In 2008, the Environment Agency published a fascinating study which calculated that 89% of the carbon emissions associated with water use resulted from what consumers did with water in their home.
Reproduced from: Environment Agency (2008). Greenhouse gas emissions of water supply and demand management options. Science Report SC070010.
Shortly after this, the Energy Savings Trust (EST) commissioned Nick Grant, Alan Clarke and myself to undertake a project to better understand the carbon emissions (i.e. to subdivide the 89% element of this pie chart). We did rather a lot more besides, including calculating the impact of water metering on carbon emissions, demonstrating the importance of behaviour change and exposing the limitations of the Code for Sustainable Homes water calculator.
One of the things we were keen to investigate was at what point in an energy efficient building would carbon emissions due to hot water use exceed those due to space heating. We calculated that in houses built to 2006 Building Regulations, emissions due to space heating would still be approximately double that due to hot water use. It isn’t until you reach Passivhaus levels of building insulation and airtightness that emissions from hot water use exceed those from space heating.