as a criterion, would require the inclusion in the study of all inputs that cumulatively contribute more than a defined percentage to the mass input of the product system being modelled.
Energy: similarly, an appropriate decision, when using energy as a criterion, would require the inclusion in the study of those inputs that cumulatively contribute more than a defined percentage of the product system’s energy inputs.
Environmental significance: decisions on cut-off criteria should be made to include inputs that contribute more than an additional defined amount of the estimated quantity of individual data of the product system that are specially selected because of environmental relevance.
Notes that cost isn’t one of the ISO criteria but it’s obviously still useful. One guiding principle for combining LCA data with life cycle costing, for instance, is that all the same stuff is included in both the cost and emissions accounts. I would say that the strength of the LCA approach is that it’s comprehensive and prevents people from using cut-off criteria to exclude inconvenient emissions.
If the ISO standard is the “letter of the law,” the RICS standard (pdf) gives a more practicable approach, including a (simple) method to account for the missing data:
A minimum of 95 per cent (EN 15804; 6.3.5, p.25) of the cost allocated to each building element category (0–7 of Table 3) should be accounted for in the assessment. Items excluded should each account for less than 1 per cent of the total category cost. In case the coverage is lower than
recommended, this should be clearly indicated and the actual percentage of coverage stated alongside the carbon calculation results. Cost has been selected over physical characteristics, such as mass, to determine the coverage cut-off point as it is more practical than determining the exact quantities of all items to be included, especially at early design stages.
The subtotal carbon budget of each category should be multiplied by the following adjustment factor to account for the impacts of the items not quantified:
Coverage adjustment factor = (100% / % of cost covered in the given category)
If cost data is unavailable, the same principles apply to the mass or area of elements as appropriate.
Obviously an expedient method, but it also assumes that the excluded materials will have a similar carbon intensity by cost than the ones included. But the carbon intensity here could also be much higher.
A third approach is outlined by the UBC methodology for generating bill of material data for wbLCA (pdf). They avoid cut-off criteria by making assumptions about ancillary material and material waste:
After the Modified BoM is input into the LCA tool, the tool’s internal algorithm may apply further modifications to the material quantities. It is important to distinguish the Output BoM from the others, as these further modifications are performed automatically by the LCA tool, not the practitioner, and therefore contain assumptions that may be harder to track than those in previous steps. These modifications can include:
• Addition of construction waste factors
• Addition of extra materials (e.g. paint, screws, connections, etc. added through Athena IE4B assembly input method)
These calculations are a few (of many!!!) differences between using a wbLCA tool and simply summing GWP values from EPDs. But this describes how your question is treated on the backend of some wbLCA modeling tools in order to avoid the need for a cut-off criteria.
Like many things, there’s no one size fits all answer. But I hope this if food for thought and I’d love to hear more about your experience determining this on your projects!
[edited to add UBC reference and correct typos]