Modeling Existing Buildings for Adaptive-Reuse in eTool

For modelling existing buildings for adaptive-reuse purposes in eTool it’s important to launch into your LCA with full clarity on how to account for the existing building in a fair, reasonable, and repeatable manner.  There are three main scenarios that our users encountered and we’ve provided some guidance below on how to attack these.

Jump to the following sections in this post:

Foot Printing “As Built” or Existing Building

An “As Built’ audit of an existing building is essentially conducted in a similar fashion to a standard LCA except that the inventory collection will focus on the actual materials, processes, energy consumption, water consumption etcetera.  At eTool, we also encourages users to put forward recommendations for improvement that can still be achieved by retrofitting the building.

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Retrofit or Refurbishment of Existing Building

If the primary purpose of the LCA is to improve the environmental performance of the building for the remainder of it’s life, the study should be approached as per a normal LCA study.  Care should be taken to assign appropriate design lives to the components in the building depending on whether they are new versus existing, and/or will recur during the remaining lifespan of the building.

Some strategies investigated in the study may ask the question “is it better to keep this existing component or replace it?”.  For example, keeping the existing windows (poorly sealed, high conductivity) may have a lower embodied impact, but what is the trade off in terms of higher energy use for heating and cooling.  In this instance, the best way to model this is to include the existing windows in your base case, and when modelling the improved case add the additional new windows.  Set the design life of both components to accurately reflect the number of replacements you expect to occur over the whole building life.  This could require a bit of a “hack”, for example, if the building is half way through it’s predicted 100 year life span, and you expect the old windows to never require replacement but the new ones to require replacement after 25 years use, you would set the service life of the windows as such:

  • New windows: 50 years (will force eTool to count 2 x the windows)
  • Old windows: 100 years (will force eTool to only count 1 x the windows)

Update your energy use with the simulated energy savings for each scenario.  This should provide a relatively accurate life cycle picture of the net benefit (or impact) of the strategy.

When modelling existing buildings it can be tricky to comply with current standards when a very old building is being modeled (+50 years) as there would not be LCI data available for the materials and processes used in that building.  That said, as long as the study authors (you) are transparent about this shortcoming it is better than exiting the study.

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New Building that Retains Parts of Existing Building

The approach our users normally take for this scenario is:

  • Business-As-Usual Design: Model the existing building along with all the new parts of the building i.e. assuming the whole building was a new construction. All parts of the building that are new/additions should be modelled as per normal although to facilitate the next step, it would be a good idea to keep the ‘existing’ parts of the building in a separate template.
  • Proposed Design:
    • For elements that are being preserved and will NOT BE REPLACED in the life time of the new building, they can be deleted to demonstrate the ‘benefit’ of reusing those parts of the building.
    • For elements that are being preserved and will BE REPLACED in the life time of the new building, their upfront carbon (Module A1-A5) should be offset. However, there may still be required to account for all other embodied impacts for the use stage (B1-B5), for the end-of-life stage (C1-C4), and for benefits beyond life cycle if Module-D is included in the study.

In the section below, we’ve provided a step-by-step method for offsetting the upfront carbon (Module A1-A5) while accounting for embodied impacts for the use stage (B1-B5), for the end-of-life stage (C1-C4) of the retained building parts.

Using EPDs to offset A1-A5 impact

The quickest ways is using EPDs, which can be created to offset the upfront carbon of the retained building parts while keeping all other embodied impacts still as a part of the study.

We demonstrate how this method works in eTool in the example below. In this case, the existing windows will be retained as part of the construction of the new building. However, the service life of existing windows is 45 years while the service life of the new building is 60 years. It is anticipate that the existing windows will be replaced over life cycle of the new building. Therefore, the upfront carbon (modules A1-A5) of the existing windows will be offset while embodied impacts of the use stage (B1-B5) and end-of-life stage (C1-C4) will be accounted in the LCA of the new building.

Step 1: Modelling Business-As-Usual Design – Model the existing building assuming the whole building was a new construction. All parts of the building should be modelled as per usual design practices.

You can consider renaming the templates that represent existing building parts to be retained to differentiate them with the new building parts.

Step 2: Modelling all other material improvements before considering the impacts of retaining existing buildings as a separate scenario. This is to ensure that there is no double  counting of the savings among scenarios.

Step 3: Once all the improvement strategies have been considered, the analysis tool can be used to collect information on A1-A5 impacts for existing window templates. Below is an example of extracting GWP impact of A1-A3, A4 and A5 modules. 

Step 4: The figures for other indicators in the scope of the assessment (e.g. Ozone Depletion Potential, ODP; Acidification Potential for Soil and Water, AP; Eutrophication potential, EP; Photochemical Ozone Creation Potential, POCP; Abiotic Depletion Potential – Elements, ADPE; Abiotic Depletion Potential – Fossil Fuels, ADPF; etc.) should be considered and extracted in a similar way.

Step 5: Once all the values are calculated, a mock EPD can be created with negative figures for the A1-A5 impact. Please ensure that you provide the purpose of using the EPD in the description. An example of a mock EPD can be found here.

For more information on how to create an EPD in eTool, please refer to this support post.

Step 6: Running a scenario design for retaining existing windows. In this scenario design, adding the mock EPD. You can use Analysis tab to check that the A1-A5 impact of existing windows has been offset while the impact of other modules (B, C & D) is still accounted for in the study.



  • Service life – Some elements of the retained building might be in the middle of their service life and will need to be replaced within the life of the building. For instance, the windows might a good for another 10 years but have to be replaced after that. This means the windows will be replaced twice for this building while usually these are replaced only once in the building’s lifetime. This can be considered by shortening the service life of such elements that will reflect the number of replacements accurately.
  • Energy Efficiency of the Building – Existing buildings might have old leaky windows, low insulation, and high energy-consuming lighting. LCA study can be used to identify potential savings by replacing these elements and their additional upfront emissions associated with the replacement. Decisions can be made regarding whether it will be better to keep or replace these elements.
  • Demolition of the existing building – Note that any demolition impacts of the existing building to facilitate the construction of the new building should not be included in the LCA study of the ‘new’ building as those impacts should be accounted for in the LCA study of the existing building’s own study. Including those end-of-life impacts would be double counting the impacts according to EN 15978 methodology.

The figure below demonstrates the boundaries for modelling existing buildings in eTool.

In this scenario, it would be somewhat pointless (or not be of significant impact) to try to model the benefit of retaining elements that have a very short service life (i.e. gets replaced quickly like internal finishes/linings) as you will still need to account for their recurring impacts that occur in the rest of the building’s service life. Retaining structural elements that will last for the rest of the building’s life (i.e. foundations, structural walls/floors etc) or will only be replaced once or twice over the life of the building would be of much bigger benefit to model.

Remember to record the removal of the existing building parts in the recommendation tab to document the benefits of the adaptive-reuse strategy. This will then quantify the benefit of retaining the existing building.  In effect the study is quantifying the benefit of the avoided material use that was achieved by retaining the part/s of the existing building.

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