Home Insulation

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Case study - 92 properties in SK31

Energy performance certificates (EPC) are a legal requirement when properties are built, sold or rented in the UK and the information is in the public domain. FLUKE has made a case study of 92 properties in the East Midlands, England, ordnance survey grid area SK31. The housing stock consists of a mixture of older Victorian and early 20th century terraced housing and some more modern, estate built houses. (The data are anonymised and given the grid area covers 100 square kilometres; this should provide adequate data protection to any individual householder). An EPC is issued following a detailed survey of the property and contains much useful information. This case study mainly focusses on the Standard Assessment Procedure (SAP) scores for the current and potential energy efficiency ratings. The latter score indicates the potential improvement in efficiency that might be achieved, if all the energy-conserving measures including insulation, low energy lighting, solar water heating etc. recommended in the report were indeed carried out. The comparison between these 'before' and 'after' SAP rating figures provides a simple method for assessing the potential improvement achievable if the entire housing stock were renovated to all the recommended measures in the EPC.

Histograms for the current and potential SAP ratings

The following two charts show the distribution of SAP ratings for the 92 houses:



As would be expected, when energy efficiency is improved the distribution shifts to the right hand side of the histogram, as energy saving measures are implemented. A more detailed way to view the data at the level of individual properties, is to correlate the SAP ratings before and after improvements:


There is considerable scatter in the data. Some of the lowest rated properties would benefit greatly from insulation, on the other hand mid-range rated houses may be old stock that have already been insulated to a good standard - or new houses that haven't been improved. The correlation between SAP values is positive, that is, the more energy efficient the house is to begin with, the higher the final achievable value. This relationship is subject to diminishing returns since an adequately insulated house can only be improved so far by further insulation, whereas an old and poorly insulated property has great scope for improvement in both absolute and percentage terms yet still not achieve the highest score even after all the recommended improvements. The thinner black trend line is the 'best-fit' to the data points and the bolder, red 45° line represents the current situation. The GAP between the red and black lines may be regarded as the realistic potential for improvement in SAP rating

Relationship between SAP, energy costs, energy consumption and CO2 emissions

A convenient relationship between SAP rating and household expenditure on energy … and by extension, the households' energy consumption and CO2 emissions … was required. The data set from EAG Charitable Trust was used to produce the following correlation and trendline. The exponential equation appears to be reasonable fit and the semi-empirical form can be justified from the observation that improvements in insulation and other energy-saving measures will reduce but never eliminate the need to purchase energy i.e. the law of diminishing returns

Cost correlation

The final step is to tabulate the results, making use of the two correlations. First assign mid-point SAP values to category bands: B=86, C=74.5, D=61.5, E=46.5 F=29.5, G=10.5. Use the cost correlation to convert SAP to current cost and a combination of both correlations to calculate the potential cost (after energy saving measures). Then weight these costs to take into account the percentage of houses in each EPC band and compute the weighted saving.

Band % in band Current Weighted current Potential Weighted potential Weighted saving
G 9% £1,753 £152 £802 £70 £82
F 20% £1,315 £257 £727 £142 £115
E 16% £1,017 £166 £667 £109 £57
D 42% £811 £344 £617 £262 £82
C 12% £666 £80 £578 £69 £11
B 1% £560 £6 £545 £6 £0
TOTALS 100% £1,005 £657 £348


Energy performance certificates (EPC) and Standard Assessment Procedure (SAP) ratings provide a valuable data source for assessing existing household energy consumption and the scope for reduction that consumption by energy saving measures

The average energy consumption cost for this case study of 92 houses is £1,005 with considerable variation due to the age and state of insulation of individual properties. If ALL the 'recommended measures' detailed in the EPC were implemented for all the houses in the study there would be a saving of £348 (35%) reducing the annual energy consumption to £657 (all figures on a weighted-average basis)

Energy costs can be regarded as a proxy for energy consumption and CO2 emissions. Large scale programs for retrofitting home-insulation for the existing UK housing stock have been proposed as a measure to 'de-carbonise' home heating. Based on this case study, a 35% reduction seems practicable