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Accounting for carbon impacts in buildings: a rating procedure

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Din, Asif (2020) Accounting for carbon impacts in buildings: a rating procedure. Doctoral thesis, London Metropolitan University.

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Abstract

The construction industry’s influence on climate change needs to be quantified in order to assess the carbon impact of dwellings. The appropriate measure of carbon within dwellings is the Greenhouse Gas (GHG) impact. The influences that determine how much GHG impacts residential buildings needs to be understood by designers and reduced if national targets are to be reached. Current UK building regulations consider the annual operational energy of dwellings using historic climate data.

Accounting for carbon (GHG) impacts within buildings can be achieved through conducting a Life Cycle Assessment (LCA). A review of different LCA standards for buildings has shown inconsistent methods in determining the GHG impact of dwellings over their lifespan. The LCA process is also too complicated to quantify GHG at the concept design stage. Current tools consistently underestimate the GHG of dwellings at an early design stage. Academic studies vary significantly making it difficult for direct comparisons to be made.

A rating procedure has been defined to create a concept design methodology. The rating assesses designs taking account of design features over the proposed lifespan of the dwelling. The relative rating of components and life cycle stages within their design indicates the significance of the design choices made. A percentage value for each component was used as the rating. Dwellings were chosen as a building type due to the large impact they have on the design industry as well as the similarities of unit types used for assessment.

Studies were conducted to test and verify the simplifications made. This included a detailed LCA of a PassivHaus built house in Wales. The simplification uses information available in current regulatory data used in the design process. It was found that a 75% accuracy can be achieved from only 5% of input data. The detailed LCA showed that the most significant quantities of GHG were found within the external walls and the structural frame as well as the roof and substructure elements. These were then analysed as part of the outputs for materials from the rating system.

Operational GHG within dwellings is significantly affected by thermal mass, air velocity and shading. Overheating makes active cooling necessary in a dwelling, resulting in an increase in operational GHG. Therefore, future quantities of GHG are highly dependent on future climate and the need for air conditioning. Room function also impacts overheating with living rooms dependant on physical building features and bedroom overheating determined predominantly by the external environment. Future heating reduces but is not as significant as the increase in potential cooling loads. These factors were used to define dwellings within the rating process to calculate future operational impacts.

An apartment block typology was explored as an archetype to quantify a wider range of life cycle stages. This clarifies which design features are significant for the GHG quantification, outperforming the accuracy of existing tools. In apartment buildings shared features show a large variation of the quantity of GHG between dwellings. Sample results from the rating procedure show examples of the early design priorities. A breakdown of life cycle stages by construction type and lifespan needs to be considered within the design process in order to understand the implications GHG in the design choices made by building designers.

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