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.


1. Source: Implementing the Housing (Scotland) Act 2006, Parts 1 and 2: Advisory and Statutory Guidance for Local Authorities: Volume 4 Tolerable Standard :

2. Different baselines were used for different gasses.

3. Under the Guidance for the Tolerable Standard, it is the 'presence' of some roof insulation, rather than any specific thickness or standard across the roof space that is defined with the standard.

4. See Appendix 12 for membership.

5. It does not include vacant dwellings, dwellings that are used as second homes, or dwellings that are used for short-term holiday lets.

6. All figures in this section are based on the SAP 2005 methodology. The evolution of the SAP methodology is discussed in the next section.

7. The SHCS is designed so that these key characteristics match with the bandings in RdSAP

8. The figures in this section are based on the SAP 2005 figures as SAP 2012 where only available for the modelled archetypes

9. For post-1964 aged properties, the NHER age is the "building warrant age" of a dwelling and is used to select the appropriate default U-values, given the Building Regulation standards applying at the time. For earlier periods, the dates reflect approximate changes in construction or housing legislation. For most cases, NHER age will be the same as the construction age banding, with the exception of conversions from non-domestic buildings or where considerable alterations have been made to the original dwelling. There will be a small number of dwellings however, where the Building Warrant was applied for just before a change in the energy efficiency standards and the dwelling built in the next construction age period.

10. See Appendix 2 for details of the different wall types

11. These fuel and heating combinations covered all cases found in the SHCS. Less prevalent types have been combined in Figure 2.6 but are expanded in the typology detailed and discussed in Section 3. Note that solid fuel boilers can use various types of coal or wood as fuel types.

12. Main heating system is defined as the system that usually provides the heating to most rooms and the hot water. Where more than one system is present, the main heating is the one that heats the main living room. Where there is a system and individual room heaters present, then the room heaters are treated as secondary / supplementary heating appliances. Where there are only room heaters, the main heating is taken as the room heater heating the main living room. Electric storage heaters (even when there is only 1 storage heater) are considered in SAP & RdSAP to be a system.

13. Electric peak room heating is the default within RdSAP where no heating system is present.

14. SHCS 2010-2012,question L1 "What mains services does the dwelling have".

15. Based on SAP 2012

16. This is a slight underestimate due to the 1.3% of cases with missing data on tenure.

17. Based on 1.736 million private sector dwellings in Scotland. This is based on the grossing figure for SHCS 2010-2012 and only includes dwellings that are used as a main residence.

18. Consideration of the technical feasibility will include examining the risks associated with particular interventions. Measures are discussed in more detail in the next section.

19. A U-value is a measure of heat loss. It shows the amount of heat lost per square metre of material - including various wall, roof and floor materials. The lower the u value, the better the insulation provided by the material.

20. It may seem contradictory that a highly efficient heating such as direct acting electricity heating is expensive to run and has a high CO2 co-efficient attached to it. This reflects the difference between the efficiency of the use of the heating system in the home (where electric heating is 100% efficient at converting electricity into space heating)and that the price of electricity and the associated CO2 emissions take account of amount of energy used to generate electricity at the power station (which is much less than 100% efficient) and its transportation to the home. Direct electric heating refers to the heating coming on at the touch of a button, compared to storage heating where the heat is stored over the night before you want to use it.

21. Detached, Semi/End Terraced, Mid-terrace/Terrace with passage/Enclosed end, Tenement, 4-in-a-block, Tower/Slab, and Flat from converted house

22. Pre-1919,1919-1964, 1965-1975, 1976-1983, 1984-1991, and Post-1991.

23. Solid brick/Cob, Sandstone, Granite, Cavity Brick, Timber, and System.

24. Mains Gas boiler, Mains gas room heater, Oil/Gas/Electric warm air, LPG boiler, Oil boiler

Solid fuel boiler, Solid fuel/wood room heater, Wood boiler, Electric peak/off-peak boiler, Electric peak room heater, Electric peak/off-peak heat pump, Off Peak Electric storage heating, Community Heating

25. This was to account for the high proportion of target dwellings in EPC band E.

26. Except for groupings with a small sample size

27. Dwellings closest to the 33% and 67% percentile of floor size will therefore have been chosen as the archetypes

28. As there was only one dwelling in EPC band G

29. Based on SAP 2005 figures

30. The eventual change took place on December 7th, 2014.

31. National Energy Services' Plan Assessor v6.1 program was used. This is a fully approved SAP 2012 program.

32. RdSAP programs use a geometric model to calculate the gross heat loss wall areas, roof areas and floor areas from the floor area, exposed perimeter, and storey height of each storey. Within RdSAP programs are algorithms to calculate window areas and to convert external dimensions to internal dimensions. While RdSAP programs assign U-values to most wall and roof constructions by using look up tables, it calculates the U-values for stone walls and for heat loss floors individually. By comparison, SAP programs require the use of internal dimensions, and the specific areas and U-values for each heat loss wall, roof, and floor in a dwelling. The necessary SAP data entry items were produced by a combination of individual hand calculations (e.g. the wall areas and roof areas), using the RdSAP default U-values, or entering the base data into an RdSAP 2009 v9.91 program to obtain the relevant data entry item (e.g. stone wall U-values, floor U-values, window areas). Where external dimensions were collected in the SHCS data set, these were converted to internal dimensions using the published RdSAP algorithms Some SAP data entry items were assumed using the RdSAP assumptions (e.g. window orientation, the draughtproofing of windows and doors, the hot water cylinder details where no access to the cylinder is available, and party wall construction).

33. National Energy Services Plan Assessor v5.5.6 SAP program and NES-One RdSAP v9.91 program

34. Parity Projects (2014) Analysis for WWF and UKGBC: achieving minimum EPC standards in housing, version 1.4, 22 May 2014, Parity Projects, London

35. The assumption at this point has to be that if an improvement measure is being installed it is being done appropriately with regard to both the technical requirements of the building, and the procedures for fitting it so as to not to put the building fabric at risk. Research sponsored by Historic Scotland has raised concerns about the appropriateness of the use of, for example, some wall insulation materials within 'traditional' buildings (i.e. stone) and problems with interstitial condensation. Appropriate, natural, breathable insulation materials are available on the market. The question is one of appropriate design and installation, not one of technical feasibility.

36. See Scottish Office Building Directorate (1987) A Guide to Non Traditional Housing in Scotland, HMSO, Edinburgh

37. This is not intended as a recommendation for low efficiency biomass or wood boilers to be installed. Rather, it is intended to be illustrative of the impact of the trade-offs that occur between fuel prices, carbon conversion coefficients and heating efficiencies within SAP and RdSAP. Much higher efficiency wood pellet and wood log boilers are available in SAP and RdSAP.

38. Source: The Scottish Government (2014) Towards Decarbonising Heat: Maximising the Opportunities for Scotland, The Scottish Government, Edinburgh, p9 available at

39. The REEPS RAG took the decision not to include the "connection to community heating" as one of the improvement options because its applicability would be much more limited than the other improvement options. A case study of the potential impact has been prepared and submitted to the Scottish Government REEPS team.

40. e.g. low energy lighting and insulated doors, where the number of each present in the dwelling is multiplied by this variable cost to get the total cost of the improvement measure Electric storage heaters have a cost range multiplied by the number of storage heaters to be installed.

41. See Appendix 6 on SAP and RdSAP

42. In fact, it actually scored '-24' within the SAP 2012 calculation, but within SAP methodology all ratings of less than '1' are quoted as 1.

43. (Sources: the Product Characteristics Database File,

44. For loft insulation the mean cost was not used in the modelling. Instead the low range cost (i.e. £100 was used where the loft insulation modelled was a top up of 100mm or less on existing loft insulation; where the amount of loft insulation added was more than 100mm (up to 300mm for a virgin loft, the high range cost (i.e. £350) was used in the modelling

45. At the time that the modelling was being carried out in the latter part of 2014, the PCDF did not include a cost of the Quantum storage heaters. A cost of £700 was identified by searching the internet.

46. Parity Projects (2014) Analysis for WWF and UKGBC: achieving minimum EPC standards in housing, version 1.4, 22 May 2014, Parity Projects, London

47. As already noted, within the SAP methodology SAP scores calculated to be less than '1' are quoted on the EPC as a SAP score of '1'. Where the publically quoted SAP score is 1, the calculated SAP score can be found on the SAP worksheet. In all other cases, where the SAP score is 1 or higher, the rating on the SAP worksheet and that on the EPC will be the same (rounded to the nearest integer).

48. The fuel prices used in calculating the fuel costs for the base case and all of the subsequent improvements were those published in Table 12 of the SAP 2012 manual (see Appendix 8).

49. available at

50. For example, where loft insulation, draught proofing and a new heating system were assessed, the saving for the loft insulation was multiplied by 42 years, then discounted by 35%; the additional saving attributed to the draught proofing was calculated and then multiplied by a lifetime of 10 years and discounted by 15%; the remaining saving calculated and attributed to the new heating system, and then multiplied by the heating systems lifetime (e.g. 12 years for a boiler and 20 years for electric storage heating) and the in-use factor applied (which is 0 for heating systems).

51 DECC (2013) Valuation of energy use and greenhouse gas (GHG) emissions, DECC, London

52. In this report, the CO2e figures are reported in kg of CO2e per year for individual dwellings. When they are aggregated for the stock, they are reported in tonnes of CO2e per year (1 tonne = 1,000 kg).

53. In para 3.8 of the 'Developing an Energy Efficiency Standard for Social Housing: A Consultation' (2012, p9), it states "In one particular instance, (detached houses and bungalows with electrical space heating), the proposed standard is below that expected for the SHQS to reflect the difficulty of reaching the requires energy efficiency rating of 60 for such properties."

54. calculated using the values set out in Table 4.2 and Table 4.2a above

55. See para 5.50 for more discussion about in-use factors and 'the rebound effect'.

56. Ofgem's lifetime guidance for ECO is 12 years for a wet central heating system, and 20 years for electric storage heating.

57. Appendix 2 provides full details of how the status of each measure was defined.

58. Primary energy consumption includes losses that occur in the generation, transmission and delivery of energy before reaching dwellings. Delivered energy consumption only takes account of energy used from the point that it reaches the dwelling.

59. Appendix 10 provides details of the work undertaken by the Scottish Government to estimate turnover rates in the target stock.

60. Table A8.10 to A8.13 provide further details.

61. For Scenario 4, the average reduction in CO2e among dwellings in urban areas is around 25% less than for dwellings in rural areas.

62. Paragraph 5.53 to 5.62 were provided by Scottish Government analysts

63. DECC, "How the Green Deal will reflect in-situ performance of energy efficiency measures"

64. Ibid.

65. The SAP 2012 manual, which sets out the full methodology and calculation algorithms, is available at the BRE website:

66. The SAP age bands for Scotland, as set out on p115 of the the SAP 2012 manual, are: Age Band A: pre-1919; Age Band B: 1919-1929; Age Band C: 1930-1949; Age Band D: 1950-1964; Age Band E: 1965-1975; Age Band F: 1976-1983; Age Band G: 1984-1991; Age Band H: 1992 -1998; Age Band I: 1999-2002; Age Band J: 2003-2007; Age Band K: 2008-2011; Age Band L: 2012 onwards

67. Source: (2014) The Government's Standard Assessment Procedure for Energy Rating of Dwellings (2012 edition), p132, BRE, Garston

68. Source: (2014) The Government's Standard Assessment Procedure for Energy Rating of Dwellings (2012 edition), p134, BRE, Garston

69. Source: (2014) The Government's Standard Assessment Procedure for Energy Rating of Dwellings (2012 edition), p135, BRE, Garston

70. Source: (2014) The Government's Standard Assessment Procedure for Energy Rating of Dwellings (2012 edition), p136, BRE, Garston

71. Source: (2014) The Government's Standard Assessment Procedure for Energy Rating of Dwellings (2012 edition), p137, BRE, Garston

72. Source: (2014) The Government's Standard Assessment Procedure for Energy Rating of Dwellings (2012 edition), p231, BRE, Garston

73. See for example: /conservationpublications /technicalpapers.htm or and

74. See Leaflet 01 of the EPC guidance, for further details.

75. BRE (2002) Thermal Insulation: Avoiding Risks, 3rd edition, BRE, Garston was first published in 1989, updated in 1994 and revised again in 2002

76. British Standards Institute (2014) PAS2030: 2014 edition: Improving the Energy Efficiency of Existing Dwellings: Specification for the installation process, process management and service provision, BSI

77. See

78. See for further details

79. More information on solid fuel cassettes can be obtained from the web e.g.

80. As already noted, for loft insulation only, the mean cost was not used. Rather the low range cost (i.e. £100) was used in instances where the loft insulation top up was 100mm or less; the high range cost (i.e. £350) was used in instances where the loft insulation top up modelled was more than 100mm thick.

81. Parity Projects (2014) Analysis for WWF and UKGBC: achieving minimum EPC standards in housing, version 1.4, 22 May 2014, Parity Projects, London

82. Davis Langdon (2013) SPONS Architects' and Builders' Price Book 2014, Taylor Francis, Abingdon

83. Assuming the mid floor flat has a top floor version of the same dimensions

84. Not including any wall backing on to a corridor or circulation area

85. Assuming the mid floor flat has a ground floor version of the same dimensions

86. SAP 2005 figures

87. Source: (2014) The Government's Standard Assessment Procedure for Energy Rating of Dwellings (2012 edition), p231, BRE, Garston

88. Collated from the PCDF / SAP 2009 / SAP 2012. Acknowledgment to Scott Restrick, Energy Action Scotland for the initial work on collating the data included in this table. Additional data was added subsequently by the authors of this report.


Email: Silvia Palombi

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