The performance gap is the observed phenomena that show a difference between the expected value of heat transfer or energy use for a material, or part, or whole of a building, and the actual value observed (Asdrubali et al. 2014; Byrne et al. 2013; Roetzel et al. 2010).
The observation is that the constructed building uses more energy than the analysis of the design indicated (Johnston, Miles-Shenton & Farmer 2015). The Zero Carbon Hub (2014) states that a performance gap undermines a government’s carbon reduction plan, and that identifying the origin, size and extent of that gap should be the priority.
Evidence of the performance gap has been shown by Bell et al. (2010), Lowe et al. (2007), Ridley et al. (2014), Thompson & Bootland (2011) and Wingfield et al. (2008). Additionally, it is reported to exist by Gupta & Dantsiou (2013), Ryan & Sanquist (2012) and De Wilde (2014). Notably, Bell et al. (2010), Ridley et al. (2014) and Wingfield et al. (2008) observed the performance gap under experimental conditions, and when buildings are in use by the
15 residents. This indicates there are multiple elements to the performance gap. As Wingfield et al. (2008) stated,
…the root causes of the gap in performance are much more complex than a simple list of design flaws, construction faults and system inefficiencies and relate to the interrelationship of the various parts of the construction process from design conception right through to occupation.
Accurately predicting heat loss and energy use is vitally important for making well informed decisions regarding energy efficiency actions and policy changes (Byrne et al. 2013). The presence of the performance gap shows that the predictive methods used, whether calculations based on reported material properties or simulations of building performance, are underestimating the level of heat transfer, and/or the amount of energy being used. The major effect of this is in reporting of meeting energy efficiency standards (Byrne et al. 2013;
Roetzel et al. 2010), and at least in the UK action has been taken to address this issue by implementing the Building Performance Evaluation competition in 2010 (Gupta & Dantsiou 2013). As these standards are generally based on buildings ‘as-designed’, limitations in the estimation methods can result in an inflated measure of the building’s energy efficiency (Roetzel et al. 2010). This indicates that the ability to evaluate the buildings in situ performance is very important if we are to realise actual energy savings targets from the housing industry.
The performance gap is caused by two distinct areas: incorrect assumptions about the materials and construction, causing an underperforming thermal envelope, and incorrect additional assumptions about occupant behaviour and weather conditions.
Underperforming Thermal Envelope
The performance gap has been observed in both specific elements of the home, such as heat loss through a wall, and in whole house performance. In relation to the thermal envelope, this is the observation that parts or the whole of the building transfer more heat than was expected, based either on calculations from reported material specifications, or from simulating the building’s thermal responses.
Studies such as Asdrubali et al. (2014) show how the in situ wall does not meet the expected standard based on the data reported by the material manufacturer. In this series of case
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2. Thermal performance of building elements and materials are measured in controlled laboratory conditions;
3. The actual installation of the materials in real buildings is imperfect;
4. External conditions (for example, rain and wind) can influence measurements.
Doran (2008) also provides evidence of a specific element underperforming, finding that thermal resistance was “around 38% less than that expected on the basis of measured cavity width”.
Bell et al. (2010), Ridley et al. (2013) and Guerra-Santin et al. (2013) all show examples of the entire thermal shell underperforming in a co-heating test compared to the designed heat loss. The estimations made by Ridley et al. (2013) and Guerra-Santin et al. (2013) were based on calculations inherent in the PassivHaus Planning Package, and Bell et al. (2010) by a calculation based on thermal properties. All these examples, however, were based on high efficiency homes, and the validity of the co-heating test in this setting is questioned by Guerra-Santin et al. (2013) on the basis that the building’s sensitivity increases in solar radiation, similar to the overheating observed by Stamp (2013). Bell et al. (2010) did conclude that this discrepancy was due to a combination of underestimating the R-value of the walls and the effect of thermal bridging. This presents a combination of underperforming thermal shell and possible errors in the standard assumptions: the actual wall R-value was influenced by a significant amount of additional timber, and the thermal bridging assumption from the Standard Assessment Procedure (SAP) was deemed inappropriate (Bell et al. 2010). The gap disappeared when these two elements were accounted for, but forensic analysis was required. In addition to these exceptionally high efficiency buildings, a review of the Leeds Beckett co-heating database, conducted by Johnston, Miles-Shenton & Farmer (2015), showed fabric losses exceeded designed loss by a not negligible amount in 22 of 25 homes. It is not clear how the designed loss was determined, but the homes were built to “exceed the fabric requirements of Part L1A
17 2006”. The comparison of predicted and measured HTC is shown in Figure 2.1. Note that since publication of Johnston, Miles-Shenton & Farmer’s (2015) study, terminology has been changed from ‘Heat Loss Coefficient (HLC)’ to ‘Heat Transfer Coefficient’ (Jack et al. 2017).
Figure 2.1 Performance gap, Johnston et al. (2015)
Tests conducted by Thompson & Bootland (2011) for the Good Homes Alliance also observed a 30% increase in measured heat loss over designed heat loss. Each of these examples demonstrates a thermal envelope performing well under the design specification, and indicates issues with design assumptions that surround the thermal envelope’s physical properties. Again, this is highlighted by the Zero Carbon Hub (2014) with their recommendation for a review of conventions for calculating inputs relating to the building fabric. At present, it appears that performance of the thermal shell as a whole system is overestimated.