A life-cycle cost model for green commercial office buildings with optimal green star credits

The green building is a widely discussed topic worldwide as a solution to increasing adverse impacts on the environment. The paradigm shift from conventional to green buildings is expected to yield environmental, social, and economic benefits. However, green building implementation is adversely affected by initial cost premiums although there are significant savings throughout the life-cycle of green buildings in terms of water, energy, and so on. Therefore, there is a clear need to analyse the initial stages of green building development regarding life-cycle impacts, capturing massive savings in energy, water, and other resources. Although it may be cheaper to select inappropriate technologies during the initial decision-making stages, more importantly, this may preclude life-cycle savings and the desired outcomes of green buildings. In order to aid the initial decision-makers with the selection of credit points considering the lowest life-cycle costs of green buildings, this research develops a life-cycle cost model that incorporates developer constraints while maximising the number of credit points achieved when using the Green Star Australia environmental rating system. The model is based on Green Star Design and As-Built version 1. 1 rating tool. Initially, an extensive analysis is carried out for all the key criteria and credits of Green Star Design and As-Built version 1. 1 rating tool. Based on the identification of different types of credits, certain credits were eliminated. Afterwards, interdependencies among various credits were established. For all the selected credits, life-cycle cost is calculated considering six main central business districts (CBDs) of Australia. The life-cycle cost calculation followed ‘Building and construction assets – service life planning – Part 5: Life-cycle costing standard’ published by the International Organisation for Standards (ISO) as a guideline. The net present value (NPV) technique is used to calculate life-cycle costs. Further, a sensitivity analysis is also carried out for selected credits to identify the changes to life-cycle cost to the changes in discount rate. Once all the life-cycle cost data is calculated, the proposed model was developed. The proposed model is developed considering a set of rules for exclusions, selections, and inter-dependencies. It initially collects user information and user constraints. Based on the user information, the model provides customised solutions to the users. The user can define the discount rate and even select the regional areas, and based on that information, the life-cycle cost is calculated by the proposed model. The user constraints select or eliminate credits, consider inter-dependencies, and calculate the optimum solutions for a specific green certification level. This model can provide optimum solutions for four-star or five-star certification levels considering Green Star rating. Finally, the proposed life-cycle cost model is validated in terms of cost and optimum credit selections. Cost is validated using costs comparisons with cost databases, industry reports, and actual green-certified buildings and interviews. To validate the credit selections, four case study buildings with Green Star certifications are considered. Based on the validation results, the cost calculations are within the range accepted by various sources. Further, the optimum credits proposed by the life-cycle cost model coincide with the credits obtained by the certified green buildings except for minor changes. Most of the credits that are proposed by the model yet not implemented by the case study buildings happen to have higher initial costs and lower life-cycle costs. This further strengthens the importance of using life-cycle costs during the initial decision-making stages for green building implementation. Further, credits with lower life-cycle costs are mostly eliminated owing to higher initial costs, which can be addressed by using the proposed life-cycle cost model. The model identified green building credits with cost savings, such s the use of photovoltaic panels, which are ignored during the initial stages owing to high initial costs. Further, this model proposed passive methods such as natural ventilation in buildings, using daylight and rainwater tanks to be considered for green building implementations. Out of all the key criteria in Green Star Design and As-Built v1. 1, credits representing management criterion are widely achieved in green building implementation. This perfectly coincides with the proposed life-cycle cost model.