Use of material in residential house design: an optimisation approach balancing life cycle cost &amp; life cycle environmental impact
Creators: name: Islam, Hamidul
Publisher: name: RMIT University
Year: 2024

description: There is a broad consensus that environmental impact of buildings is reduced through use of more environmentally friendly materials in conjunction with conventional insulating materials, careful design, assembly, and climate. However, there is no certainty that use of such materials ensures a positive impact on the environment unless a whole life cycle perspective is taken.<br><br>Popular house designs in Australia include brick, concrete, timber and timber composites. Insulating materials are used to improve the building envelope thermal performance. Stakeholders generally select the “best” design after considering only star rating and construction costs. However, what is the certainty that this is the optimum design, from both an environmental and cost perspective? Is the optimum design the same if a narrow or wide range of building materials and assemblage designs is considered? If a narrow or wide range of environmental and economic impact indicators is considered, what is the value for stakeholders? What is the most useful optimisation model to identity the best designs? <br><br> In this study, a sample recently built Brisbane house was adopted as a case study. Its envelope components were varied to achieve different star ratings. Materials (wall cladding, rooftops and floor tops) and assemblage dimensions (insulation and air gap thickness, roof inclination) were selected based on their importance and relative effectiveness in achieving a particular star rating. The environmental and cost impacts were assessed across a 50-year life cycle. The Bill of Quantity (BOQ) was calculated for each alternative design, and then the annual operational energy requirements were modelled using <i>AccuRate</i> software. These data were then used to build a life cycle assessment (LCA) model using <i>SimaPro </i>software. Life cycle costs (LCC) were calculated using the BOQ and industry standard prices to find the present value of the house using suitable inflation and discount rates. Finally, various optimisation models were trialled to assess their usefulness in identifying the best design, one where life cycle cost and environmental impacts were balanced subject to sets of constraints. Various sets of constraints were evaluated. <br><br>The impacts of material selection and assemblage design on selected environmental and economic indicators were assessed across the life cycle stages of construction, operation, maintenance and disposal. Four environmental impacts category indicators were selected including greenhouse gas (GHG) emission, cumulative energy demand (CED), water usage and solid waste. One economic indicator was selected, life cycle costs including present value of future costs.<br><br>The results show that GHG, solid waste and LCC are significantly affected by wall assemblage design; water use is affected by roof assemblage design; and GHG and CED are significantly affected by floor design. The house designs with higher star ratings have lower GHG and CED emissions and higher LCC, as expected. The best design was a dwelling with weatherboard walls, a skillion flat roof and a timber/tiled floor, all at the highest star rating. House designs with modifications to just one assemblage had limited benefits in terms of life cycle environmental impacts and costs (with reductions in environmental impact up to 21%). <br><br>However, when incremental design improvements for wall, roof and floor assemblages were combined into one house design, the results showed remarkable reductions in LCA and LCC impacts (up to 43%) compared to the base case house. A small investment in construction costs yielded equal savings in operation and maintenance costs. <br><br>Several optimisation-modelling techniques were used to identify the “best” design, including graphical approach and single-objective optimisation (SOO) and multi-objective optimisation (MOO) using Mathematical Modelling linear programming (LP). A graphical approach as well as LP/SOO approach identified the same set of “best” designs, with no design best for all indicators. A graphical approach and LP/MOO approach using highest constraints also identified similar sets of best designs with no single best design. An LP/MOO approach using average constraints identified a single best design. However, the “best” design also depended on which set of indicators and weightings were selected. The preferences of the decision maker can be included when multiple indicators are considered so that a trade-off can be made. Hence, LP using a MOO approach with average constraints was found to be the most useful approach to predict a single best design amongst a wide variety of designs.<br><br>In conclusion, great savings can be made in terms of environmental impact if design of residential building envelope is optimised. Building materials have varying thermal performances, water usage and solid waste and this can be optimised only by considering the whole building and whole life cycle context. Evaluating both LCA and LCC across a range of indicators identifies a design with a minimal increase in construction costs while delivering significant benefits to the environment. Optimisation of wall, floor and roof designs can yield a single “best” design significantly better than a conventional design using the industry standard criteria of minimum construction and operations cost. LP using a MOO approach with average constraints is the most useful optimisation approach.