32 THE BUILDING CONSERVATION DIRECTORY 2025 CATHEDRAL COMMUNICATIONS HOW IS AN EPC USED? The government publishes EPC statistics on a quarterly basis with the chart below showing the distribution of EPCs lodged over time. In 2024, approximately 2.5 per cent of EPCs had a rating of F or G and thus would not be rentable2. It is at this level that EPCs have their greatest value in giving a performance overview of the building stock of England and Wales and how together we are moving towards the government’s net zero target. The EPC rating of a building can have far-reaching implications. The EPCs are used for government incentives such as the boiler upgrade scheme or the former renewable heat incentive. They also feed into retrofit frameworks such as PAS2035 to help improve the energy efficiency of buildings. A building’s EPC rating may mean a homeowner can apply for a green mortgage, loan-to-buy or be eligible for a green homes scheme. HOW IS THE EPC CALCULATED? EPCs are generated via two methods: SAP The Standard Assessment Procedure is used to generate an EPC for new builds and are more accurate as the materials and design specifications are known. RdSAP The second method is called the Reduced Standard Assessment Procedure and this is what Domestic Energy Assessors (DEAs) use to assess existing buildings. It is here where EPCs begin to show their limitations particularly with their lack of detailed assessment of older buildings. All of the approved SAP and RdSAP calculation methodology is publicly available through the BRE website3. DEAs gather information which is entered into the RdSAP modelling software to generate an EPC. The EPC then generates opportunities for improvements to the building’s energy performance rating using a computer algorithm. In order to remain competitive within the current market, DEAs need to assess a domestic building in under an hour and at a price between £65 and £120 per building. The EPC assessment does not consider measured energy usage data. The assessment is purely theoretical and in turn may differ from reality. EPC LIMITATIONS RdSAP makes a significant number of estimations and assumptions to allow an EPC to be generated. If a piece of information is unavailable, an estimate based on the building’s age and its assumed basic construction will be made. Over 50 per cent of homes in England and Wales were built before 19654 and the vast majority have construction detail information unknown or inaccessible during basic surveys. This commonly results in these types of properties appearing to have worse ratings than they would have if all the information had been available during the assessment. There are three key areas of sensitivity with RdSAP which have a large influence on the overall rating: • Fabric performance (which is heavily defined by age) • Space heating efficiency • Hot water efficiency The RdSAP utilises age bands to categorise buildings based on their construction date. These age bands, such as ‘pre-1900’, ‘1900– 1929’ and ‘1930–1949’, are used to assign default values for building components like walls, roofs and windows. While this approach simplifies the assessment process, it can lead to inaccuracies in older buildings due to the significant variations in construction methods and materials within each age band. Furthermore, a building could have undergone significant renovations, rendering the default values associated with its age band inaccurate. If information about a specific building component is unavailable or inaccessible during the assessment, the assessor has to assume the property has performance values of the building’s original build date, further compromising the accuracy of the assessment for heritage properties. This reliance on broad age bands can obscure the unique characteristics of an older building and result in an EPC that doesn’t truly reflect its energy performance. The Society for the Protection of Ancient Buildings (SPAB) published a report in 2012 where it compared theoretical U-values for walls with measured in situ U-values. They concluded that 77 per cent of theoretical U-values were overestimates and thus the fabric of these older walls were generally performing better than expected. Solid walls showed the largest discrepancy between in situ U-value measurements and their theoretical U-values5. At present, an in situ U-value measurement cannot be used in an RdSAP calculation. To ensure historic homes are given the best fabric values by the assessor you can: • ensure the assessor can ascertain and evidence the building’s age accurately • provide access to as much of the property as possible including loft spaces, crawl spaces, room in roof inspection hatches and others to assess insulation • provide evidence of building modifications. An assessor would evidence modifications using photographs showing measurements, building control certificates, architects’ drawings showing insulation specifications, FENSA certificates for windows, CIGA insulation certificates and more. If the assessor can’t evidence an improvement, they have to assume original construction. The most common example of insufficient evidence being available to an assessor relates to rooms in the roof. These have often been retrofitted and in many cases there is no access to measure the insulation installed. Therefore the assessor has to assume original construction and, for a historic building, this usually notes no insulation. Arguably, the biggest influence on an EPC rating for a heritage building relates to the fuel type and methods used for space heating. If we firstly consider fuel types, the RdSAP model’s cheapest fuels are mains gas, coal and wood chips with electricity being the most expensive. Depending on the tariff used, electricity can be up to four times the price of gas. As the EPC rating
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