Are buildings that consume less energy really more energy efficient as a result? As the University of Cambridge begins Switch Off Week, researcher Scott Kelly explains how predicting energy efficiency is easier said than done, especially once human behaviour becomes part of the calculation.

Dwellings are heterogeneous. A decarbonisation strategy that works well for one may not work for another.

Scott Kelly

Improving the energy efficiency of the UK’s existing building stock is vital to meeting the UK’s climate change mitigation targets. Currently at least 30% of all end use Green House Gas (GHG) emissions are from the UK residential sector. If the UK is going to meet its legally binding target of 80% reduction in GHG emissions by 2050 from 1990 levels, emissions from the UK residential sector will have to be eliminated almost entirely.

This means the ‘right’ mix of policies, technologies and behavioural changes with the greatest potential for emissions abatement is crucial. However, this is much more complicated than it appears at first sight. New research from the University of Cambridge has started to highlight the importance of understanding how the interaction between energy efficiency technologies, socio-demographic characteristics, environmental conditions and human behaviour all combine in unique ways to create very different carbon mitigation opportunities for each dwelling. Therefore, maximising the potential for carbon mitigation requires much deeper understanding for how different factors interact for each dwelling.

Recent studies have shown that human behaviour is at least as important as the physical characteristics of a building in influencing energy use, and that carbon emissions from dwellings are most sensitive to internal temperature changes, largely dependent on human behaviour. By understanding the interaction between human behaviour and the physical variables of buildings they occupy, we can untangle the complex relationships affecting energy use and get a clearer idea where energy and emissions savings can be made.

Predicting the impact that human behaviour and energy efficiency measures may have on residential energy consumption is complex. Energy consumption is affected by the income, age group, lifestyle and behaviour of occupants as well as the physical characteristics of dwellings and the environment where the building is located. As dwellings are non-homogenous, maximising emissions reductions for each dwelling can only be achieved with knowledge of how the combination of variables of the buildings physical material, environment and behaviour of its occupants all come together uniquely.

There is good evidence to show that improving building efficiency will lower energy expenditure and therefore emissions, even after the “rebound effect” is accounted for where energy savings achieved through energy efficiency measures are “taken back” as higher internal temperature or increased plug load .  Although model estimates including the rebound effect suggest overall energy demand will decrease, the level of reduction will vary for different income groups and building efficiency levels.

For my PhD research in the Cambridge Centre for Climate Change Mitigation Research (4CMR) I developed a building stock model of the UK residential sector and made several important innovations on existing methods. The most important innovation is the inclusion of human behaviour, income, age-group, lifestyle and environmental factors down to the dwelling level. With this new model it is possible to estimate the level of emissions reductions that can be made from very specific subsectors within the building stock. As an example, it is now possible to estimate the level of energy and emissions reductions possible by improving the loft insulation in all end-terrace dwellings belonging to people aged over 65.

My research shows there is an equivalent, opposite effect to the rebound effect; that the building’s propensity for energy use affects the overall efficiency of a building; people living in a building with high energy bills are motivated to install energy efficiency measures and save money – the more money saved the more that is available to be spent on energy efficiency measures. My research suggests that dwellings that consume less energy have poorer energy efficiency, but homes that consume more energy have higher energy efficiency. Following the law of diminishing returns, buildings that are already more energy efficient are more expensive to make more efficient because the majority of low cost energy savings have already been adopted. For these homes the most effective strategy for reducing energy and emissions will be to change the energy consuming behaviour of occupants.

On the other hand, homes that consume less energy are less energy efficient when compared to the rest of the building stock and benefit from improvements in energy efficiency measures such as improved loft and wall insulation, triple glazing and improvements to the energy efficiency of heating systems. Unfortunately these homes are also the most susceptible to the rebound effect reducing the expected energy savings. However improvements to energy efficiency in these dwellings will lower the overall cost of energy and therefore lead to a reduction in fuel poverty. In addition warmer and less draughty dwellings improve the health of those belonging to the lowest income groups and thus lower the overall cost of health care for all UK taxpayers.

In summary, in order to meet future emissions reduction targets the question should not be about the role of either energy efficiency or human behaviour within the building stock. Rather, it is important to recognise that dwellings are heterogeneous and therefore a decarbonisation strategy that works well for one dwelling may not work for another. It is the complex interaction of variables occurring at the dwelling level that ultimately determines the optimal carbon mitigation solution.  Policy instruments therefore need to reflect the diversity within the building stock so that emissions reductions can be maximised across the building stock.

Scott Kelly is a research associate in the Cambridge Centre for Climate Change Mitigation Research.
