Scottish Energy Performance Certificates - new metrics: research

Executive Summary

EPC ratings are currently based on the calculated running costs per m² of floor area of the dwelling. This takes into consideration how well insulated the dwelling is, but also the choice of heating system and the type of fuel it uses.

The use of a cost-based metric has worked reasonably well in the past, but in recent years with the rapid decarbonisation of the electricity grid, this correlation between the cost and CO₂ emissions of fuels has broken down to the extent that a cost-based metric now favours some relatively high carbon outcomes, particularly the continued use of fossil fuels over electric heating systems. The Scottish Government (SG) are therefore considering moving to or supplementing this with a new metric based purely on how well insulated the dwelling is, with the intention that this could be used alongside other policy levers that ensure low carbon systems will be used to provide that energy.

To support SG’s requirements, a number of possible metrics for this purpose have been considered. Of these, one (‘useful energy’^[1] (UE) required per m² of floor area) was examined in more detail, looking at what value would be equivalent in ambition to the current band C threshold.

The impact on the existing and proposed metric of applying energy efficiency improvement measures to the Scottish housing stock was then examined in the context of how many homes would be likely to pass the band C threshold as the housing stock is upgraded.

Key findings of this work were:

1. If a new EPC metric based on the amount of energy required to maintain a comfortable temperature in dwellings were to be adopted, setting this at a level of 162 kWh/yr per m² would be equivalent in ambition to the current EPC band C threshold. Around 40% of homes currently meet this level. The equivalent value using an alternative UE definition (excluding water heating energy) is 120 kWh/yr/m².

2. When a basic set of measures was added (loft insulation, cavity wall insulation) the number of homes reaching the band C equivalent figure of 162 kWh/yr per m² threshold increase from 40.8% to 46.9%. Adding suspended floor insulation) increased this to 64.7%. This shows that, while challenging, it is possible using currently available solutions to bring the majority of the Scottish housing stock past the proposed threshold level.

3. During the analysis it was noted that a significant proportion of the homes receiving highest EPC ratings at present appear to be using means other than further improving fabric insulation (e.g. by having PV systems). These would not improve the rating under the proposed new metric – suggesting the options for achieving high rating (As and Bs) might be limited, or at least expensive – e.g. deep retrofits.