G-04 Comparative assertion for building or product LCA

Aspect G-04 Comparative assertion for building or product LCA

Comparative assertion is an environmental claim regarding the superiority or equivalence of one building product (or one building design) compared with a competing building product (or an alternative building design) that performs the same function. Usually, the comparison of alternative products, systems or buildings is the main goal of a comparative LCA study. Different aspects have to be taken into account to conduct a meaningful and robust comparative assertion. One of these is the functional equivalence between the systems under comparison (see the ‘functional equivalent’ aspect), but there are other aspects that need to be considered, such as the system boundary, the background database, the life cycle inventory, and the life cycle impact assessment methodology applied.

How can a comparative assertion be made in building LCA applications?

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

In a comparative study, it must be ensured that the systems being compared are equivalent in function. According to ISO 14044 and ILCD, the same functional unit, system boundary, data quality requirements and allocation procedures have to be applied.

EN 15978 provides the practitioner with rules to be considered in the comparison of two buildings. The comparison also has to use the same environmental indicators, using those defined in the standard and in the ILCD Handbook.

The provisions of EN 15804 may be used under very specific conditions to compare cradle-to-grave product alternatives (cf. guidance).


Rules from:
EN 15978:

Purpose of the assessment

EN 15978 states that it can be used for “assistance in a decision-making process, for example: comparisons of the environmental performance of different design options”.

EN 15804:

5.1 Objective of the PCR

EN 15804 states that the “declarations based on this standard are not comparative assertions.”

ISO 14040:

4.3 Key features of an LCA

5.2 Goal and scope definition

6 Reporting

7.2 Need for critical review

ISO 14044:

4.2.2 Goal of the study

4.2.3 Scope of the study Comparison between systems

4.4.5 LCIA intended to be used in comparative assertions intended to be disclosed to the public

5.3 Further reporting requirements for comparative assertion intended to be disclosed to the public


6.10 Comparison between systems

6.10.3 Considered alternatives, the functional unit, and assumptions

6.10.4 Methodological, assumptions and data consistency

6.10.5 Data quality requirements

6.10.6 Identical parts of the compared systems

6.10.7 Scenarios in support of comparisons

6.10.8 Carbon footprint studies and other selected comparisons


Main provisions for all comparative studies (summary of the ILCD Handbook):

I) ISO 14044:2006 provisions for comparison assertions shall also be applied to non-assertive comparative studies.

II) All aspects of the goal and scope definition shall be addressed consistently especially for: LCI model, assumptions, data quality.

III) Uncertainty and accuracy calculation shall support this analysis.

IV) The completeness/cut-off (in%) shall be met for mass and energy, next to the overall environmental impact.

V) The excluded processes that are identical for all alternatives may be left out of all models.

VI) An LCIA shall be performed for LCA studies intended to support comparative studies (for public communication).

VII) If only one impact category (e.g. carbon footprint) is considered for those studies, it shall highlight that the comparison is not suitable to identify environmental preferable alternatives. This applies unless it is demonstrated that the two compared systems do not differ in other relevant environmental impacts to a degree that would change the main conclusions.


Main provisions for studies with similar functional equivalent (summary of the ILCD Handbook):

VIII) The compared systems shall have the same functional unit.

IX)  The study should include potentially environmentally better market relevant and available alternatives, as otherwise the study would be considered misleading.

X) The selected functional unit should reflect well-justified typical or average production/operation/use scenarios; it shall be agreed with the affected stakeholders.

XI) If the system needs to be replaced to meet the required duration of performance of the compared functional unit, the replacement should consider that potentially a newer system will replace the initially used model.

XII) For comparative micro-level studies (Situation A), each compared scenario shall be complemented with assumption scenarios of reasonably best and worst cases. Uncertainty calculation shall be performed.

XIII) For comparative meso/macro-level (situation B): the scenarios for each of the analysed alternatives shall apply the modelling guidance of situation A, except for processes affected by large-scale consequences of the analysed decision.

XIV) Involvement of interested parties in review.


For building LCA applications, comparative assertions are typically conducted to choose among design alternatives.

The major focus of the LCA practitioner has to lie on defining two alternatives that are truly comparable. Therefore special attention should be directed to identifying all functions that a design alternative has. This may include technical functions, e.g. structural functions, and building physical functions, such as ensuring specified quality levels of the indoor environment, but also architectural and aesthetic functions. Therefore design alternatives should always be agreed within the planning team, including the architect, structural engineers, building physics and energy performance experts. If the functions of two design alternatives are not truly comparable, the planning team should identify additional solutions to ensure comparable features, and these should be included in the comparative LCA study.

Users who are not LCA experts typically use dedicated LCA tools for comparing their building’s design alternatives. Such tools may support the choice of adequate – comparable – design alternatives, and assist in conducting the LCA study, especially with regard to the comparability of background data, etc. (see below).


1) Practical guidance when comparing building design alternatives

Because design software contains built-in methodological decisions (functional unit, system boundaries, cut-off criteria, etc.), as well as built-in databases, it is generally important not only to apply generally uniform methodological decisions, but also to use the same software when comparing two building design alternatives.

Concerning the background database used for the comparative assertions, the user can look at the methodological report or user manual of the LCA software for buildings to check for consistency according to the ICLD handbook.

Useful information on the use of LCA in the design of buildings can be found in one of the deliverables of the LoRe-LCA European project (FP7), available online.


2) Comparative assertion for product LCA based on EN 15804 in E2B EI research projects

Under specific conditions, it may be possible to compare a regular product and an innovative product following EN 15804 provisions, if the product model is from cradle to grave. All the other provisions of EN 15804 should be applied, if necessary, to comply with the comparative assertion rules according to ILCD; otherwise use the ILCD provisions if no provisions exist in the EN 15804 standard.

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