G-11 Definition of system boundaries for existing buildings

Aspect G-11 Definition of system boundaries for existing buildings
A clear definition of the system boundaries is needed to understand and interpret the LCA results better, as well as to use them for comparative assertions (e.g. choice of design alternatives) or stand-alone LCA (e.g. for benchmarking purposes). How should this system boundary be defined in the case of existing buildings?

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 For an existing building, the system boundary should include all stages representing the remaining service life and the end-of-life stage of the building.EN 15978 gives clear rules for setting the system boundary of a building. These rules follow the ‘modularity principle’: that is, where processes influence the building’s environmental performance during its life cycle, they should be assigned to the module in the life cycle where they occur.
Rules from:

EN 15978

7.4.2 Boundary of the product stage (Modules A1 to A3)
7.4.3 Boundaries of the construction process stage (Modules A4 and A5)
7.4.4 Boundaries of the use stage (Modules B1 – B7)
7.4.5 Boundary of the end-of-life stage (Modules C1-C4)

7.4.6 Boundary for the benefits and loads beyond the system boundary (Module D)

The assessment should concern the building and its site. For existing buildings, the main issues are the use stage (B2 to B7) and the end-of-life stage. The system boundary should also include the production and transportation of any component and ancillary products used for maintenance (including cleaning) and transportation of any wastage from maintenance processes and the end-of-life processes.
The rules given in EN 15978 should be applied (see above). The LCA of an existing building can address different goals: for example, to compare different refurbishment scenarios and the current state of the building, or to compare a refurbishment scenario with an operation of demolition/new construction.For an existing building, five possible goals may be considered:

  • Scenario 1: Comparison of a rehabilitation operation with an operation of demolition/new construction.
  • Scenario 2: Comparison of a rehabilitation operation with no rehabilitation.
  • Scenario 3: Comparison of a rehabilitation operation with a reference rehabilitation operation.
  • Scenario 4: Evaluation of the absolute performance of an existing building without modification until demolition.
  • Scenario 5: Evaluation of the absolute performance of a rehabilitation operation.

Thus four different types of operation may be identified:

  • rehabilitation;
  • reference rehabilitation;
  • complete demolition and new construction;
  • maintenance of an existing building (maintaining performance during a given period).

A rehabilitation operation consists of a deconstruction operation (removal of products to change) and a reconstruction operation (adding new products to replace discarded ones). Reference rehabilitation is a generic rehabilitation that implies the use of standard construction materials, designs and processes. It is to be used for comparison purpose with regard to a specific rehabilitation scenario.

Defining the boundaries of the study implies selecting the items or contributors to the environmental impacts relevant to each use. The following contributors should be considered:

  • new building products (those of a new construction after demolition, or those that replace products discarded during rehabilitation);
  • products discarded during the operation;
  • products not discarded during the operation;
  • energy consumption of the building before the operation;
  • energy consumption after the operation;
  • water consumption of the building before the operation;
  • water consumption after the operation;
  • demolition operations;
  • deconstruction operations (discarding of products to be replaced);
  • reconstruction operations (replacement of discarded products);
  • new construction operations.

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