Developing regulation of energy efficiency of private sector housing (REEPS): modelling improvements to the target stock - Main Research Report
This report describes how the least energy efficient dwellings in the private sector were identified and how their ratings could be improved by a range of improvement measures. Modelling was used to ascertain the least cost way of reaching different standards, with findings presented on capital costs, fuel cost savings, carbon and energy reductions.
2 Summary of the Target Stock
2.1 In this section we provide a very brief overview of energy efficiency ratings, before giving an overview of the target stock - private sector dwellings in the lowest three EPC bands EFG - before improvements.
Overview of energy efficiency ratings of domestic dwellings in Scotland
2.2 Much like the rating of the efficiency of new electrical appliances, Energy Performance Certificates (EPCs) show the efficiency of buildings, with ratings ranging from A (very efficient) to G (inefficient). They show a current rating and a rating if improvements were made. All domestic buildings in the UK that become available to buy or rent must have an Energy Performance Certificate. An example is shown in Figure 2.1.
Figure 2.1: Example Energy Efficiency Rating from a standard Energy Performance Certificate (EPC).
2.3 EPC Energy Efficiency Ratings are underpinned by the SAP (Standard Assessment Procedure) methodology. As the name (partially) implies, this is a standard way of assessing the energy consumption of dwellings.
2.4 SAP works by assessing how much energy a dwelling will consume, when delivering a set level of comfort and service provision (with regard to heating, lighting etc.). This is based on standardised assumptions for occupancy and behaviour and enables a like-for-like comparison of homes. Related factors, such as fuel costs and emissions of carbon dioxide (CO2), can be calculated from the assessment.
2.5 SAP quantifies a dwelling's performance in terms of energy use per unit floor area. This is based on estimates of annual energy consumption for the provision of space heating, domestic hot water, lighting and ventilation combined. In a nutshell, the rating is determined by how well a dwelling holds energy (such as retaining heat, storing hot water etc.) and how efficient its systems are at providing heat, hot water, etc.
2.6 The SAP scale ranges from 1 (least energy efficient) to 100 (the most energy efficient). In Figure 2.1, the numbers in brackets are the corresponding SAP scores for each EPC band: EPC band G ranges from 1 to 20, F from 21 to 38, and E from 39 to 54. Thus the target stock, private sector dwellings in the lowest three EPC bands, equates to all private sector dwellings with a SAP score of 54 or below.
2.7 The SAP methodology has been constantly evolving. The impact of these changes to this research is summarised later on in this chapter while a fuller description is given in Chapters 4 and 6.
2.8 The analysis in this report is based on data from the Scottish House Condition Survey (SHCS). Data has been combined from the 2010, 2011 and 2012 surveys to provide sufficient sample sizes for analysis. The SHCS is designed to be representative of all dwellings in Scotland that are used as a main residence[5]. Estimating energy use has been at the core of the survey for almost two decades and SAP ratings have been calculated for all dwellings covered by the survey since 1996. A summary of the SHCS is provided in Appendix 1.
Energy efficiency profile of Scottish residential dwellings
2.9 There were around 2.4 million dwellings in the residential housing stock in Scotland used as a main residence over the 2010 to 2012 period. Around three quarters (74%) of these dwellings were in the private sector with 62% owner occupied and 12% privately rented.
2.10 The energy efficiency of private sector housing stock overall was poorer than that of the public sector. Overall, 23% of private sector dwellings were in EPC bands EFG compared to 12% of public sector dwellings (Figure 2.2)[6].
2.11 This means that while 74% of dwellings are in the private sector, 85% of all dwellings in EPC bands EFG are in the private sector.
Figure 2.2: EPC bandings by sector
2.12 The difference in energy efficiency between the public and private sectors was also reflected in the average SAP scores. The median SAP score among all private sector dwellings is 64, compared to 69 among public sector dwellings.
Figure 2.3: Summary of EPC bandings by tenure breakdown within the private sector
2.13 Figure 2.3 shows the distribution of EPC bandings within the private sector. A higher proportion of private rented dwellings than owner occupied dwellings were in EPC bands EFG (29% compared to 23%).Overall, 23% of the private sector residential stock was in the lowest three EPC bands, EFG. This equated to 400,548 dwellings.
2.14 As detailed in the next chapter, four key dwelling characteristics - dwelling type, dwelling age, wall type and heating type - that are closely associated with energy efficiency were used to create a typology of the REEPS target stock and to choose archetype dwellings for modelling potential improvements[7]. Below we summarise the make-up of the target stock with regard to these four characteristics.
2.15 Figure 2.4 summarises private sector dwellings by dwelling type[8]. Detached properties tend to be less energy efficient than other dwelling types. While detached properties account for around 29% of all private sector dwellings, they account for 45% of dwellings in EPC bands EFG, and 58% of dwellings in EPC bands FG. (Tables A8.1 to A8.4 in Appendix 8 give further details of this breakdown and the number of dwellings).
Figure 2.4: Distribution of private sector dwellings by dwelling type
2.16 Semi-detached properties account for 31% of dwellings in EPC bands EFG, and 18% of those in bands FG. Tenements account for 13% of the target dwellings, and 16% in bands FG.
Figure 2.5: Distribution of private sector dwellings by NHER Age
2.17 Figure 2.5 shows a similar analysis by NHER age[9]. Older dwellings were much more likely to be energy inefficient than younger dwellings. Pre-1919 dwellings account for 46% of the target stock compared with 24% of the private sector stock overall. Additionally, pre-1919 dwellings accounted for almost two thirds (63%) of dwellings in the lowest two EPC bands, F & G. In contrast, only 11% of the target stock was post-1976 dwellings and only 7% of dwellings in the lowest two EPC bands were post-1976.
2.18 With regard to wall type, dwellings with cavity walls were more likely to be energy efficient than dwellings with solid walls. As shown in Figure 2.6, while dwellings classed as having cavity brick wall type[10] accounted for half the private sector dwellings (52%) in total, they only accounted for just over a third (36%) of the target stock, and only 24% of the EPC bands FG. By contrast, sandstone and granite/whinstone combined accounted for only 25% of all dwellings, but comprised 46% of the target stock and 61% of all dwellings in EPC bands FG. Solid brick/cob accounted for 7%, timber for 6% and system wall types for 5% of the target dwellings.
Figure 2.6 Distribution of private sector dwellings in EPC bands EFG by wall type
2.19 Figure 2.7 summarises private sector dwellings by their main heating systems[11]. While 77% of the private sector stock overall had a mains gas boiler, this type of heating was only present in 40% of dwellings in the target stock and only accounted for 9% of all dwellings in bands F and G. In contrast, other forms of heating, particularly electricity-based heating system, account for a larger proportion of dwellings in the lower EPC bands. For example, while peak electric room heaters were the main heating system[12] for only 2% of all private sector dwellings, they account for 6% of the target stock and 18% of dwellings in bands FG[13].
Figure 2.7: Summary of private sector dwellings in EPC bands EFG by heating type
2.20 The prevalence of different heating types is closely linked to access to mains gas (see Figure 2.8). Inefficient dwellings are more likely to lack access to mains gas. Across all private sector dwellings in Scotland, 20% of dwellings did not have access to mains gas[14]. Among the target stock, this proportion rose to 52%. Moreover, 76% of dwellings in EPC bands F and 81% of dwellings in band G did not have access to mains gas.
Figure 2.8: Summary of access to mains gas among all private sector dwellings and by EPC bands EFG
2.21 This prevalence of non-gas heating was also significantly different from the social rented sector where very little heating is not either mains gas or electric. The social sector has very little solid fuel or oil heated dwellings. This is not the case in the poorer performing dwellings in the private sector - 28% of all dwellings of the EFGs are using something other than gas or electric heating.
The evolution of SAP rating methodology
2.22 The methodology underpinning SAP is constantly being improved and refined. The original methodology, SAP 1998, has been replaced on a number of occasions, by SAP 2001, SAP 2005, SAP 2009, and SAP 2012 version 9.92.
2.23 At the start of the research project in May 2014, the latest available SHCS data was the 2010-2012 waves. (Three years of data were combined to allow robust sub-group analysis.) This data included the 2005 version of SAP. The initial stage of the research - estimation of the total size of the target stock, the development of a typology of dwellings and identifying associated archetypes in the data - used this version of SAP.
2.24 The latest version of SAP, SAP 2012 version 9.92, was released mid-way through the research project in December 2014. It was agreed with the REEPS RAG that the analysis of improvements and grossing of results should use this, the most up to date version of SAP.
2.25 A SAP 2012 score was calculated for all dwellings that were chosen to act as archetypes. Table 2.1 details the extent of the match between SAP 2005 and SAP 2012.
Table 2.1: Comparison breakdown of target stock by EPC band between SAP 2005 and SAP 2012 figures. (Total %ages)
| EPC Banding based on SAP 2012 | |||||
|---|---|---|---|---|---|
| E | F | G | All | ||
| EPC based on SAP 2005 | E | 214,172 (53%) |
85,832 (21%) |
5,996 (1.4%) |
306,000 (76%) |
| F | 15,217 (4%) |
52,240 (13%) |
8,896 (2%) |
79,353 (19%) |
|
| G | 451 (0%) |
2,960 (1%) |
14,784 (4%) |
18,195 (5%) |
|
| All | 229,840 (57%) |
141,032 (35%) |
29,676 (7%) |
400,548 (100%) |
|
2.26 There was a reasonable match between the two versions with 70% being within the same EPC bandings. More dwellings were in EPC bands F and G using SAP 2012 than were using SAP 2005. In the final section of this chapter we summarise the distribution of the target stock using the SAP 2012 scores.
Size and distribution of the target stock
2.27 Table 2.2 summarises the SAP 2012 EPC banding profile among the target dwellings. The majority (57%) were in EPC band E, 35% are in EPC band F and 7% were in EPC band G.
Table 2.2: Breakdown of SAP 2012 EPC bands among private sector stock in bands EFG[15]
| EPC Banding (SAP score) | Dwellings[16] | % of REEPS Target population | % of total Private Sector main residential dwellings[17] |
|---|---|---|---|
| E (39-54) | 229,840 | 57.4% | 13.2% |
| F (21-38) | 141,032 | 35.2% | 8.1% |
| G (Up to 20) | 29,676 | 7.4% | 1.7% |
| All | 400,548 | 100% | 23.1% |
2.28 Around four-fifths of the target stock was owner-occupied (81%) with the remainder (19%) privately rented (Table 2.3). Privately rented dwellings were more prevalent in the lower bands - a higher proportion of dwellings in EPC bands G and F than in EPC band E were privately rented (40% and 21% compared to 16%).
Table 2.3: Breakdown of SAP 2012 EPC bands by tenure
| EPC Banding | ||||
|---|---|---|---|---|
| G | F | E | All GFE | |
| Owner Occupied | 17,776 (60%) |
110,904 (79%) |
194,158 (84%) |
322,838 (81%) |
| Private Rented Sector | 11,900 (40%) |
30,128 (21%) |
35,682 (16%) |
77,710 (19%) |
| All | 29,676 (100%) |
141,032 (100%) |
229,840 (100%) |
400,548 (100%) |
2.29 Although 42% of the overall target stock is in rural areas, the proportions vary considerably by EPC band. (Table 2.4) Overall, 67% of G band properties and 57% of F band properties were in rural locations compared to 29% of E band properties.
Table 2.4: Breakdown of SAP 2012 EPC bands by rurality
| EPC Banding | ||||
|---|---|---|---|---|
| G | F | E | All GFE | |
| Rural | 19,910 (67%) |
80,999 (57%) |
66,149 (29%) |
167,057 (42%) |
| Urban | 9,767 (33%) |
60,032 (43%) |
163,692 (71%) |
233,491 (58%) |
| Total | 29,676 (100%) |
141,032 (100%) |
229,840 (100%) |
400,548 (100%) |
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Email: Silvia Palombi
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