G-36 (Buildings) / G-34 (Products) Weighting of indicators

Aspect G-36 (Buildings) / G-34 (Products) Weighting of indicators

According to the ILCD Handbook, the weighting step refers to an optional step of the ISO 14044 standard to support the interpretation of the profile as a fully aggregated result. Generally speaking, it is a subsequent step to the normalization of indicators (see aspect ‘normalization’), where each normalized indicator is multiplied by a specific weighting factor that is intended to reflect the relative importance of the various impact categories. Different weighting methods can be applied in practice to help the interpretation of results in the building sector.How can this issue be considered within this guidance document?

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

new buildings existing buildings building products screening LCA simplified LCA complete LCA
Provisions The weighting of normalized indicators (which are the LCA results of products and buildings) remains an optional stage in this guidance document, in accordance with ISO 14044. However, in certain circumstances the weighting of indicators may be asked for by the decision-maker in order to ease the interpretation stage.
Rules from:
EN 15978
11. Calculation of the environmental indicators
“The standard does not present any methodology for the aggregation of the individual indicators.”

“Provisions: 6.7 Preparing the basis for the impact assessment […]

Normalization and weighting: […]

XIII) MAY – Results interpretation: Normalisation and weighting are in addition optional steps under ISO 14044:2006 that are recommended to support the results interpretation (see part 6.3.6)”

“Provisions: 10.3 Three levels of reporting requirements […]

IV) SHALL – Main report, with the following aspects […]

IV.e) Life cycle impact assessment results calculation, where applicable: […]

IV.e.vi) data and indicator results reached prior to any normalization, grouping or weighting shall be made available together with the normalized, grouped or weighted results.

IV.e) Life cycle interpretation: […]

IV.f.iv) full transparency in terms of value choices, rationales and expert judgements

Guidance If weighting is chosen in the case study, this should be carried out with regard to the ILCD Handbook. The LCA practitioner should keep in mind that the weighting step in LCA is always a subjective step, which should be documented as transparently as possible. The LCA practitioner should not use weighting indicators in comparative assertions; and, whenever such indicators are used, should include a statement that ‘It should be recognized that there is no scientific basis for reducing LCA results to a single overall score or number.’.In practice, different approaches can be used to weight the indicators.– The first approaches are typically already included in final LCIA methods that are used by LCA practitioners. For example, weighting factors used in the LCIA methods (such as Eco-indicator 99, IMPACT 2002+ and ReCiPe) to go from midpoint to endpoint indicators (reflecting three areas of protection: human health, ecosystem and resources) may be based on different assumptions. For example, the developers of the Eco-indicator99 method, as well as ReCiPe, used the cultural perspective based on [Hofstetter 1998] to adapt the weighting factors, i.e. egalitarian, hierarchical and individualistic perspectives. Thus the midpoint categories, e.g. for the area of protection of human health (such as carcinogenic, respiratory effects, climate change, radiation and ozone depletion) are weighted differently according to each perspective.In the LCA of buildings, the practitioner should be cautious when using a damage indicator (if relevant for the study), as it is based on a series of assumptions.– The second approach uses expert panels. They are defined within an LCA study, and this should be done on a case-by-case basis. Different mechanisms can be used, such as setting the weighting factors by public policymakers or industry panels, broad stakeholder panels, expert panels, and so on [ILCD 2011a]. Examples of weighting in the LCA and construction sector can be found for the European context in the BRE Environmental Profiles Methodology, for example. Other weighting methods are also implemented in some building LCA tools, such as EcoEffect in Sweden.

A comprehensive list of weighting methods is described in detail in the LoRe-LCA project.

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