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The comparison provided the following outcomes:

- A different sharing extent was reported for the LCA modules: 100% sharing for Raw Material Supply (A1), Transport (A2), Manufacturing (A3) and Disposal (C4). 83% for Maintenance (B2), Replacement (B4), Demolition (C1), Transport (C2) and Water processing (C3). Instead, greater discrepancy was reported for the remaining modules.

- With respect to impact categories: 100% sharing was reported for Climate Change (GWP), Stratospheric Ozone Depletion (ODP), Eutrophication (EP), Photochemical Ozone Creation (POCP) and Acidification (AP). 67% for Fossil Fuel Depletion (ADP), while minor agreement was reported for the other impact categories. This outcome seems to confirm the indications expressed in the EN 15804:2012 and EN 15978:2011, which define seven impact categories for buildings LCA: GWP, ODP, AP, EP, POCP, ADP-non-fossil, ADP-fossil.

- The building elements considered by the majority of GBRSs are: ground slabs, floor slabs, external envelope, windows and doors (100%), footings, foundation and roof assemblies (83%), while sharing to a lesser extent was reported for the other building elements.

- Four out of the six considered GBRSs include EPD-specific credits, generally rating the number or the percentage of building components with EPD. Other GBRSs include EPD within the LCA criteria as a possible source of data to perform the analysis.

Regarding the rating methods, the majority of the analyzed GBRSs rate the LCA impact reduction with respect to a reference building that must be specifically designed according to protocol requirements (usually according to national regulations). In particular:

- LEED (USA): The building to be used as a reference must reflect the project building with regard to: size, function, orientation and operating energy performance. The significative differences between the baseline and the project building are represented by the structural typology and the technological configurations such as the external envelope and technical installations which must, however, be compliant with the local minimum regulations for energy and thermal performance. LEED requires a (at least) 10% improvement for (at least) three indicators, of which GWP is mandatory. In addition, all other environmental indicators must not increase more than 5%. It is therefore not clear what can be considered a good result in absolute terms, as the improvement achieved depends on the initial conditions, which are conditioned by the designer’s choices.

- Green Star (Australia) enables two pathways for the LCA analysis interpretation. The first one is similar to the one proposed by LEED, in which the performance of the project building has to be compared to a standard practice reference building which is a hypothetical standard contemporary construction with the same structural requirements, scale, function, location, tenant requirements, materials, aesthetics, site conditions (including underlying geology), planning constraints, orientation and construction season. The reference building, in addition, must be compliant with the national codes and regulations on materials and energy performance. The second pathway requires the project building to be compared with an actual reference building, constructed in the last five years that is similar to the usage, construction and operation of the

of the structural requirements, scale, function, location and site conditions. Unlike LEED, which only permits one level of improvement (10%) with respect to the reference building, assigning only 3/110 points, Green Star awards up to 6/100 credits (+ 1extra point) based on increasing the cumulative impact reduction from 30% to 130%.

- Green Globes (Canada) does not require detailed specifications for a reference building but rather limits the selection, through an LCA tool compliant with ISO 14040:2006 and ISO 14044:2006, to the building with the least anticipated environmental impact between (at least) two different core and shell designs. Green Globes awards up to 33/1000 credits, identifying three different reduction levels (10%, 15%, 20%) with respect to three impact indicators, one of which must be global warming potential. Another requirement to obtain the credits is that only one impact indicator can exceed the reference building result.

- DGNB (Germany) is one of the few GBRSs setting predefined benchmarks for the lifecycle impacts of buildings. The threshold values were identified through research on the German building stock, promoted by the German Federal Ministry of Transport, Building and Urban Development (BMVBS) and they were updated to align with new national regulations (Ganassali et al, 2016). Similarly, Active House also defines specific benchmarks, but it is not clear how they are determined.

- BREEAM requires the use of a specifically developed tool (“Mat.01 Calculator”) to rate a variety of LCA analysis aspects.

One of the first consequences emerging from this study is the impossibility of comparing the LCA outcomes from different GBRSs as the calculation and rating methods adopted differ greatly from one system to another. With respect to threshold values, two main approaches were identified: one is based on the comparison with a reference building, while the other one relies on predefined benchmarks. In the first case the thresholds vary from project to project and credits are awarded depending on the relative improvement achieved with respect to one or more design alternatives which, however, still depend on the designer’s choices. This method, therefore, cannot provide an objective measure of the severity of the actual impacts deriving from the project. The approach specified by DGNB, conversely, aims to provide objective values to refer to in order to understand the relevance of the environmental impacts over the building’s life cycle.

The DGNB methodology was used in the study conducted by Gervasio et al. (2018) as part of the research project “EFIResources: Resource Efficient Construction towards Sustainable Design” within the Horizon 2020 context.

Inspired by the European energy performance certification, Gervasio et al. (2018) aimed to set benchmark values for LCA outcomes at the building scale, based on the statistical evaluation of a sample of purposely selected buildings, discerning “conventional” practice from “best practice”. In order to set reliable and representative threshold values, they considered three main factors such as: the building typology, the seismic area and the climatic area.

This kind of approach likely represents the proper way to determine objective reference benchmarks for the building stock.