G-12 (Buildings) / G-11 (Products) Cut-off rules for screening, simplified, complete LCA

Aspect G-12 (Buildings) / G-11 (Products) Cut-off rules for screening, simplified, complete LCA
Description
Cut-off rules enable LCA practitioners to conduct LCA without having to model 100% of the product system. According to the ILCD Handbook, the cut-off criteria refer to the omission of non-relevant life cycle stages, activity types, specific processes and products and elementary flows from the system model. Cut-off rules are quantified in relation to the percentage of environmental impacts that have been approximated to be excluded via the cut-off. The apparent paradox is that one must know the final result of the LCA to be able to know which processes, elementary flows etc. can be left out [ILCD, 2011a]. But, if one knew the 100% impacts of a product, then there would be no need for a cut-off. In practice, the total inventory is always unknown, but must be extrapolated from the measured or calculated data.What should the cut-off rules be for a product LCA or a building LCA?

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 construction products screening LCA simplified LCA complete LCA
Provisions In general, ILCD provisions should be followed for the cut-off rules.For product and building LCA studies complying with EN 15804 and EN 15978 goal and scope definition, the cut-off rules should be adopted.For product and building LCA studies that comply with EeBGuide predefined study types, the cut-off rules should be adopted. They are defined to allow for a simplification of LCA, and in particular to reduce the complexity of the study.For screening and simplified building LCA, the practitioner should refer to the list of building products and technical equipment to take into account. The others remain optional – to date – owing to potentially missing data. However, they may be included if default values exist for these building products or technical equipment at the European level.For complete LCA studies, cut-off rules should be applied primarily according to the ILCD Handbook, but if this is not achievable in practice,  they may be applied according to EN 15804/EN 15978 (in this case, the study type is not entirely a complete LCA).
Rules from:

EN 15978

7.4.3 Criteria for the exclusion of inputs and outputs

EN 15804

7.3.5 Criteria for the exclusion of inputs and outputsThe cut-off rules given in EN 15804/EN 15978: “All inputs and outputs to a (unit) process shall be included in the calculation, for which data are available. Data gaps may be filled by conservative assumptions with average or generic data. Any assumptions for such choices shall be documented; In case of insufficient input data or data gaps for a unit process, materials and processes can be omitted, if the process contributes with less than 1% of mass or renewable or non-renewable primary energy of the total, and all excluded materials and processes do not exceed 5% of total energy use and mass.”

ILCD

6.6.3 Quantitative definition of system boundaries – the cut-off criteria
ILCD states: “The application of cut-off criteria has to consider two main aspects: the translation of the cut-off criteria into operational criteria during data collection of the individual unit-processes and – before that – the procedural issue of how to overcome an apparent paradox:The apparent paradox is that one must know the final result of the LCA (so one can show that the omission of a certain process is insignificant for the overall results) to be able to know which processes, elementary flows etc. can be left out. This paradox is solved through the iterative approach used when performing an LCA, as described in ILCD Chapter 4 and with more details on the inventory part in ILCD Figure 5: the initial settings are to be revisited once or several times and refined in view of the outcome of the subsequent LCI data collection, modelling (including of alternative scenarios), LCIA results calculation, and interpretation (especially contribution, sensitivity, completeness checks and uncertainty analysis). These iterative steps are to be repeated until the results meet the completeness, accuracy and precision requirements as needed for the intended applications of the LCI/LCA study.” Provisions: 6.6 Deriving system boundaries and cut-off criteria (completeness)“VIII – SHALL Quantitative cut-off criteria

VIII.a) Overall environmental impact

VIII.b) Identify the aimed-at cut-off”

ISO 14044

4.2.3.3.3 Criteria for the exclusion of inputs and outputs
Guidance
Cut-off rules should not be used to hide results. Therefore it is mandatory to provide verifications for excluded parts.The application of cut-off rules is always highly dependent on the specific circumstances and the product; no general approaches can be defined, but some PCR documents for EPD or labelling schemes define specific cut-off rules, or the extent of the materials and products that should be included. Information from these sources can be stated when the exclusion of materials and products is justified. If appropriate data are available, they should be included in the LCA. If not, then conservative assumptions should be made, and documented transparently. If no appropriate data are available at all, cut-off rules can be applied and the respective aspect can be left out, which should be documented transparently with very special care when comparing product systems (ISO 14044: 4.2.3.3.3).

1) Specific guidance for product LCA studies
Cut-off rules for building product LCA can be defined, e.g. for the exclusion of capital equipment of the production plant, or the exclusion of ancillary materials such as plastic packaging of a raw material in the upstream processes.Practitioners should refer to the existing common rules in the background LCI database for the cut-off rules that should be applied. Generally speaking, the background data should not be modified to comply with the cut-off rule of simplified approaches such as EN 15804. The cut-off rules are defined mainly to ease data collection and modelling for the foreground data. For specific applications such as EPD, generic background data may be pre-verified, i.e. the cut-off rules are compliant with the PCR/EPD programme, so that the practitioner does not need to be concerned with this issue.

Practical guidance on the cut-off rules can be found in dedicated PCR for building products and technical equipment. Generally speaking, simplified approaches such as EPD may have a 5% cut-off rule in mass or in energy for the corresponding life cycle stage. However, it is generally recommended that the available LCI of the raw materials be accounted for, if available, rather than systematically neglect them to comply exactly with the 5% cut-off. The cut-off rules linked to mass and energy are also limited if the scope of the study is to assess toxicity and ecotoxicity indicators. Some raw materials, such as paints, may have a low energy or mass input while having a high toxicity effect. In this case they should be taken into account in the LCI of the building product. In current PCRs (e.g. the French PCR NF P01-010 standard), these substances are identified – e.g. in France as very toxic (T+), toxic (T), noxious (Xn), or dangerous (N) – and should be included in the inventory if there is an LCI available; otherwise they should be included in the methodological report.

 

2) Specific guidance for the cut-off rules for complete LCA of buildings

There is a difference in terms of cut-off rules between product LCA and building LCA. For product LCA, the cut-off rules are generally defined in an existing PCR, e.g. EN 15804 (see the  ‘Rules from’ section), or in the database. For building LCA studies the practitioner is likely to use already calculated LCA or EPD data. These data have already a cut-off rule linked to the product LCA studies (see the guidance above). At the building level, there is an additional layer of cut-off rules, which correspond to the number of building products quantified to model the full building.

EN 15978 states that the total excluded materials and processes should not exceed 5% of total energy use and mass. However, the cut-off should be used differently between screening, simplified and complete LCAs. The mass of the building is driven mainly by the structural components which can lead some products such as floor coverings being neglected. However, these are likely to have higher environmental impacts than the structural components for some specific impact categories. Therefore the cut-off rules should be extended in complete LCA to account for ILCD provisions.

The provisions in EN 15978 are somewhere in between a simplified and a complete LCA for the cut-off rules aspect.

 

3) Specific guidance for the cut-off rules for screening and simplified LCA of buildings

For building LCA, it is important to ease the process of doing an LCA study, as it is a time-consuming task to account of possibly hundreds of building products.

In the EeBGuide, the cut-off rules for screening and simplified LCA are less strict than for complete LCA, as some building products or technical equipment may not have LCA or EPD data in some countries.

In the current version of the EeBGuide these may be neglected, owing to potentially missing data, although the general rule for simplification would be to conduct a complete LCA, and then see whether  it is relevant to neglect these items (see 2.4 Study types). In any case, the limitation of building products to include should always be justified by the practitioner.

As an alternative for optional building products, it is also recommended that default values be used for these optional building products, as far as possible (see 2.4 Study types), as there are likely to exist in Europe, even if they are not very representative of a specific product. This enables the cut-off rules to be limited, even for screening or simplified LCA (validation step), while easing the completion of the study by practitioners through the use of default values.


Comments are closed.