All buildings should become zero carbon by 2019: this is one of the ambitions of the UK government regarding energy and the built environment. However, new buildings often fail to perform as expected, while energy retrofitted buildings also tend to underperform compared to the professionals’ calculations. This disparity between the estimated and the actual energy consumption, which is observed both in new and retrofit buildings, is defined as the “energy performance gap”. This article defines the energy gap, explains its causes and tries to quantify it. It continues with an explanation of its importance and ends with the measures currently taken and their targets.
The energy performance gap is related both to technical and behavioural elements of buildings and the ways in which they are used. These features of the energy performance gap are relevant not only to new buildings, but also to existing ones where energy retrofit measures are implemented. In the technical category, discrepancies and changes during initial estimation and design development, as well as during the construction and the commissioning processes, are identified as top reasons for this gap. In some cases poor workmanship and in-situ installations can also be to blame, especially when it comes to the building fabric. Specific causes might include problems with insulation, ventilation, solar and daylight characteristics of the building envelope which can have a major influence on the building’s energy consumption.
The situation is perhaps more challenging when it comes to retrofitting traditional buildings, where it is even harder to make accurate assumptions regarding the way in which buildings and building materials perform with regard to heat and moisture movements through their mass. For example, recent research on the thermal performance of traditional buildings, focusing on in-situ and laboratory measurements of U-values, estimates U-values totally different to the ones currently used for existing solid walls (U-values express the amount of heat that passes through the building envelope. The lower the U-value is, the less heat loss). Moreover, thermal bridging, which is defined as the additional heat loss at junctions of plane elements, is not included in the U-value calculation, with the risk of locally having significantly different U-values, which would have a substantial impact on the building’s energy performance. Thermal bridging can be understood as the extra over heat loss at that point not measured by U-value.
According to the research mentioned before, the average U-values of stone and brick walls are 1.42 W/m2K and 1.24 W/m2K respectively, as opposed to the 2.4 W/m2K and 2.1 W/m2K used in the Standard Assessment Procedure, which is currently used for energy calculations in the UK. This research, and other results from similar studies, suggests that there is a lot more to know regarding not only retrofitted, but even unmodified buildings.
Furthermore, it is often the case that the buildings’ occupancy is different than the one assumed; or the occupants have different behaviours compared to those modelled during the building design. It is, for example, difficult to estimate what temperature is considered comfortable in UK dwellings at the moment; although all sources agree that this is increasing compared to a few years back. In some cases it is described as 17.5°C, 19°C, or 20.4˚C, perhaps even with differentiations between rooms of a specific dwelling.
A 1°C temperature difference between calculations and actual performance might have a major influence on buildings’ energy consumption. On a bigger scale, if all dwellings’ interior temperature changed between 17°C and 18°C, this would reduce the UK’s greenhouse gas emissions by 10% or 8% respectively, compared to 2007 levels. Therefore, the assumption of an interior temperature being 17.5°C or 20.4°C, as mentioned above, is crucial for the buildings energy consumption calculations as a whole.
The energy performance gap is important for a number of reasons. Buildings’ thermal envelope is often the basis for professionals’ assumptions for building thermal performance. If the U-values from the studies described above are accurate, we are currently overestimating the potential savings from insulation retrofit measures. This would mean that initiatives and legislation promoting insulation retrofit will not be as successful as thought. Moreover, on a larger scale, this imposes a considerable risk on the UK’s estimations and plans regarding its carbon reduction targets in the long term.
Besides, the energy performance gap jeopardises energy efficiency legislation, such as the Green Deal or the ECOs, whose success depends on the accurate calculations of retrofit energy savings. This can prove very challenging for the companies in the sector, creating a lack of confidence for the homeowners who consequently doubt the effectiveness of energy retrofit measures.
Being unable to accurately estimate new buildings’ energy consumption or the energy savings incurred due to the energy retrofit of existing buildings, means there is a difficulty in achieving our carbon reduction aims. This “gap” doesn’t allow for long term planning and strategies or policies to be well informed and properly developed.
The figure below refers to non-domestic buildings and describes the difference between the design prediction and the actual total energy use, trying to identify its causes. Unregulated CO2, different occupancies, inefficiencies and special functions not included in the design prediction, are the reasons behind the energy performance gap.
The energy performance gap is very hard to quantify and it is even harder to attribute different amounts to various causes and to differentiate fully between technical and behavioural reasons causing this phenomenon.
The comparison of four different papers, each one based on numerous case studies of dwellings that have been insulated and monitored, shows that the reduction factor (between estimated and actual energy consumption) varies from 40% to 60% The reduction might be due to a combination of insulation performance, outside temperatures and ventilation, but also due to an amount of energy (approximately 15% according to the same research) taken by the occupants as improvement on their comfort levels. A study undertaken by the Buildings, Energy and Sustainability group at Leeds Metropolitan University suggests that the performance gap can be over 100% in some cases. It is worth noting that none of the dwellings in this study had a measured value lower than the predicted one. However, a variety of studies and monitoring cases find different percentages of failure to achieve the expected energy performance.
As long as we don’t have adequate knowledge to bridge the gap between the theoretical and the actual energy consumption of buildings the DECC has decided to apply in-use factors. More specifically, in order to reduce the risk on the Green Deal energy retrofit, the savings’ estimates will be revised down by a specified percentage based on evidence and research, or where this does not exist, on the basis of expert judgement on the scale of the potential difference in performance. In-use factors will be revised in the future, depending on the progress of research and industry experience and their ability to accurately estimate energy savings after energy retrofit.
Research on existing buildings and their in-situ performance is crucial for professionals in the built environment sector as well as for policy makers. The former need the information in order to accurately estimate buildings’ energy consumption on a project by project basis; the latter ones have to make informed decisions based on the actual performance of dwellings. The energy performance gap has to be bridged if we want our ambitious energy and carbon targets to be achieved.
Image Credit: By Ha17 (Own work) [Public domain], via Wikimedia Commons