Publication - Research and analysis

Energy Efficiency Standard for Social Housing: peer review

Published: 22 Oct 2013
Energy and Climate Change Directorate
Part of:
Environment and climate change

Peer review scrutinising the example dwellings in the Energy Efficiency Standard for Social Housing consultation document.

Energy Efficiency Standard for Social Housing: peer review
Case Study Group A:

Case Study Group A:

1. Pre 1919 solid wall ground floor flat, 55m 2
2. Pre 1919 solid wall mid floor flat, 58m 2
3. Pre 1919 solid wall top floor flat, 67m 2

Case Study Group A: 1990 Baseline


  • Currently only 2% of the 7.9 million solid wall properties in Great Britain have any form of insulation [1] .
  • Only 40 of the 487 social rented, pre-1919, solid wall tenement dwellings, surveyed for the 1991 SHCS, had insulation on all external walls. 'No insulation' is therefore considered an appropriate assumption for the 1990 baseline.
  • The RdSAP methodology v9.91 assumes U-values between 1.6-2.1 W/m 2.K for uninsulated solid walls constructed pre-1919 in Scotland. The range for v9.90 is 1.5-2.1 W/m 2.K, therefore some dwellings may experience slightly greater fabric heat loss under the latest version.

Ground Floor

  • Where it is not possible to determine the level of floor insulation, RdSAP assumes 'suspended timber floor construction, with no insulation', for pre-1919 Scottish dwellings.


  • Based on observations from the 1991 SHCS, the majority of pre-1919, top-floor tenements from the social housing stock featured a satisfactory level of loft insulation, defined then as 100mm (13.3% of the dwellings where it could be accommodated by 16.8%). This has required further investigation to determine the validity of the baseline assumption, as discussed in Appendix E of the main report of the main report.


  • The properties surveyed for the 1991 SHCS indicate that the majority of social rented, pre-1919 tenements featured single glazing, as per the baseline assumption. Only 20% were noted to have fully upgraded to double glazing.

Ventilation: Chimneys

  • Almost three quarters of the social rented, pre-1919 tenements, surveyed for the 1991 SHCS, featured a gas or solid fuel fire, and would require some method of ventilation to remove the combustion products. Chimneys should therefore be accounted for within the RdSAP calculation (unless any of the exceptions detailed in section 'S4 Parameters for ventilation rate' of Appendix S apply). Exclusion of these will underestimate the level of infiltration associated with the property, and consequently will calculate a reduced space heating requirement.

Space and water heating

  • Almost half (48.9%) of the pre 1919, social rented tenements surveyed for the 1991 SHCS did not have central heating. Regardless of this, the RdSAP methodology operates on the basis that the entire dwelling is heated. It should be remembered that the calculated RdSAP data is not representative of actual energy consumption and CO 2 emissions.
  • Case study group A specifies 25mm factory applied foam insulation for the hot water cylinder. This is better than the default performance specification (12mm loose jacket) assumed by RdSAP for hot water cylinders in pre 1919 Scottish dwellings. It is not until 1984 that 25mm factory applied foam insulation is the default RdSAP assumption.

Electric heated dwellings
Space and water heating

  • Based on observations from the 1991 SHCS, 93% of pre-1919, electric heated, social rented tenements featured storage heaters, and 86% were provided with hot water via electric immersion. This is in line with the 1990 baseline assumptions.

Gas heated dwellings
Space and water heating

  • Over a third (35.9%) of the pre-1919, social rented tenement dwellings, surveyed for the 1991 SHCS, featured a gas fuelled heating system.
  • It is unlikely that data will be available describing the efficiency of the boiler installed in the 1990 property, therefore it will be necessary to refer to the assumptions detailed within the SAP methodology: Appendix S assumes pre-1998 gas boilers are not fan-assisted, which corresponds to an efficiency of 66% (with the exception of a regular floor mounted boiler installed pre 1979 with an assumed efficiency of 56%).
  • Should any of the efficiency adjustments outlined in Table 4c of the SAP methodology be applicable, the boiler efficiency may be reduced by 5%. This means a regular, non-condensing, pre-1998 gas boiler (not fan assisted) without any thermostatic control of room temperature is assumed to have a winter efficiency of 61%.


  • The remaining number of open chimneys and/or flues should be considered for pre-1919 dwellings.
  • The minimum efficiency assumed for the winter boiler efficiency (applied for the space heating requirement) should be no less than 61%.

Case Study Group A: SHQS 2015 Assumptions

  • Element 31 of Annex C is not applicable to case study group A, where the dwellings cannot accommodate cavity wall insulation.
  • Element 32 of Annex C is not applicable to case study 1 or 2 of group A, where they cannot accommodate loft insulation.
  • The case studies detail 25mm of factory applied foam insulation on the hot water tanks. As long as the pipes are insulated, or contribute to the space heating requirement, case study group A complies with element 33 of Annex C.
  • Sub-elements 34A and 34B require the dwellings to feature a full central heating system (addressing all habitable rooms, excluding the kitchen and bathroom), and that it be deemed efficient. Both the storage heaters and gas boilers specified for the baseline dwellings meet the necessary criteria.
  • Element 35 of Annex C potentially creates the greatest challenge for case study group A, where it specifies a minimum NHER or SAP rating. Solid wall dwellings are 'hard to treat', therefore likely to require additional energy efficiency measures beyond the minimum standards prescribed by elements 31 to 34 of Annex C. These are discussed under the headings for electric and gas fuel sources.


  • Loft insulation (of minimum 100mm thickness) must be installed for case study 3, in line with element 32 of the SHQS 2015 criteria.

Electric heated dwellings
Space and water heating

  • Free-standing, large volume, electric storage systems may be deemed efficient if installed after 1984, however it may be necessary to upgrade to an improved system to help exceed the minimum SAP rating for element 35 of Annex C.
  • The proposed fan storage heater will have the same efficiency, but will benefit from increased 'responsiveness' [2] and automatic controls. This will contribute to reduced space heating requirements, fuel costs and consequently a better SAP rating.
  • The improvement to the hot water tank insulation, increasing the factory applied foam insulation thickness from 25mm to 80mm, identifies an additional method by which landlords can help improve the dwellings SAP rating for element 35.


  • Case study 1E is reliant on the installation of double glazing to help achieve the minimum SAP rating. Specification of pre-2003 glazing allows landlords to view the potential benefits associated with this measure, whilst providing flexibility to choose better glazing specifications to realise the SAP rating should they not be able to accommodate any of the other proposed energy efficiency measures.
  • Case study 2E specifies the installation of double glazing. Application of all the ' SHQS 2015' improvements demonstrates a SAP rating of 67, which is 9 points above the minimum criteria for element 35 of Annex C.
  • Many older dwellings experience difficulties accommodating improvement measures such as replacement glazing, where they are subject to listed building status or considered to be of historical interest. Such instances will require correspondence with the local planning authority and consideration of any additional costs that may be incurred.

Gas heated dwellings

  • The 1990 baselines for case study 1G and 2G meet all the criteria for ' SHQS 2015': cavity and loft insulation are not applicable, the properties feature a full central heating system with an efficiency greater than 55%, and the SAP rating is better than the minimum specification.
  • These case studies demonstrate that 'hard to treat' properties can still comply with the SHQS 2015 criteria, without relying on measures which can be restricted in their application due to factors such as listed building status.
  • The 1990 baseline of case study 3G is reliant on further improvement measures to meet elements 32 and 35 of Annex C, which include 100mm of loft insulation and improved glazing.


  • Despite meeting compliance, the ' SHQS 2015' stage still consider the benefits of installing pre-2003 double glazing for the gas heated case study group A. This allows landlords to view the potential benefit of this measure.
  • It should be noted that whilst the properties feature a 61% efficient boiler, the magnitude of the fuel bill and CO 2 savings, subject to the installation of double glazing, will appear greater compared to the application of this measure in a dwelling featuring a more efficient boiler. The overall fuel bills and CO 2 emissions will be lower for the property with the more efficient boiler.


  • Additional case studies should identify strategies to meet compliance for hard to treat buildings, which may be subject to listed building status and therefore unable to accommodate typical measures such as improved glazing.

Case Study Group A: Further measures 2020

  • The EESSH outlines a minimum EI rating to be achieved, which for an electric and gas heated dwelling are:
    • E (50) and D (65) respectively for a ground floor flat,
    • C (70) and C (80) respectively for a mid- floor flat, and
    • D (60) and C (70) respectively for a top floor flat.
  • Case study 1E and 2E do not consider any additional efficiency measures under the 'Further Measures 2020' stage, even though they fall short of meeting the minimum EI rating.
  • A number of additional measures are specified for case study 2G, however these are not successful in exceeding the minimum EI rating.
  • A number of additional measures are specified for case studies 1G, 3E and 3G which are successful in exceeding the minimum EI ratings.


  • The loft insulation is increased to a thickness of 250mm for both electric and gas heated dwellings (only applicable to case study 3). Combined with the other specified improvements, the electric heated case study just exceeds the minimum EI rating, whereas the gas heated case study is 9 points above the minimum criteria.

Electric heated dwellings

  • In addition to increased loft insulation, case study 3E specifies post-2003 glazing as an improvement measure under the 'Further Measures 2020' stage.
  • The specification of post-2003 glazing appears critical to meeting the EESSH criteria, where the case study only just exceeds the minimum EI rating.
  • The installation of post-2003 glazing specification may be limited in its application due to factors such as listed building status, or the prior upgrade of double glazing to pre-2003 standard. If this is the case, social landlords may need to identify alternative technologies to help reduce the building's energy consumption.

Gas heated dwellings
Space and water heating

  • The indicative life expectancy for a condensing boiler is in the region of 15 years [3] , therefore installation of a new system is appropriate for both the ' SHQS 2015' or 'Further Measures 2020' improvement stages. This also provides an opportunity to upgrade the control strategy.
  • Table 4b of the SAP methodology specifies a non-condensing combi-boiler to have an efficiency of 70-74%. Any installations after 2005 are likely to be condensing systems, thus assume an efficiency of 80-84%. The case study control specification indicates that the system does not qualify for any efficiency adjustments detailed in table 4c of the SAP methodology.
  • An improved boiler efficiency will yield considerable space heating and hot water energy reductions. Standard 6.3 of the Domestic Technical Handbook specifies a minimum seasonal efficiency of 88% ( SEDBUK 2009) for gas boilers. It is proposed that this value is used to describe the efficiency for any future installations, where it represents the current minimum standards.
  • A more efficient boiler will use less energy to address the load on the heating system. Any savings realised by a measure which reduces the load on the heating system will be greater under the operation of a less efficient boiler, compared to a more efficient system. This is important to bear in mind when considering the cumulative effect of implementing energy efficiency measures over a number of stages: the benefits of individual measures cannot be added together.


  • Additional efficiency measures should be considered under the 'Further Measures 2020' stage to ensure case studies 1E, 2E and 2G meet the minimum EI ratings for their corresponding dwelling type.
  • Consideration should be given to the potential benefits of other fuel sources and heating systems to realise the proposed targets, particularly for hard to treat dwellings. Should this involve a change of fuel type ( e.g. from electric to gas where feasible), consideration must be given to the efficiency and carbon intensity associated with the new system, to ensure an overall reduction in CO 2 emissions [4] .
  • An assumed gas boiler efficiency of 88% is proposed, in line with the minimum standards outlined by Standard 6.3 of the Domestic Technical Handbook.
  • Social landlords should be reminded that the case studies demonstrate the cumulative application of efficiency measures over three stages, and the potential benefits realised by a single measure are subject to changes relative to the other measures in place.
  • Additional case studies should identify strategies to meet compliance for hard to treat buildings, which may be subject to listing building status and therefore unable to accommodate typical measures such as improved glazing.

Case Study Group A: Advanced measures 2050


  • Solid wall properties are identified as 'hard to treat' ( HTT) therefore it is unlikely that insulation of the external walls will be a measure considered prior to the 'Advanced Measures 2050' stage.
  • Within Scotland many of the older domestic properties are listed, which may prohibit the application of external insulation. Further complications may also be introduced in trying to address a mixed tenure block. Whilst internal insulation may be considered a more appropriate approach in such instances, there are again limitations in its application, whether due to the reduction in internal floor area or incompatibility with internal features.


  • The RdSAP calculation used to determine the U-value for a suspended timber floor does not appear to accommodate insulation within the equation. This means that social landlords may wish to provide documented evidence of the improved floor U-value, if they wish to override the default values used in the calculation.
  • The installation of floor insulation is a particularly disruptive process, and would likely incur additional expense (where occupants may require alternative accommodation and storage of their possessions for the duration of the work). The installation of other improvement measures which require the floorboards to be raised, such as the installation of a central heating system, may provide an opportunity to install floor insulation with minimal additional disruption.
  • The process of insulating suspended timber floors requires careful consideration, in terms of damage limitation when lifting floorboards, and ensuring a sufficient level of cross ventilation is maintained to mitigate the development of damp or rot.


  • It is useful to demonstrate the benefits associated with further improved glazing to post-2003 standards, where this is a feasible measure (for case studies 1, 2, and gas heated case study 3G). Considered alongside other technologies under the 'Advanced Measures 2050' stage of improvements, it is difficult to identify the glazing contribution towards the reduction in energy consumption and CO 2 emissions, however it is unlikely it will be a measure undertaken earlier in the process where the glazing was previously assumed to have been upgraded for the ' SHQS 2015' stage.


  • Social landlords should be advised to acquire documented evidence detailing the U-value associated with an insulated suspended timber floor should they wish to override the default assumptions used in the RdSAP calculations.


Email: Agnes Meany