Modelling Reference Design for Green Star

In accordance with EN 15978, the reduction of a building’s life cycle impacts can be evidenced through a comparative Life Cycle Assessment (LCA). Green Star requires a comparative LCA study, wherein you must showcase the reduction in environmental impacts by comparing the project building (Proposed Design) with the reference building (Reference/BAU Design). This article explains how to model the Reference design in eTool to fulfill Green Star requirements. For guidance on how to model the Proposed design for Green Star projects, please refer to this support post.

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What is a Reference/BAU design?

According to Green Star, the Reference/BAU design is defined as below:

The reference building is a hypothetical building that represent standard contemporary construction and operation practices. 

The Reference/BAU design and the Proposed design must have the same:

  • Structural requirements
  • Scale
  • Function
  • Location
  • Tenant requirements
  • Aesthetics
  • Site conditions including underlying geology
  • Planning constraints
  • Orientation
  • Season of construction

The Reference/BAU design must be a standard practice and code-compliant design (e.g. building fabric & energy consumption), which is fit-for-purpose and operating conditions of the Proposed design.

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Modelling Reference Design

1/ Identifying the difference between Reference/BAU Design and Proposed Design

The aim of the Reference/BAU design is to provide a point of comparison for reductions against, by developing a building that could have been built in place of the Proposed design provided no work was done to reduce its carbon emissions. For Green Star projects, you would usually need to develop your Reference/BAU design based on the Proposed design. In doing so, you would first identify standard-practice strategies VS improvement strategies in your Proposed design.

If you are unsure about what are improvement strategies, exploring eTool “Recommendations” library can gives you some suggestions. For more information, please refer to this support post: Guide to Recommendations: Modeling Improvement Strategies from the Library

For instance, BAU ready-mix concrete usually contains 100% Portland Cement Blend. If your proposed design specifies ready-mix concrete with 30% GGBS Blend, you can model BAU concrete in the Reference design and Concrete Replacement 30% GGBS Blend in the Proposed design to demonstrate the savings (see example below).

Reference design

Proposed design

Savings Percent

For more information about modelling the Proposed design for Green Star projects, please refer to this support post: Modeling the Proposed Design for Green Star

2/ Materials in Reference/BAU Design

eTool library contains various material templates that can be used to represent all building components in the Reference design, ranging from Facilitating works, Substructure, Superstructure, Internal finishes,
Fittings, furnishings and equipment, Services equipment, External works, etc. You can search using relevant keyword to find the appropriate templates. If there are no available templates that suits your needs, you are encouraged to create custom templates either from existing templates or from scratch. For more information, please refer to this support post: Creating Templates.

To ensure that you don’t miss any items in the Reference design, it is recommended to refer to the target construction scope at the Structure level as a checklist of what should be included in the design.

For higher quality of LCA, please ensure that materials in the templates are entered in the right categories, leading to impacts being allocated to the right categories for analysing and reporting purposes. For more information about this topic, please refer to this support post: Template Categories.

When modeling EPDs in your study, please ensure that the LCI source used in the EPDs are consistent with the rest of your study to ensure alignment with EN 15978. For more information on the different types of data, please download and read this article: LCI Data.

3/ Operational Energy and Water in Reference/BAU Design

For GS projects, Operational Energy input is based on Green Star GHG calculator/Energy Modelling Report/NABERS reverse calculator while Operational Water input is based on Green Star Potable Water calculator, which are calculated outside of the software. You will be entering the Standard Practice/Reference building input into the software for the Reference design.

Note that some building services templates such as HVAC, HWS, etc. may contain both material and operational impact. Please delete the operational impact of these templates to avoid double counting. 

When entering this input into eTool, it is best practice according to EN 15978 standards to have all Operational Energy & Water inputs split up into their end-uses. The granularity adds quality and transparency to your study which is beneficial to the study audience. Note that when you model your Proposed Design, you’ll also need to record the improvements as individual strategies for transparency and repeat-ability (i.e. upgrade to high efficiency HVAC system; change to LED lighting with timers etc). Therefore, you can consider using the following blank templates: ‘GS Energy Input Template‘ & ‘GS Potable Water Input Template‘ for ease of entry. 

It is important to note that Green Star Energy GHG calculator typically excludes tenancy lighting which in EN 15978 (referenced by Green Star) is considered a building integrated operational energy load (B6). As such, you may have to check whether tenant lighting loads have been included. If this has not been included, there are a number of estimated tenancy energy load templates available for use in the eTool template library (search for ‘Office Tenancy’) or eTool BAU lighting templates. You can select the most appropriate template to add to the study and check under ‘Operational Energy’ that these templates have not double counted some energy entries.

4/ EPDs 

It is uncommon to see EPDs in a Reference Design for Green Star as EPDs are normally used to demonstrate an improvement over the Reference Design in the Proposed design. However, depending on the different circumstances, if you wish to proceed with EPDs in your Reference design, we recommend to check with GBCA to ensure that your EPDs meets Green Star requirements for Reference design and include all relevant supporting documents for the design certification.

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