2. Modelling Domestic Energy Use
6. Estimating energy use in dwellings is at the core of assessing the energy efficiency of the housing stock, the greenhouse gas emissions for which it is responsible and the risk of fuel poverty faced by residents. A number of measures produced through the SHCS are based on modelling energy use in the home:
- Energy Efficiency Ratings: SAP, EPC band, and NHER ratings;
- Carbon emissions and Environmental Impact Ratings; and
- Fuel Poverty.
7. The general methodology that underpins all of these estimates is known as BRE Domestic Energy Model (BREDEM). It was first developed in the early 1980s and has been continuously updated as a result of changes to our understanding of dwelling energy consumption and the use of energy in the UK housing stocks.
8. Prior to the 2013 SHCS Key Findings report, domestic energy consumption in SHCS statistics was estimated through the use of 'Auto-Evaluator', a software programme developed by National Energy Services Ltd (NES) and National Energy Foundation (NEF) which is based on an earlier version of BREDEM, BREDEM-12, published in 2001. Details on this methodology and the outputs it provided are available in the Scottish Government's Technical Note on Fuel Poverty under the old definition.
9. With the publication of the 2013 Key Findings report, the SHCS moved to an updated methodology which reflects the current industry standard of assessing home energy performance. The new approach is based on BREDEM 2012, and is implemented by the Building Research Establishment (BRE) with the use of proprietary energy models. This improvement incorporated several years of cumulative change and resulted in a substantial impact on all energy consumption-based indicators. The 2013 SHCS Methodology Notes provided detailed information on the key measures which were affected by the methodological update.
10. Further changes were introduced for the 2014 Key Findings report to reflect the updated version of BREDEM 2012, version 1.1. Details of what these changes involved were provided in the 2014 Methodology Notes. Further Key Findings reports continue to use the same version of BREDEM 2012.
2.1 Standard Assessment Procedure (SAP)
2.1.1 Energy Efficiency Rating
11. The Standard Assessment Procedure (SAP) is a BREDEM-based methodology which provides the UK Government's recommended system for assessing the energy and environmental performance of dwellings, accounting for the energy cost associated with space and water heating, ventilation and lighting, and where relevant, energy generated by renewables. SAP ratings allow comparisons of energy efficiency between different dwellings to be made.
12. The Energy Efficiency Rating (EER) is expressed on a scale of 1 - 100 where a dwelling with a rating of 1 will have very poor energy efficiency and high fuel bills, while 100 represents very high energy efficiency and low fuel bills. Ratings can be greater than 100 for dwellings that generate more energy than they use, however these are rare in the existing stock. Extremely inefficient cases can result in a negative rating. These are reset to a value of 1.
13. Ratings are adjusted for floor area so that they are essentially independent of dwelling size for a given built form. They give a measure of the floor area-weighted fuel costs for the dwelling under standard occupancy and heating regimes. The fuel prices used are averaged over the previous three years across the different areas of the UK. The SAP rating takes into account a range of factors that contribute to energy efficiency, the main of which include:
- the dimensions of the heat loss surfaces of the dwelling;
- materials used for construction of the dwelling;
- thermal insulation of the building fabric;
- efficiency and control of the heating and hot water systems;
- fuel used for space and water heating;
- ventilation and solar gain characteristics of the dwelling;
- renewable energy technologies.
14. SAP is used to compare the energy performance of dwellings and so is not affected by the individual characteristics of the occupying household, nor by the dwelling's geographical location. The calculation is based on a fixed heating pattern of 21°C in the main living area and 18°C elsewhere and 9 hours of heating on a weekday and 16 hours at the weekend.
15. The heating season occurs during the months of October to May. It is based on standard occupancy assumptions with the household size correlating with the total floor area of the dwelling. The dwelling is assumed to be located in the East Pennines region.
16. SAP is updated periodically by BRE on behalf of BEIS in order to reflect developments in our understanding of energy consumption, to update data for factors such as prices and temperatures, to incorporate new systems and technologies and to address applications across an increasing range of carbon and energy reduction policy areas. Alongside this, there is a 'reduced data' version of the methodology, RdSAP, which is applied to the assessment of existing buildings. Modelling of the SHCS data follows the conventions set out in this RdSAP methodology.
17. This report uses two editions of SAP to describe the energy efficiency of the Scottish housing stock, SAP 2009 and SAP 2012. SAP 2009 was adopted in the 2013 SHCS Key Findings report and applied to data back to 2010. The 2019 Key Findings report continues to publish energy performance statistics on this basis to allow an analysis of change over time.
18. SAP 2012, which is the current edition of the SAP methodology, is used from the 2014 Key Findings publication onwards to report on the energy efficiency and the environmental impact of Scotland's homes.
19. SHCS energy modelling for SAP 2012 in this report is based on two versions of RdSAP. The first, RdSAP v9.92 which was released on 7 December 2014, introduced some technical updates and broadening of scope (for example, enabling assessment of 'park homes' as a dwelling type) as well as updating UK carbon factors and fuel costs based upon recent research undertaken by the Department for Business, Energy and Industrial Strategy (BEIS).
20. The latest version of RdSAP (v9.93) was released on 19 November 2017 and contains revisions to the underlying assumptions used within the SAP calculations. The most notable update to the methodology in v9.93 was a change to the default U-values of cavity, solid and stone walls, built prior to 1976. Compared to v9.92, U-values for solid, insulated stone and uninsulated cavity walls have improved, whereas they have declined for insulated cavity walls. These U-values are used to calculate the rate of heat loss through the walls, which contributes to the overall thermal performance of the building fabric of the dwelling. Data on the basis of RdSAP v9.93 is presented for 2018 and 2019 only.
21. The key differences between SAP 2009 and SAP 2012, as well as the different RdSAP versions, are summarised below. The full documentation can be found on the BRE website.
- Climatic data has been extended to allow calculations using regional weather for some elements.
- An allowance for height above sea level is incorporated into external temperature data.
- CO2 emission factors have been extensively revised.
- Fuel price and primary energy factors have been revised.
- The options for heat losses from primary pipework have been extended.
- Default efficiencies for heat pumps have been revised.
- Adjustments have been made to the solar water heating methodology (lower savings if electric shower present).
- A new heating type has been added: high heat retention electric storage heaters.
- Low temperature heat emitter options have been extended (previously only underfloor).
- Thermal bridging details have been updated.
- Appendix R (reference values for calculating TER) has been updated.
- Solar radiation calculation has been updated (effects solar gains, solar water heating).
- Provision for dealing with park homes added
- Party wall heat losses included
- Post 2012 age band added
- More wall types included in tables
- Flue gas heat recovery and waste water heat recovery now recognised
- Ability to enter specific data in place of defaults where documentary evidence is available for the U-values of most items and some other features, e.g. solar water heating
- Additional improvement measures included such as glazing only upgrade (keeping existing frames)'.
24. The main changes in RdSAP v9.93 compared to RdSAP v9.92 include:
- Revision to the thermal performance of external walls; namely updates to the U-values of solid, stone and cavity walls built prior to 1976.
25. Each year, small improvements are made to the modelling accuracy of RdSAP and details can be found in the relevant year's methodology notes.
26. In 2019, more detailed information on combi boilers from the Product Characteristics Database (PCDB) have been included. The PCDB is a large repository of boiler information, used to assign efficiencies to boilers matched as part of the SHCS modelling process. For certain combi systems, more detailed information is available concerning additional losses associated with hot water. Improvements to the BRE boiler model have allowed this information to be extracted from the PCDB and used as part of the SAP calculation, to improve the accuracy of calculations surrounding hot water losses. The mean BREDEM energy consumption is expected to increase by around 33 kWh per year due to this change.
2.2 Environmental Impact Rating
27. The Environmental Impact Rating (EIR) represents the environmental impact of a dwelling in terms of the carbon emissions associated with fuels used for heating, hot water, lighting and ventilation. This calculation is based on SAP and uses the same heating regime and occupancy assumptions as for the calculation of EERs. EIRs are adjusted for floor area so they are independent of dwelling size for a given built form. They vary between 1 and 100, where 1 represents very high carbon emissions and 100 very low carbon emissions.
28. EI ratings for 2018 and 2019 in the 2019 Key Findings report have been described in this report based on SAP 2012 under both RdSAP v9.92 and v9.93. EI ratings for 2015-2017 are based on RdSAP v9.92 and EIR relating to 2010-2013 are based on SAP 2009.
2.3 Energy Performance Certificates
29. Energy Performance Certificates (EPCs) were introduced in January 2009 under the requirements of the EU Energy Performance Building Directive (EPBD). They are required when a property is either sold or rented to a new tenant.
30. EPCs are generated through the use of the SAP methodology. For EPCs, Energy Efficiency (EE) and Environmental Impact (EI) ratings are presented over 7 bands, labelled A to G. Band A represents low energy cost and high energy efficiency (or low environmental impact), while band G denotes high energy cost and low energy efficiency (or high environmental impact).
2.4 National Home Energy Rating (NHER)
31. The NHER system was extensively used in SHCS reports prior to the 2013 Key Findings publication to describe the energy efficiency of housing. Detailed discussion of this methodology can be found in the Scottish Government's Technical Note on Fuel Poverty under the old definition.
32. The NHER methodology used in the SHCS was based on an earlier version of the BRE Domestic Energy Model, BREDEM-12. This has now been superseded and under the updated method, BREDEM 2012, it is no longer feasible to reproduce the original NHER measure accurately. To provide some degree of continuity, an emulated NHER measure was developed for the 2013 Key Findings report and has been reported in the Key Findings reports since then. Details on how this measure compares to the original NHER measure are provided in the 2013 SHCS Methodology Notes.