5 Home energy assessment
So far this course has described a wide range of different technologies. But how do you assess what makes a really energy efficient home without actually reading the fuel bills? Which retrofit measures are likely to be most cost-effective or produce the largest reduction in CO2 emissions? Fortunately home energy rating computer models can help.
The need for an energy model to assess housing energy use was identified in the UK in the 1980s. The Building Research Establishment developed a domestic energy model (BREDEM) in the 1990s. This forms the core of the Standard Assessment Procedure (SAP). Since 2006 this has been the key calculation tool to show compliance with the Building Regulations for houses and flats.
It has been through several revisions and the current (2023) version is SAP 2012 (BRE, 2022). Although the actual calculation procedure only covers about six pages, the need to deal with all possible types of houses and flats and their heating systems requires a large amount of explanatory tables.
Its use in setting the UK Building Regulations has been a little controversial. From a point of view of setting enforceable standards, it would be simplest to say that the elements of buildings should use a given thickness of insulation or achieve a given U-value. This is known as the ‘elemental’ approach. If the requirement is to set the overall fuel cost or CO2 emissions, then a more complicated calculation is required to take into account the different sizes of buildings, the heating system efficiency, etc. Finally, there is the problem of what to do about the different climates across the country. A house on a hilltop in Cumbria is likely to have a colder climate than one on the coast of Cornwall. Should it be required to have better insulation standards? Although the 2009 version of the SAP methodology did not distinguish between different climate zones, the revised 2012 version has now taken this into consideration.
Then there is the question of how much of house energy use should be ‘regulated’. The choice of a gas or electric cooker obviously influences energy use and carbon dioxide emissions, but is a matter of choice for the occupant, not the builder. As a result this is left out of the SAP calculations.
The full version of SAP is used in assessing new buildings, but for assessing existing homes a ‘reduced’ version, know as ‘RdSAP’ is used. This is used in ‘home energy audits’, particularly to produce Energy Performance Certificates (EPCs). These have been required for homes for sale in the UK since 2007 and newly rented properties since 2008. They can only be issued by qualified energy auditors.
In its original form the SAP assessment was only concerned with the cost of energy for space and water heating, ventilation and lighting. It is adjusted for floor area. It is expressed on a scale of 1 to 100, the higher the number the lower the running costs.
For example a pre-1919 house with solid walls, no loft insulation and no central heating would have a SAP rating of just over 20. This was about the average rating for the UK housing stock in 1973 (BEIS, 2021c). A newer house meeting the 2006 Building Regulations for England and Wales would have a SAP rating of about 85. Its fuel costs per square metre of floor area would be about a fifth of those for the older uninsulated house. Since the rating is cost based, electrically heated houses tend to have worse SAP ratings than gas-heated ones.
The rating also allows for houses to have their own generation technologies such as solar water heaters, PV panels or micro-CHP units.
However, the need to reduce overall CO2 emissions has now become the main driving force of the Building Regulations. An Environmental Impact Rating (EIR), based on CO2 emissions per square metre of floor area, was added. This, like SAP, is expressed on a scale of 1-100. The higher the number the better the standard.
In order to make things easier to understand for the consumer, the scales have been simplified into A-G bands (as shown in Table 14).
|SAP or EIR rating||Label band|
|81 – 91||B|
|69 – 80||C|
|55 – 68||D|
|39 – 54||E|
|21 – 38||F|
The sample energy performance certificate shown below in Figure 44 gives an ‘Energy Efficiency’ (SAP) rating of 55 (a D rating) with a potential to increase it to 85 (B rating). The house was also given an Environmental Impact Rating (EIR) of only an ‘F’ (not shown in the figure). This was mainly due to its high CO2 emissions because it was totally electrically heated.
It is estimated that the average SAP rating of the UK housing stock has improved from about 18 in 1970 to about 60 in 2017.
A thorough assessment of the potential savings in a home is likely to require a careful energy audit. This video shows how one set of homeowners in Oxford made changes to the energy efficiency of their home. It also shows that home energy efficiency is only one factor in a wider appreciation of a low carbon lifestyle.