Article
Reference
In search of optimal consumption: a review of causes and solutions to the Energy Performance Gap in residential buildings
COZZA, Stefano, et al.
Abstract
The assessment of building performance through energy certificates is important for tracking and improving the energy efficiency of the building stock. The reliability of these assessments is critical for achieving future energy targets. However, there is evidence of a significant Energy Performance Gap (EPG) in buildings, defined as the difference between measured and calculated energy consumption. This work performs a systematic review of EPG causes and reduction strategies in the context of heating of residential buildings. It introduces the concept of “optimal” consumption, in contrast to “theoretical” (i.e. calculated with standards) and “actual” (i.e. measured) consumption, which enables a more rigorous classification of causes and potential solutions to the EPG. This review found that inaccuracies in modelling of building characteristics and occupant behaviour has been most studied by researchers. It found that many EPG reduction strategies have been proposed, which can be categorized into two groups. The first aims to improve the energy consumption calculation by correcting the standard assumptions [...]
COZZA, Stefano, et al . In search of optimal consumption: a review of causes and solutions to the Energy Performance Gap in residential buildings. Energy and Buildings , 2021, vol. 249, no. 111253, p. 1-15
DOI : 10.1016/j.enbuild.2021.111253
Available at:
http://archive-ouverte.unige.ch/unige:153644
Disclaimer: layout of this document may differ from the published version.
In search of optimal consumption: A review of causes and solutions to the Energy Performance Gap in residential buildings
Stefano Cozza
a,⇑, Jonathan Chambers
a, Arianna Brambilla
b, Martin K. Patel
aaEnergy Efficiency Group, Institute for Environmental Sciences (ISE) – University of Geneva, Switzerland
bSchool of Architecture, Design and Planning – The University of Sydney, Australia
a r t i c l e i n f o
Article history:
Received 1 February 2021 Revised 20 April 2021 Accepted 3 July 2021 Available online 7 July 2021 Keywords:
Energy certificates Governance improvement Measured consumption Theoretical consumption Environmental target
a b s t r a c t
The assessment of building performance through energy certificates is important for tracking and improving the energy efficiency of the building stock. The reliability of these assessments is critical for achieving future energy targets. However, there is evidence of a significant Energy Performance Gap (EPG) in buildings, defined as the difference between measured and calculated energy consumption.
This work performs a systematic review of EPG causes and reduction strategies in the context of heating of residential buildings. It introduces the concept of ‘‘optimal” consumption, in contrast to ‘‘theoretical”
(i.e. calculated with standards) and ‘‘actual” (i.e. measured) consumption, which enables a more rigorous classification of causes and potential solutions to the EPG. This review found that inaccuracies in mod- elling of building characteristics and occupant behaviour has been most studied by researchers. It found that many EPG reduction strategies have been proposed, which can be categorized into two groups. The first aims to improve the energy consumption calculation by correcting the standard assumptions and/or considering new approaches to create energy certificates. The second group focuses on improving the actual performance of the building’s energy systems, through better monitoring, maintenance, and gen- eral usage of the building. A range of practical strategies were identified, which are relevant to a range of stakeholder groups. At the same time, this work also highlights that understanding of the relative impor- tance of EPG causes and the potential impact of the corresponding solutions is incomplete.
Ó2021 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
Contents
1. Introduction . . . 2
1.1. Background . . . 2
1.2. Aim and scope . . . 2
2. Method . . . 2
2.1. Systematic review . . . 2
2.2. The concept of optimal consumption. . . 3
3. Causes of the EPG. . . 4
3.1. Theoretical consumption deviation . . . 4
3.1.1. Inaccuracy of inputs and assumptions for building modelling . . . 4
3.1.2. Inaccuracy of climate data . . . 5
3.1.3. Inaccuracy of occupant behaviour modelling . . . 5
3.2. Actual consumption deviation . . . 5
3.2.1. Malfunctioning equipment. . . 5
3.2.2. Measurement systems limitations. . . 5
3.2.3. Execution of the work . . . 6
3.2.4. Non-optimal use of the building by the occupant . . . 6
3.3. Assessing the causes . . . 7
https://doi.org/10.1016/j.enbuild.2021.111253
0378-7788/Ó2021 The Authors. Published by Elsevier B.V.
This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
⇑Corresponding author.
E-mail address:[email protected](S. Cozza).
Contents lists available atScienceDirect
Energy & Buildings
j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / e n b
4. Solutions to address the EPG . . . 8
4.1. Improvements to theoretical calculations . . . 8
4.1.1. Approaches to energy labelling and calculations . . . 8
4.1.2. Accuracy of standard values . . . 8
4.1.3. Additional considerations for retrofitting . . . 9
4.2. Improving actual consumption. . . 9
4.2.1. Technical factors to improve actual consumption . . . 9
4.2.2. Monitoring practices and continuous iteration. . . 9
4.2.3. Collaboration between stakeholders . . . 10
4.3. Solutions at different building life stages . . . 10
5. Conclusions. . . 11
Declaration of Competing Interest . . . 11
Acknowledgement . . . 11
References . . . 12
1. Introduction 1.1. Background
Improving building performance is key to tackling the chal- lenges of climate change, considering that buildings account for 40% of the global energy consumption [1]. The urgent need for energy efficiency improvement calls for the implementation of suitable policy instruments, including performance certificates [2]. The European Union’s Energy Performance of Buildings Direc- tive (EPBD)[3]establishes general energy performance certificates calculation requirements, while implementation details differ between countries. Performance certificates were introduced to raise awareness of building performance among owner and tenants and to motivate performance improvements, as well as to ensure that the performance of the building stock meets national require- ments and complies with broader energy policy[4].
Energy and climate policy targets for the building sector are fre- quently based on the evaluation and improvement of building energy performance through certification schemes [5–7]. It is therefore critical to determine to what extent these can be consid- ered reliable tools for fulfilling this purpose[8]. The Energy Perfor- mance Gap (EPG), generally defined as the difference between expected energy consumption calculated by a building perfor- mance assessment and the actual consumption, is a significant challenge to the achievement of energy efficiency targets[9–11].
Although the concept of the EPG is broadly accepted, its interpre- tation and implementation differs considerably between authors [12]. While there is a range of literature on the EPG in buildings, there is only limited consistency between different works in terms of definitions and methodologies[13,14]. The lack of consistency in definitions reflects the challenge to find suitable solutions to close the EPG. Some solutions are in fact addressed to different stake- holders, in different contexts, for different building types, and for the different design phases of a building. Therefore, not all of them can be readily implemented in the context of energy performance certificates in Europe. Indeed, previous extensive review in Europe addressed only non-residential buildings [15], while a review which also included residential buildings considered all regions of the world and did not target the distinctive and specific nature of the EPG identified by the use of EU energy certificates/standards [16]. This review addresses this gap in the existing research by proposing solutions for closing the EPG in the context of energy certification practice over the buildings’ life. Finally, although the EPG has been investigated in numerous publications, little work to date has aimed to combine the findings and to propose a consis- tent strategy, in spite of the growing interest in the subject[17,18].
1.2. Aim and scope
Energy performance certificates certify the design more than the actual building, relying on standards and assumptions to com- pare buildings in different local contexts[19]. However, energy performance certificates are frequently used in many applications in an incorrect manner (e.g. to predict the savings of an energy ret- rofit[20,21]), under the assumption that they are a reliable indica- tor of the buildings’ measured energy consumption. This has resulted in observations of differences between the theoretical energy consumption according to certificates and standards on the one hand and the actual energy consumption on the other, i.e. the EPG. This work reviews research identifying the causes of the EPG and synthesises research findings that give solutions to reduce the EPG.
To enable a rigorous classification of causes and potential solu- tions to the EPG we introduce the concept of ‘‘optimal” consump- tion, in contrast to ‘‘theoretical” (i.e. calculated with standards) and
‘‘actual” (i.e. measured) consumption. This definition is developed inSection 2.2and is essential for identifying which aspect of the energy performance and certification process should be targeted by the various solutions.
We apply systematic review methods, considering only the research on space heating of residential buildings in Europe. We focus on a) studies that identify causes of the EPG and b) studies on practical solutions to reduce the EPG. These works are cate- gorised following the optimal consumption concept definition.
This classification approach allows us to synthesize the common findings between research coming from different European coun- tries into a single conceptual framework. From this review, we pre- sent concrete strategies which may be applied for different stakeholders and at different stages in building construction, reno- vation, and use. These are highly relevant to construction practi- tioners and policy makers.
2. Method
2.1. Systematic review
We apply a systematic review method to investigate causes of and solutions to the EPG, composed of three steps as follows[22].
(1) Collection of relevant publications: We considered publica- tions including journal articles, conference papers, reports, build- ing standards, and national guidelines. An initial list of four review publications on the EPG was identified[15–17,23]as a basis to generate keywords for electronic searches. The search has been directed using the keywords ‘‘energy performance gap”, ‘‘measured consumption”, ‘‘building actual consumption” that are included in
the article title, abstract, and keywords, by year and country. Fur- thermore, publications within the reference lists with titles fitting the search criteria were also reviewed. The keyword search was conducted using Google Scholar and Scopus databases. All the jour- nal or conference papers were required to meet the following cri- teria: peer-reviewed publications; focused on Europe; relevant to the topic; focus on recent research findings. Publications were fil- tered according to the inclusion criteria outlined above on the basis of title and abstract.
(2) Content analysis: The publications selected were reviewed with regards to nine aspects: building type; number of case stud- ies; origin of the dataset; method applied to calculate the theoret- ical consumption; data used for the actual consumption; energy
use considered (i.e. space heating, domestic hot water,. . .); EPG size; EPG causes; and approaches for reducing the EPG.
(3) Content synthesis: After analysing relevant publications, the underlying causes and solutions of the EPG were identified and grouped. Future research and policy directions were identified to make the energy performance certificates a more robust and com- prehensive method in building energy assessment.
To conclude,Fig. 1describes how the records identified through the searches were processed for this review. A total of 160 publica- tions were included in the analysis, divided in: 108 Journal articles;
20 Conference papers; 20 Reports; 9 Standards and directives; and 3 PhD theses.
2.2. The concept of optimal consumption
Building energy performance is influenced by several intercor- related factors, making it particularly challenging to identify and quantify each individual contribution to the EPG. In this review we introduce a new concept to classify the causes and solutions of the EPG: the optimal consumption(seeFig. 2). The optimal consumption is the energy consumption when the building is per- forming in an ideal way with respect to its intended design and construction quality, while satisfying the reasonable needs of its occupants. These conditions are usually different from both the theoretical ones defined by the standards and the actual ones.
We introduce this concept because there is a lack of clarity in the literature about whether the causes for the EPG should be ascribed to problems in the certificate calculations or problems with con- struction or use of the building (often, albeit incorrectly, described as ‘occupant behaviour’). While it is trivial to note that the reality can be a mixture of all of those, there is a need to introduce the Fig. 1. PRISMA flow diagram of review process indicating the number of publications included in the analysis at each stage following filtering based on the exclusion criteria, from the initial identification through the screening and eligibility steps.
Fig. 2.Overview of the optimal consumption. The EPG is the sum of the theoretical and actual deviations relative to the optimal consumption. The first is defined as the difference between the optimal and the theoretical consumption, the second is the difference between the optimal and actual consumption.
optimal consumption concept to allow a clear organisation of find- ings. Following this approach, the EPG can then be broken down into I) the difference between theoretical consumption and opti- mal consumption and II) the difference between actual consump- tion and optimal consumption. The first difference is associated with the limitations of using a theoretical model to describe the real conditions of use of a building, while the second difference is associated with malfunctioning and unrecommended use of the building. The sum of the two, as shown in Fig. 2, gives the EPG - the difference between actual and theoretical consumption.
A common approach to reduce the EPG is to perform field mea- surements to obtain values for model parameters based on actual operating conditions, in order to improve the prediction of actual energy use ; thereby reducing the level of uncertainty in the results [12]. This kind of reasoning, however, is based on current stan- dards, both normative (e.g. ventilation rate) and in terms of expec- tations of the actual use of a building (e.g. comfort level), which are very difficult to modify. In order not only to close the EPG, but also to significantly reduce the consumption, it is important to consider the level of service that buildings should provide[24]. This is the role of the optimal parameters: to challenge both theoretical and actual values, in order to define the optimal consumption for each building whereby efficiency and comfort needs are reached bearing in mind realistic building usage.
The below is an example that can further clarify the concept of optimal consumption. Consider the three types of consumption (i.e. theoretical, optimal, and actual) as a function of indoor tem- perature. The calculation of the theoretical energy consumption assumes an indoor temperature of 20 °C according to Swiss national standards [25], while Flourentzou et al. [26] report an observed value of around 23.5 °C, resulting in a correspondingly higher actual energy consumption. Since an indoor temperature of 20 °C is typically considered as uncomfortably cold in central Europe[23,27], Flourentzou et al.[26]propose 21.5°C as the ‘‘op- timal” indoor temperature for which they report no significant complaints about cold/discomfort by the occupants, while the energy consumption is not increased excessively. We refer to the corresponding energy consumption as optimal (Fig. 2). Another example is the efficiency of the heating system, such as the coeffi- cient of performance (COP) of an air source heat pump. In this case the theoretical energy consumption may assume a COP of 4.5[28], while an ‘‘optimal” value may amount to 3.0, based on empirical evidence of the real-world performance of a well-maintained heat pump[26]. In contrast, the actual COP may only reach a value of 2.0 due to poor installation or maintenance of the heat pump [29]. The difference in energy consumption under theoretical as opposed to actual conditions represents the EPG.
3. Causes of the EPG
By introducing the concept of optimal consumption, we propose a new, clearer classification of causes of the EPG, as shown in the Fig. 3. In this section we separately discuss the causes found for the deviation between I) the optimal consumption and theoretical consumption and II) the optimal consumption and actual consumption.
3.1. Theoretical consumption deviation
Building performance certificates rely on simplified building energy models for determining the theoretical energy use[30].
These models are based on standards assuming uniform conditions and allowing the comparison of buildings which differ significantly [31]. However, these models are subject to simplification of reality, overlooking the complexity underlying building energy use[32].
This section focuses on reasons why a simplified building energy model (as used to determine theoretical consumption according to the building energy certificate) may not describe the energy consumption of a building as accurately as it should.
3.1.1. Inaccuracy of inputs and assumptions for building modelling As basis for certificates, usually simplified ‘‘white-box” models on the building’s thermal balance are used[33]. White-box models require detailed physical input data to predict the energy con- sumption of a building based on standard thermodynamic equa- tions, and their accuracy is determined by the correctness of the physical model and the quality of the inputs[34]. There is a general consensus in the literature as to which of the building model parameters are most commonly based on assumptions that have a significant impact on energy consumption[35]. These are: the mean indoor temperature[36,37], the building elements’ assumed U-values[38,39], and the ventilation rate[40,41]. Indoor tempera- ture has been shown to have a large impact on energy consump- tion estimates and therefore on the EPG [36]. Real indoor temperature are typically higher than required by the various stan- dards[42,43], e.g. with values of 23°C measured for Germany[44]
or the 22°C in Denmark[45]. One study estimated that an increase in room temperature by 1°C resulted in approximately 5% higher energy consumption[46]. A comprehensive review on the relation- ship between indoor temperature and building energy consump- tion can be found in[47].
In new constructions, U-values are calculated in the design phase and therefore variations during the building process will add uncertainty to the actual U-values[48]. In existing buildings on the other hand, typical values for the period of construction Fig. 3.Classification of the Energy Performance Gap’s principal causes, categorised into theoretical and actual deviation.
and the type of building are used. In both cases (also in carefully performed U-value calculations in which these are corrected for thermal bridges) the uncertainty of the theoretically calculated U-value and the corresponding actual U-value is very high [49].
Research in UK statistically analysed (along with a review of ther- mal imaging data) this difference revealing widespread deviations from the design intent across the majority of dwellings [50].
Another study showed a 2.5% difference in the heating consump- tion for each building element (e.g. window) for which the U- value was poorly assessed[51].
Defining a standard value for the ventilation of the building has also been found to be very complex[40]. The value for natural ven- tilation is constantly changing due to the weather, with a large impact on the heating consumption [51]. In one study it was shown that an increase rise in ventilation rate by 0.2 m3/(m2h) leads to an increase of the heating demand by about 5 kWh/
(m2y)[52]. A large part of the prediction error is due to the free- dom of choice between default and measured values as input for the calculation, especially for the air tightness of the building envelope[53].
It has also been shown that current static calculations on the energy consumption of buildings with high thermal mass lead to different results than those measured[54], especially during inter- mittent occupancy[55]. In standard calculations, the importance of the dynamic thermal properties is neglected or underestimated [56], leading to up to 26% of the variation in theoretical predictions [57]. Inaccuracies in the geometrical representation of the building in the model (i.e. dimensions of its internal and external spaces), have been indicated as another important cause of the EPG [36,58]. Research found a discrepancy in the buildings’ surface area overestimating the theoretical area of 8–10%, which impacts the resulting energy calculation [59]. To conclude, because there is not a clear trend found for the errors, the issues with these param- eters are not easy to resolve. Strong variations are indeed noted between countries and across different buildings types.
3.1.2. Inaccuracy of climate data
Building energy models have been shown to be especially sen- sitive to the weather data used[60]. Several studies showed the substantial performance difference depending on the weather data (up to 40% in[61]), and the need to use climate adjusted data for an energy certificate[62–64]. The main challenges in the generation of weather data include how to deal with uncertainty, urban heat islands, climate change, and extreme events[65]. An energy label- ling simulation of a residential building in seven different cities was performed with the Norwegian certification tool and the stan- dard weather file, and gave a D-label as result [66], while when using typical reference years from the local site the energy label for three cities was upgraded to a C.
Current standard weather files for building models are not sui- ted to the assessment of the potential impacts of a changing cli- mate [67,68] which impacts the EPG [61]. Kocˇí et al. [63]
investigated the effect of warming trends on the calculated energy demand and found the 2013–2017 warming trend reduced average heating demand by 4% relative to the Test Reference Year temper- atures used for Czech Republic. As a consequence, the current Test Reference Year for Prague, (and likely for other European cities [69]), no longer reflects the actual weather conditions. In summary, energy certificates currently have a systematic problem in terms of not using the best locally adjusted climate data that represents the local conditions, including the effects of climate change.
3.1.3. Inaccuracy of occupant behaviour modelling
Even when a building is correctly designed and used, occupant behaviour characterisation is a major challenge in building mod- elling[70,71]. The actual use by the inhabitants is different from
what was planned in the design of the building[72], and usually it varies over time[73]. In this section we refer only to differences in occupant behaviour between standard conditions, as repre- sented by the model, and optimal conditions in the sense of accu- rately modelled conditions [74]. A major challenge is the representation of thermal comfort with current models (e.g. pre- dicted mean vote and adaptive models), which still have many lim- itations and are largely inadequate for predicting individual responses [75]. Other examples are the number of people per metre square or the daily presence in the building [76]. Sec- tion 3.2.4deals with the non-optimal use of the building by the occupants (e.g. overheating of rooms). In their review, Zou et al.
[16]listed the numerous factors that can affect the occupant beha- viour, including occupants’ characteristics (e.g. lifestyle, cultural background, geographical origin, and gender[77]), the interaction between occupants, their cost-consciousness, and occupants’ com- fort. Other authors pointed out the importance of the building characteristics (e.g. windows orientation) on the use made of it by its occupants[78], occupants’ knowledge[79], and occupants’
experience with the building technical system[80].
3.2. Actual consumption deviation
The optimal energy performance of a building relies on certain design assumptions, such as set-point temperature, control sched- ules, and performance of the technical systems. In reality, many of these assumptions do not apply due to malfunctioning of the building systems, non-optimal use by occupants, or other reasons, contributing to the EPG. InSection 3.1we discussed why optimal values are different from theoretical values used in standards, now we will investigate what generates the deviation of actual val- ues from optimal ones (Fig. 3).
3.2.1. Malfunctioning equipment
Malfunctioning equipment is a common issue, where the differ- ence between the theoretical and actual consumption is no longer caused by problems in modelling or data collection, but by a real technical problem in the building[16,81]. Heating systems in par- ticular are frequently identified as the cause of an EPG[44,82]. The examples of technical problems in literature are numerous and diverse, highlighting the critical importance of monitoring and maintenance [83]. Reimann et al.[82] demonstrated that issues with these systems can have a much larger influence on energy use than user behaviour. Building HVAC systems, especially for innovative systems (e.g. smart building controls), often suffer from control systems malfunctions and lack of fine-tuning of control strategies[84]. Fine-tuning may be omitted by the project devel- oper but even if performed, the results may be disappointing because proper execution is time-intensive and implementation according to state of the art may conflict with requests made by occupants[85]. The absence of a facility manager (typically only present in larger buildings) has also been found to be an important cause of the EPG[16]. Poor building operation practise has been found to increase energy use by 50–80% relative to baseline, while good practice may save 15–30%. However, this is contingent on managers having access to information and tools to apply optimi- sation – without these, expectations for optimisation are less likely to be met[86]. Before concluding, it is important to mention that in the event of a malfunctioning equipment, the impact on the energy consumption, and therefore on the EPG, could also be positive (i.e.
a lower actual consumption due to a partial failure of the heating system)[37].
3.2.2. Measurement systems limitations
Monitoring systems, while critical to identifying performance problems, can themselves be the source of inaccuracies[87]. Incor-
rect installation or malfunction of monitoring systems leads to errors in the interpretation of the building energy performance [88]. Metered energy use obtained from measurement systems needs to be validated to ensure accuracy of the data[15]. The most common sources of errors identified are mislabelling (i.e. sensors not measuring what it was thought they were measuring), incor- rect installation [89], and lack of calibration of sensors[90]. For certain types of measurement, for example measuring building ele- ment U-values, the need for a qualified expert using sufficiently capable measurement devices has been highlighted[91].
3.2.3. Execution of the work
The building energy performance is greatly influenced by the quality of construction, which is difficult to account for during
the design phase, resulting in a deviations from the design specifi- cations, especially with regard to insulation and air-tightness[89].
Construction quality depends on several factors[16], such as lack of expertise on-site that may, for example, cause unexpected ther- mal bridges[92]. Bell et al.[93]showed that an architect or project developer with limited experience often fails to specify all the details of the project, leaving important decisions to the builder which results in a gap between designed and as-built. Van Dronke- laar et al.[15]listed the most common issues related to on-site workmanship for non-residential buildings which can easily be extended for residential buildings: these include the imperfect eaves to wall junction insulation, the improper installation of drai- nage, air ducts and electrical pipe work, and the incorrect position- ing of windows and doors, that reduce the actual performance of the thermal envelope.
An important factor is the general lack of attention in the pro- curement of materials and construction contractors (and sub- contractors) – to the extent that this is often not even considered as a distinct phase of the design and building process[94]. This is important as all the practical implementation of designs and technical solutions ultimately rests with the construction contrac- tors. Finally, it is important for the original design to both meet the energy efficiency targets, and for it to be actually possible to build.
If insufficient attention is paid to the executability of the construc- tion, it becomes likely that flaws in the final building will arise and cause performance problems[92].
3.2.4. Non-optimal use of the building by the occupant
The way occupants operate the building in regard to opening windows, handling controls or shading, lighting, and indoor set point temperature, significantly affects the EPG[95,96]. Contrary toSection 3.1.3, this section is not concerned with inaccurate mod- elling of the occupant behaviour but instead discusses non-optimal use of the building by the occupants. A clear example is the contra- Fig. 4. Number of references analysed in this study, categorised by their main topic
(s).
Table 1
Causes of the EPG. Cause ID is used to link causes with solutions given inTable 2. *The number reported in each box represents the number of times that cause was indicated as the cause of the EPG.
Deviation (N*) Macro-causes (N*) Causes description (N*) Cause
ID Deviation of theoretical consumption
relative to optimal consumption
(272) Inaccuracy of inputs and assumptions for building modelling
(149) Inappropriate initial standard assumptions (61) 1 Inappropriate modelling of building element
(façade, roof)
(42) 2 Inaccurate representation of the exact geometry
of the building
(13) 3 Incompleteness and inaccuracy of available
project information (for retrofit)
(21) 4 Uncertainty in prediction of the actual use of the
building
(12) 5 Inaccuracy of climate data (30) Uncertainty in definition of the actual external
environmental conditions
(24) 6 Uncertainty in definition of future weather
scenarios
(6) 7
Inaccuracy of occupant behaviour modelling
(93) Uniform assumptions for occupant behaviour in the standards
(47) 8 Variability in comfort needs of the occupants
across the population
(46) 9 Deviation of actual consumption relative
to optimal consumption
(159) Malfunctioning equipment (56) Partial failure or malfunctioning of energy systems
(19) 10 Lack of fine-tuning during the operation stage (26) 11 Controls strategies/software do not work as
predicted
(11) 12 Measurement system limitations (8) Poor quality of the measurement equipment (8) 13 Execution of the work (34) Poor quality of the building envelope (12) 14
Poor workmanship (15) 15
Lack of attention to executability of construction (7) 16 Non-optimal use of the building by
the occupants
(61) Occupants’ experience with the installed technology
(26) 17 Poor communication between the stakeholders
(little information to the occupant)
(35) 18
dictory use of the thermostatic valves set at their maximum, while simultaneously keeping the windows open in the tilted position all day long[78], mainly to counteract the indoor overheating[26], and causing an increase of actual consumption by up to a factor of two[97]. However, occupant behaviour and its impact on energy consumption is notoriously hard to predict[70]. In one study, man- ual operation of shading systems by users had the effect of reduc- ing solar gains and increasing heating consumption[98], while in another, manual shading operations instead increased solar heat gains and reduced heating consumption[99].
3.3. Assessing the causes
The combination of these causes can significantly influence building energy performance. Moreover, it was found that the causes of EPG are not independent, but are often strongly interre-
lated [100]. The problem identified through this review is that most reviewed studies focus on one or two causes separately, but never in combination. Furthermore, studies dealing with theoreti- cal deviation only focus on calculation problems, and rarely worry about the actual technical problems of the buildings, and vice versa. This is even more evident fromFig. 4, in which all the studies analysed in this review are reported: 60.5% of the studies deal with theoretical deviation, 25.5% treat only the technical problems of the buildings, and only 14% deal with both causes of the EPG, those related to theoretical deviation and those related to actual deviation.
Finally, making use of the insights of the literature review pre- sented above, we prepared a full list of all the causes found, which have been classified in Table 1 according to the macro-causes defined previously. For each study, we counted the covered causes, and reported them inTable 1, whereby the number reported rep- Fig. 5.Treemap visualization of the causes of EPG. Each cause is represented using rectangles, the dimensions of which are calculated using the number of times that cause was indicated as the cause of the EPG.
Fig. 6. Classification of the Energy Performance Gap’s principal solutions identified in the literature. They allow to reduce the deviation between the theoretical or/and actual consumption on the one hand and the optimal consumption on the other.
resents the times when that cause was indicated as the cause of the EPG.
As can be observed from Table 1, all these causes have been identified with a different frequency in the literature and most importantly, they contribute differently to both energy consump- tion and EPG. These different contributions can be visualized in Fig. 5, in which the dimension of each box represents the number of times that cause was indicated as the cause of the EPG.
In conclusion, this literature review shows that it is challenging to arrive at a ranking of the causes based on their influence on the EPG. Indeed, each of these macro-causes may be the main reason for the EPG. However, while one of these occur, it is also possible that two or more of these factors are jointly present. Literature does not offer quantitative evidence whether single-factor or mul- tifactor causes are more frequent and if so, which ones.
4. Solutions to address the EPG
Several strategies can be adopted to reduce the EPG by optimiz- ing the actual consumption or increasing the accuracy of the theo- retical consumption assessment. We grouped those strategies into suggestions to I) improve the calculation of theoretical energy use to better represent optimal consumption (Section 4.1), and II) increase the actual performance of buildings to be closer to their optimal consumption (Section 4.2). This grouping is illustrated in Fig. 6. Finally, we determined the phase of building projects where the different strategies should be applied and linked those with the causes categorised previously with respect to the optimal con- sumption (Section 4.3).
4.1. Improvements to theoretical calculations
This section addresses solutions that aim to correct the theoret- ical energy consumption calculation, as well as considering approaches to facilitate the application of energy certificates.
4.1.1. Approaches to energy labelling and calculations
Even though the energy performance certificate is regarded as a key instrument in promoting energy efficiency in the building sec- tor, so far its impact is quite limited[4]. These schemes need to be improved not only concerning theoretical calculations, but also more generally in their implementation modalities. Taranu and Verbeeck [101] highlight that the framing of the key messages and of the information about the technical aspects of the certificate is an important aspect in the certificate implementation. They note that some certificates provide a confusing mix of information and recommend that each EU state should define a priori what the pur- pose of the certificate is. For example, if the purpose of the certifi- cate is to encourage the owner to seek advice on energy efficiency improvements, it is important that it clearly reports the current and optimal potential energy performance of the building[102].
Several studies have been conducted with the aim of enhancing the quality of energy performance certificates. An algorithm was developed for Spain’s certificates with the aim of improving the baseline certificates according to building type and climate zones, and it proved to reduce the EPG [103]. Another suggestion to improve the quality of the certificate is ‘‘smart auditing” [104].
To-date, energy certificates have been randomly audited, but a tar- geted procedure could audit those that meet certain criteria (e.g.
having no gas heating systems declared, but a gas supply present) or have been identified as having a discrepancy.
An alternative to smart auditing would be to allow the energy assessors to correct potential errors while the certificate is pre- pared, based on machine learning approach. For example, if a cer- tificate that is produced describes a solid wall property, but
neighbouring properties have cavity walls, then the software could ask for confirmation that the wall type is correct. A similar approach has also been applied to detect outliers in energy use pre- dictions[105]. Finally, the work of Semple and Jenkins[106]high- lighted that there is still much to be done to integrate actual data within the assessment methods used to generate performance cer- tificates. They found that only 21% of the homes in the six largest European countries can use actual consumption to generate a certificate.
This review highlights that improving the procedures for obtaining and validating certificates is necessary. It also highlights how diverse the actors involved in gathering data reported in the certificates are, and how different the expectations are for each of them[107]. Furthermore, it would be useful to add a value for target (optimal) consumption for the building in operation, sepa- rate from the value calculation used for labelling and compliance.
It has been suggested[108]to use a two sets of inputs for the cer- tification, one standardised for labelling and a second focused on the potential for energy saving measures.
4.1.2. Accuracy of standard values
It has been argued that national building performance certifi- cates require increased data quality[39]. The variability of user behaviour and the challenges to define it in building energy mod- elling is a widely acknowledged issue[53]. To address this prob- lem, the use of large-scale occupant behaviour datasets, gathered through different data collection methods (e.g. questionnaires, wearable devices), has been recommended to bring the theoretical values closer to the optimal values[71].
Currently, information on the building characteristics is col- lected from drawings and in-situ observations, and then calculated according to appropriate standards [109]. However so far only indirect approaches have been proposed to improve these calcula- tions, mainly through proposing to update standard values (e.g.
new standard U-value)[110], or by applying adjustment factors for groups of buildings (e.g. to the indoor temperature profiles) [31]. It has been demonstrated that through the substitution of the standard values in the model with values based on in-situ mea- surements of air tightness and U-value, it was possible to reduce the EPG to 2.5%[35]. However, to improve the accuracy of the U- values, a comprehensive fabric test including air permeability, U- values (heat flux) measurements, and thermal imaging surveys have been suggested [50]. It has also been proposed to replace the use of static U-value in certificates with an ‘‘effective U- value” to quantify the dynamic performance of walls[55].
Various authors have proposed increasing the indoor tempera- ture heating set-point to 21.0°C or 21.5°C[26,52], instead of the current 20°C set in the national standard[42,111,112]. Further- more, while heating setpoint temperatures can vary greatly between dwellings, research suggested that this variation is not random[43]. Systematic variations were found according to occu- pancy profiles, household characteristics, motivation, and beha- viour; this could be accounted for by modifying standard values accordingly[113]. Therefore, the optimal values should also be dif- ferent depending on the building type and the respective occu- pants. For semi-detached houses a higher set-point value (23°C), while for semi-detached bungalows a lower one (20°C), are sug- gested compared to other dwelling types[43]. In newer houses, constructed after 2007, an even lower indoor temperature set- point (19°C) should be assumed due to well-insulated building envelopes [51]. Using the household characteristics instead, in UK for single parent a setpoint temperature of 18°C was reported, while for dwelling occupied by inhabitants rating their general health as ‘‘very bad”, a setpoint temperature of 20°C was reported [43]. In this sense, personal comfort models have also been pro- posed, which can be crucial for ‘‘generating accurate predictions of
individuals’ comfort requirements and closing the loop between occu- pants and heating systems”[75]. However, these models are not cur- rently compatible with the approach to thermal comfort foreseen in energy standards/certificates. Certificates should be further developed to cater for individual difference in thermal comfort.
Several studies suggested changing the ventilation rate to reduce the EPG [52,114]. Conservative values were indeed used in certificates to encourage a better thermal performance of the envelope (walls, floor, roof) [53]. However, most of the recent new buildings have become much more air-tight, calling for an update of these values. Some authors have therefore proposed revised ventilation rates of 1 m3/(m2h) instead of 0.3 m3/(m2h) for forced ventilation [98], and 1.1 m3/(m2h) during use and 0.5 m3/(m2h) out of use instead of the default of 0.7 m3/(m2h) for hybrid ventilation[37] in order to reduce the EPG. These values are considered optimal because they take into account the air flow resulting from window opening (by counting the opened windows several times during the day), which is not sufficiently considered in the standards[26].
To address the lack of precision in the standard weather data used in the certificates, new tools that support the quantification of local and micro-climate conditions have been created [115].
Tools have also been introduced to generate weather files that take climate change into account[63]. Jentsch et al.[116]developed a tool to integrate the future UK climate change scenarios into the widely used Typical Meteorological Year file formats, using the
‘morphing’ methodology [117]. These were all found to reduce the EPG.
While the various solutions to improving the accuracy of stan- dard values have been found to reduce the EPG, the synthesis of these findings indicates that small changes to any of these values could significantly change the resulting energy consumption esti- mate. This allows to close the EPG but without necessarily being a better representation of reality. Great care must be taken to avoid over-fitting the theoretical model by performing arbitrary changes to key parameters without significant experimental evidence for these changes.
4.1.3. Additional considerations for retrofitting
The EPG gives rise to uncertainty also when property owners take decisions about energy retrofit[42]. European studies using energy performance certificates found that the ratio of actual to expected energy savings after retrofit ranged from 40% to 60%
[21,118]. Considering that standard inputs for calculations can be up to 50% higher than the optimal inputs, unrealistic expectations of energy savings and profitability can be created if recommenda- tions are based only on standardised calculations[119]. To reduce this problem, the study of Cozza et al.[20]indicated that a more realistic assessment (3.6% difference) of real energy savings can be achieved by comparing the actual current consumption with the expected theoretical consumption defined by the energy cer- tificate after retrofit. Moreover, the energy certificate has to-date a limited influence on homeowners’ energy retrofit practices. Own- ers in general regard both comfort and energy saving as important reasons when renovating[120]. Further developments to the per- formance certificate concept, such as Building Renovation Pass- ports outlining a long-term building renovation history and future roadmap, may also help[121].
Another important aspect with respect to the certificate for ret- rofit is the mode of verifying retrofits. For existing buildings, an energy performance certificate is typically only required if a build- ing or a large building element is subject to major renovation[3].
To-date, these policies do not require proof of performance after commissioning[122], i.e. existing policy does not currently take the EPG into account. A part of the challenge is to achieve energy policy goals without imposing excessive monitoring labour and
costs[123]. New methods for rapid and less intrusive performance assessment may help in this respect[124].
Finally, the role of economic subsidies that can be obtained through an energy certificate is very important. Given the high capital cost of energy retrofitting it has been argued that, in order to be more effective, subsidies should be paid before the start of the work of energy retrofitting, and not after its completion, as is the case today[125]. This would avoid that the owner has to obtain an interim loan from the bank which can act as further barrier [126]. At the same time, compliance with minimum energy perfor- mance targets could be better guaranteed if part of the subsidy is only paid once proper execution and functioning has been proven.
To conclude, simplification and acceleration of legal procedures for energy retrofits may be required[127].
4.2. Improving actual consumption
This section focuses on solutions that aim to improve the actual performance of the building’s energy systems, enhancing the design and construction phases, as well as driving better monitor- ing, maintenance, and general use of the building.
4.2.1. Technical factors to improve actual consumption
Most researchers and practitioners agree that adjusting build- ing technical systems (e.g. building controls, hydraulic balancing, etc.) is among the most effective ways to bring consumption closer to optimum and reduce the EPG [128,129]. Continuous post- occupancy data collection and performance monitoring of the building can ensure that optimal conditions are met under actual operating conditions [130] identifying deviations caused by the lack of fine-tuning during operation[131]. To solve this problem, sensors[132], advanced electricity meters[133], building monitor- ing system[134], Wi-Fi[135], and wireless camera networks[136]
can be used for data collection. It is then essential that facility man- agers are given access to detailed data on energy consumption in buildings[135]. At the same time, the cost of monitoring needs to be kept in mind. It will be necessary to develop and implement cost-effective solutions which are likely to differ depending on the type and the size of the building[137].
The optimization of the HVAC systems tailored to the occu- pants’ needs is essential to reduce the EPG[85,138]. The function- ing and comfort provision of ventilation systems has been highlighted as needing more attention during the design phase as well as improved monitoring[139]. In the context of building retrofit, field measurements indicated that the ventilation effi- ciency was significantly lower than assumed in the design phase and that simple adjustment of the settings improved the overall performance and reduced the EPG[40]. In general, it has been observed that actively tuning and maintaining the building’s energy systems, e.g. by hydraulic balancing of the heating system, adjusting heating profiles and temperatures, or correctly setting up heat recovery system, can help to attain the optimal consumption and can drastically reduce the EPG[133].
4.2.2. Monitoring practices and continuous iteration
The engineering and architectural knowledge exists to allow the construction of energy-efficient buildings and to conduct effective deep energy retrofitting. However, the legislative framework is essential to enforce good practice in construction, commissioning and operation. The EPBD has improved building energy efficiency ratings in the EU[140]but it does not mandate monitoring actual consumption of the building. The 2010 EPBD update added the possibility of inspection and monitoring of building HVAC systems [141], but did not make this mandatory[142]. The 2018 EPBD[3]
finally required Member States to ensure that residential buildings are equipped with electronic monitoring. Nevertheless, there
remains significant opportunities to integrate measurement and verification in the EPBD requirements for building performance monitoring[143]. A major limitation of the certification schemes is the absence of performance monitoring once the certificate has been issued[144]. Many authors indicate that continuous monitor- ing should lead to optimal consumption and reduce the EPG [57,92]. Monitoring should therefore be required for certification of buildings[44,145]. A positive experience has been made in this respect by the Passive House institute, which requires monitoring of consumption, as well as occupant satisfaction and thermal com- fort, even after awarding their energy standard, with the aim of reducing the EPG[146].
It has been suggested that building designers or construction contractors should be responsible for verifying energy perfor- mance of completed buildings to identify over-consumption resulting from poor construction[147]. This makes it possible to hold contractors responsible for resolving problems in the project execution. As simple solutions, requirements for certification could be the documentation of the progress of construction with pho- tographs as well as airtightness tests during the construction per- iod, as recommended by the checklist for planning and implementation of Passive Houses [148]. However, as pointed
out by Way and Bordass[149], ‘‘most designers and contractors have traditionally shown little interest in learning from how their buildings actually perform in use; and most owners have certainly not wanted to pay them to do so”. It is precisely on this aspect that specific regu- lations are needed. A solution is the implementation of a role (per- son) responsible for implementation of the energy project[150], to ensure that the energy objectives are met[23]. The core part of this approach is the support to the inhabitants during the operating phase, accompanied by an optimization phase after renovation, which further reduces heating consumption and therefore the EPG[151].
While the measures discussed above concern primarily multi- family buildings (and especially larger ones), the installation of innovative technologies such as heat pumps in single-family build- ings is generally less demanding. Accompanying measures may nevertheless be necessary in order to ensure optimal performance and to avoid a bad reputation of innovative technological solutions.
As adequate approach, quality labels for recommendable installers may be introduced or lists of accredited installers may be pub- lished, helping homeowners to make a choice[120]. Good experi- ence has been made with this approach in Switzerland[152].
4.2.3. Collaboration between stakeholders
Several studies highlight the importance of collaboration and communication between stakeholders to reduce the EPG [153,154]. However, this is complex due to the different objectives between stakeholders as well as their different knowledge and experience with technologies, and especially due to the different definition of optimal consumption among the stakeholders[16].
It is important to increase the transparency in all the phases of design and construction, from the administration to the architect, from the energy utility to the inhabitant [155]. The objective is to share the knowledge gained in various projects with a larger audience, informing all building occupants about responsible use of the building systems[156]. Schröder et al.[112]clearly stated that ‘‘the influence of individual user behaviour on the integral energy consumption will further rise, because the energy release into the liv- ing environment gets more and more disconnected from the human perception”. This means that it becomes increasingly difficult for users to understand the energy impact of their own actions (e.g.
raise the thermostat by one degree). To address this, some work has applied Behaviour Change Programs using workshops, coach- ing, and feedback programs to promote energy saving behaviours and reduce energy consumption[157].
To facilitate the collaboration, researchers have tested the application of Building Information Modelling and cloud sharing platforms[158,159]. However, these technologies are still at an early stage and need all stakeholders to be trained to cope with these new tools before they can be implemented in the energy cer- tification scheme[89]. It is indeed important to improve the skills of all the actors involved in the design process[160], especially by promoting more integrated and cross-disciplinary teamwork,[161]
in order to move towards a common understanding of the optimal consumption.
4.3. Solutions at different building life stages
The energy certificate represents a snapshot of building perfor- mance at a given time, while the causes of the EPG stem from dif- ferent stages of its design, construction and operation. It is therefore important that the solutions presented inSections 4.1 and 4.2also be applied at their most appropriate phase of the pro- ject. The final goal of these solutions is to bring the consumption of a building as closely as possible to optimal consumption. These solutions have been summarised in the form of a series of recom- mendations inTable 2. These aim to reduce the EPG by avoiding Table 2
Overview of the recommendations to reduce the EPG according to the stage of the project. Cause ID refers to the cause or set causes listed inTable 1.
Design phase Recommendation to reduce the EPG Cause ID Initial
assessment
Well defined expectations and resources available in order to set realistic objectives
1-5 Better defined the external environmental
conditions (weather data) and initial state of the building
2-3-6- 7 Take in account the experience learned from previous similar projects
1-8-9 Plan the maintenance of the building systems 10-11 Take in account the feedback and comments of the future occupants
8-9- 17 Design Unambiguous building plan/retrofit plan that can
be realistically implemented
1-4 Promote simple and reliable solutions, taking into account possible future change of use of the building
5-10- 16 Clarify the responsibility and the role of each stakeholder
18 Employ only experienced and qualified personnel 14-
15-16 Define a strategy for monitoring 11 Construction Regularly control the quality of execution and
eventually update the theoretical values with the real ones
1-13- 14 Set up a system of sensors for measurements and alarms to detect malfunctions
10-11 Encourage collaboration/communication among all stakeholders
15-18 Commissioning Adjust the systems depending on the real usage of
the building
11-12 Convene all stakeholders when the building is
commissioned and implement periodic inspections to be performed in the first year of operation
11-18
Train building managers on different aspects of energy management
11-12 Ensure that follow-up is organized and
communicated in writing
11- 15-18 Operation Monitor the performance and eventually optimize
it
7-11 Inform and raise awareness among inhabitants and assess their level of satisfaction/comfort
9-17- 18 Give feedback to the designers and all the other stakeholders involved, disseminate the good practice
18
breaks in the chain of responsibility throughout the stages of the buildings’ life. The recommendations are grouped according to the different phases of the project following the classification of Khoury et al.[23]and matched with the specific cause(s) identified inTable 1using the Cause ID. Since some causes may be tackled differently, more than one solution may refer to the same cause.
5. Conclusions
This study reviewed the causes of the Energy Performance Gap between calculated energy consumption in residential perfor- mance certificates and actual consumption, as well as strategies proposed for reducing the gap. We introduce the concept of opti- mal consumption to classify the causes and mitigation strategies for EPG. Optimal consumption is the real energy consumption of a building performing in an ideal way, in which the comfort of the inhabitants is guaranteed, their use of the building is as intended, and the technical systems function as planned. The EPG can then be broken down into i) the difference between theoretical consumption and optimal consumption and ii) the difference between actual consumption and optimal consumption. The first difference is associated with the limitations of using a theoretical model based using standardised methods and inputs to describe the real conditions of use of a building, while the second difference is associated with malfunctioning and unrecommended use of the building.
Introducing the optimal consumption concept allows to clarify how the various causes and solutions fit into the building lifecycle and what stakeholders might be involved. It highlights that a building’s theoretical consumption provided by a certificate will systematically be different from the actual consumption. It is therefore necessary to define an optimal consumption as a target, towards which improvements to both the building certification and building operation can strive. Furthermore, optimal values, as opposed to the theoretical values that are used for today’s per- formance calculations, offer the basis for a debate around what is optimal performance of a building and what services it should pro- vide. As highlighted by Shove[24]‘‘the conclusion that technologies and practices are interwoven suggests that there might be ways of crafting buildings that do not meet present needs, and that do not deli- ver equivalent levels of service, but that do enable and sustain much lower-carbon ways of life”, and the concept of optimal consumption can be seen as a step in this direction.
This work further highlighted that EPG causes are generally considered in isolation. Most studies focus on one or two causes separately, but never in combination. Only 14% of the studies deal with both deviations (theoretical and actual,Fig. 2) of the EPG with a more holistic approach. We find that the majority of studies tar- geted inaccuracy of building physical modelling (35%) and occu- pant behaviour modelling (22%) to explain the EPG.
Malfunctioning equipment (13%) and execution of the work (8%) have been mentioned less often as causes of the EPG, although they are directly related to the actual consumption of a building. This work therefore highlights the need to consider the diversity of pos- sible causes when developing strategies to reduce the EPG. Those mentioned most often in literature are not necessarily the ones with the biggest impact on the EPG. Notably, previous works have often defined the role of the occupant as the main, if not sole, cause of the EPG, but our findings indicate that this is not sufficiently substantiated by evidence, in line with other recent works[162].
This review found that many strategies have been proposed to reduce the EPG, and that these can be categorized into two groups.
The first group, which appears most often, aims to improve the energy consumption calculation by correcting the standard values and/or considering new approaches to create energy certificates.
The objective is to make theoretical values more precise and accu- rate, and therefore close the EPG by bringing theoretical consump- tion closer to the actual one. However, current certification methods should not be expected to predict actual consumptions as they are primarily designed to enable comparisons between buildings. The problem is that certificates are often used to predict actual energy consumption and energy savings from building ret- rofits. This is due to a lack of valid alternatives, which highlights the pressing need for quick and accessible performance informa- tion. To this end, improved standard values could be used to more accurately model energy consumption, although - as models are sensitive to parameter changes - this could lead to problems of over-fitting. Alternatively, methods using actual energy data could be adopted.
The second group focuses on improving the actual performance of the building’s energy systems, through better monitoring, main- tenance, and general usage of the building. In this case the objec- tive is to close the EPG by focusing only on actual consumption to bring it closer to theoretical consumption.
Our work highlights that neither approach should necessarily aim to eliminate the gap between current performance certificate calculations and measurements, since the true optimal operation point of the building may correspond to neither of these values.
A key first step is understanding to what extent observed discrep- ancies stem from the calculation side compared to the implemen- tation. This is why we propose the concept of optimal consumption as something distinct from theoretical and actual consumption because it brings greater clarity to the study of the EPG. However, further research is required to develop this concept of optimal val- ues, in order to properly represent occupant and buildings diversi- ties. This implies that the attributes of individual occupants and their buildings, such as the set-point temperature and the effi- ciency of the systems, should be used to better describe, and achieve, optimal consumption. This review also identifies the need for studies on the very definition of optimal consumption for dif- ferent conditions (e.g. climate zone, urban area, technologies), so that it can be shared, regulated, and included within the existing certification process.
An overview of possible solutions has been presented based on the existing body of knowledge. Significant work remains to refine these solutions and ensure their effective implementation. It is notably essential to ensure better alignment between the various construction phases, in combination with complementary projects and activities, leading to more specific recommendations.
The EPG remains a complex and incompletely understood topic.
The related research questions are complex and require continuous efforts across the multiple domains of research. The causes classi- fied can all influence the building’s energy performance, but researchers have not been able to reliably quantify each impact independently due to intercorrelation and case-specific variability.
Large-scale quantitative multi-parameter analyses remain scarce, and with significant limitations, rendering it difficult to make definitive judgements about the relative importance of the causes of the EPG and its solutions. Among the strategies to better under- stand causes and solutions, we recommend in-depth monitoring and analysis of sufficiently large samples of buildings that are prominent in the building stock, thereby allowing to draw conclu- sions for a significant share of today’s buildings.
Declaration of Competing Interest
The authors declare that they have no known competing finan- cial interests or personal relationships that could have appeared to influence the work reported in this paper.
