In conducting life cycle assessments, normalisation and weighting are needed to provide context and comparability, and prioritisation of environmental impacts. They aggregate life cycle impacts across multiple environmental impact indicators into a single score, facilitating comparison and informed decision making. Optional grouping of impact indicators into sub-categories prior to weighting can further refine the analysis.

Several rating schemes utilise normalisation and weighting, including BREEAM (BRE), ISv1.2 & ISv2.0 (ISC), and Green Star Buildings (GBCA) for calculating EcoPoints, EnviroPoints and Life Cycle Impacts, respectively. 

What is Normalisation?

ISO 14044 defines normalisation as: “calculating the magnitude of category indicator results relative to reference information”. This process converts the raw environmental impact values for each indicator into a relative environmental impact value compared to the selected reference point.

A common reference point is the average per capita environmental impact over a year, whether globally, nationally or regionally. This expresses the environmental impact of a product, service or project relative to that of a single person across the selected set of environmental indicators. Typically, impacts are normalised per functional unit for easier comparison with benchmarks or other studies. 

Other reference points might include industry or sector specific emissions. Additionally, internal normalisation compares impacts within the same study such as a proposed building design and an equivalent business as usual building design. Note that the normalised impact figures have no units.

Normalisation Calculation:

Normalised Impact = Raw Impact / Normalisation Factor

 

What is Weighting?

ISO 14044 defines weighting as: “converting and possibly aggregating indicator results across impact categories using numerical factors based on value-choices”. Weighting applies to normalised indicator values based on their relative importance, allowing both normalised and weighted indicators to be combined into a single aggregated environmental impact score for decision making.

Weighting Calculation:

Normalised and Weighted Impact = Normalised Impact x Weighting Factor

 

What is Grouping?

Grouping is an optional step that sorts environmental impacts by various characteristics, such as geographical scale  (e.g. global scale impacts vs regional scale impacts), or impact categories (e.g. ecological impacts, human health impacts, and resource use impacts). Grouping indicators prior to weighting helps rationalise disparate or large numbers of environmental impacts into fewer or more relatable categories.

Table 1. Examples of Impact Category Grouping

Normalisation and Weighting Factors

Table 2: Bre EcoPoints Normalisation and Weighting Factors

Table 3: Green Star Buildings (Australia) Normalisation and Weighting Factors

Table 4: ISv2.0 (ISC) Normalisation and Weighting Factors

Table 5: ISv1.2 (ISC) Normalisation and Weighting Factors

Normalisation and Weighting Examples

In this example we compare the impacts of two example buildings (A & B), using the Green Star Buildings normalisation and weighting factors.

Table 6: Normalisation and weighting of example building A impacts

Table 7: Normalisation and weighting of example building B impacts.

Conclusion

After normalisation and weighting, the total life cycle impacts of building A are 0.0799, and of building B are 0.0641. As such, using these normalisation and weighting factors it would be concluded that building B has lower overall life cycle impacts than building A.