The specification of construction strategies for low and zero carbon homes: decision-making tools and the evaluation of whole life energy, carbon, cost and performance

The UK is committed to cutting greenhouse gas emissions by 80% from 1990 levels by 2050, with intermediate targets set in the form of 5-year carbon budgets. Residential sector emissions accounted for 26% of UK greenhouse gas emissions in 2009. For the UK to meet its carbon budgets, residential sector emission reductions of 35-40% are required by 2022 from 2009 levels, with further reductions beyond. As shown here, the UK has a range of policies and initiatives to reduce carbon emissions from the residential sector; and there is a range of methods and technologies available to reduce carbon emissions from dwellings. The process of selecting the combination of methods and technologies to reduce carbon emissions from dwellings is here termed the specification of a construction strategy. Energy assessment tools (such as SAP, BREDEM and PHPP) are integral to this process. They are used to predict the energy use of dwellings, to demonstrate compliance with targets and regulations, and to compare and choose between alternative construction strategies. However, current energy assessment tools, and the associated methodologies and data, are insufficient for optimal strategy selection. This is because they do not assist in the systematic comparison of all combinations of options; they do not assist in the evaluation of whole life energy, carbon and cost; they do not always allow sufficient flexibility for key inputs and assumptions characterising the dwelling and its use; and they do not allow sufficient flexibility for the key metrics (weather data, carbon emission factors, and energy costs) or lead users to develop values for these metrics appropriate to the study period. Following detailed analysis of the current processes of construction strategy selection for dwellings, a framework is developed for the rapid identification of the optimal construction strategy (within the constraints of the tools and data used), whilst evaluating the whole life energy, carbon, cost and performance of alternative strategies. Methodologies are developed and implemented in a working tool for the systematic comparison of all combinations of options entered by the user; the evaluation of whole life energy, carbon and cost in an energy assessment tool; the use of flexibility for key inputs and assumptions to test the sensitivity of strategy performance to changes in these values; and the development and application of metrics appropriate to the study period under consideration. The use of the framework, methodologies, and tool are demonstrated in worked examples for a new dwelling constructed to the proposed 2016 ‘Zero Carbon Homes' standard, and for the renovation of an existing dwelling.