Home Insulation

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 English East Midlands, ordnance survey grid area SK31. The housing stock consisted of a mixture older Victorian and early 20th century terraced housing and some more modern, estate built houses. (The data are anonymised and 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:

Naturally, the distribution shifts to the (more energy-efficient) right, when the energy saving measures are implemented. Another way to view the data 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 rad 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 Household's 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 bit never eliminate the need to purchase energy i.e. the law of diminishing returns

The final step is to tabulate the results using the two correlations

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G	9%	£1,753	£152	£802	Example	Example
F	20%	£1,315	£257	£727	Example	Example
E	16%	£1,017	£166	£667	Example	Example
D	42%	£811	£344	£617	Example	Example
C	12%	£666	£80	£578	Example	Example
B	1%	£560	£6	£545	Example	Example