G-18 (Buildings) / G-16 (Products) Allocation

Aspect G-18 (Buildings) / G-16 (Products) Allocation
Normally, LCA studies should focus only on a single product at a time. However, systems under assessment often produce more than one product. The same problem arises when different waste flows are treated collectively in the same process (e.g. incineration). In these situations, the allocation problem arises: that is, how to allocate the environmental loads of those shared processes to the products delivered. In the building sector, owing to its long supply chain and influence, virtually all allocation cases can be found, from raw materials supply and manufacture (e.g. co-production processes) to the end of life (e.g. energy recovery during the incineration of building products) but also during the use phase (e.g. allocation of renewable energy produced in the building) The allocation rules may have a significant influence on the LCA results, and so this is a key methodological aspect. How should allocation be tackled in the case of buildings and building products?

related study objective

stand-alone LCA comparative assertion

related study phase

goal and scope definition inventory analysis (LCI) impact assessment (LCIA) interpretation reporting

relevant for

existing buildings new buildings building products screening LCA simplified LCA complete LCA
Provisions According to ISO 14040/14044, the ILCD Handbook and the EN 15804/EN 15978 standards, when dealing with systems involving multiple products and recycling processes, allocation should be avoided as far as possible; when unavoidable, allocation should be considered carefully, and justified. Allocation should be handled as mentioned in EN 15804/EN 15978 and the ILCD Handbook. Other aspects provide guidance for the most frequently found allocation problems occurring for energy-efficient buildings or products.
Rules from:

EN 15804

6.4.3 Allocation of input flows and output emissions General Co-product allocation Allocation procedure of reuse, recycling and recovery


Provisions: 7.9.2 Avoiding allocation by subdivision or virtual subdivision

  • “I) SHALL – Analyse whether allocation can theoretically be avoided by substitution
  • II) SHALL – Aim at avoiding allocation by subdivision or virtual subdivision”

Provisions: 7.9.3 Solving multifunctionality by allocation

  • “I) SHALL – Share inventory between co-functions by allocation
  • II) SHALL – Differentiate multifunctional processes and multifunctional products
  • III) SHALL – Two-step procedure for multifunctional processes
    • III.a) First step and criterion “determining physical causality”
    • III.b) First step and criterion “determining physical causality”
    • III.c) Checklist for “determining physical causality” criteria
  • IV) SHOULD – Second step and criterion “market price”
  • V) SHOULD – Two-step procedure for multifunctional products
  • VI) SHALL – Attributional modelling of reuse, recycling, recovery
  • VII) SHALL – System-wide consistent application of allocation
  • VIII) SHALL – 100% rules”

The following main allocation methods are frequently used in LCA practice:

  • allocation based on market price (e.g. a gold mine where other precious metals such as silver are also extracted as co-products);
  • allocation based on physical mass/volume/energy content;
  • allocation based on exergy (e.g. a refinery with a multitude of products with high energy value).

The mass allocation may be preferred, as it is less sensitive to market price changes. However, there are no globally uniform rules. For complete LCA, an in-depth analysis has to be made of this issue, and the allocation procedure applied has to be justified. This is also the case for a study with a special focus on a certain aspect (e.g. recycling).

The ILCD Handbook (see rules above) provides a comprehensive procedure for doing the allocation in a consistent way. The practitioner should make sure that allocation rules are applied in a consistent way throughout the assessed system.

For building LCA applications, this means that the data used for the study should have been modelled using the same allocation rules. For example, if the blast furnace slag is considered as a co-product when it leaves the system boundary of a construction steel product, then the cement should use the slag as a co-product linked to the fabrication of slag in the steel industry. The same rules apply if the slag is not allocated as co-products; then the cement should use it as a waste. If the allocation rules differ between the steel and the cement then the system is not balanced, which does not comply with the ILCD rules. Further information on this relevant example for the construction sector can be found in the corresponding aspect.

The practitioner should also check that the sum of inventories allocated to all the co-products is equal to the inventory of the system before allocation was done [ILCD 2011a].

In addition, the LCA practitioner should prove that the chosen allocation rule does not change the overall conclusions and results of the study. This can be applied by scenario analysis (see the corresponding aspect), if allocation is used in foreground systems. Scenario analyses are most likely not feasible for assessing the impacts of the allocation rules used within background datasets.

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