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Mandatory CO2 benchmarking, SAP and EPCs

All new buildings in the UK are required to submit EPC certificates  as part of their compliance with building regulations. This process is comprised of a design stage assessment, whereby the building is assessed using the UK SAP methodology. Subsequently upon completion of the dwelling, a revised assessment is undertaken to reflect the building as constructed, this may include a site visit but more commonly can be done from an as built set of plans and specification by the On Construction Domestic Energy Assessor (OCDEA), also known as energy consultants.

The overall energy performance of the building must meet the minimum performance standards as outlined in the Building regulations Part L1A 2013 the latest revision of which came into force in April 2014. The building must meet a range of minimum performance standards in terms of fabric performance; including u-values or the thermal transmittance of fabric elements, air permeability (in effect how draughty a building is), as well as minimum efficiency levels for heating plant and other non-standard systems.

Figure Example EPC certificates showing both EER and EIR ratings

SAP relies primarily on two metrics, as previously discussed the DER/TER needs to be >0, reflecting minimum CO2 performance. SAP also generates two SAP score values, that are displayed on the EPC certificate. The first is the Energy Efficiency Rating (EER), which reflects the overall energy costs for heating and hot water in the building. The second the Environmental Impact rating (EIR) indicates a score based on the overall CO2 emissions of the building. Both metrics are based on a 1-100 scale and are banded in letters from A-G with a score of 100. A being highly energy efficient building with neutral CO2 emissions from regulated sources; heating, lighting, pumps and hot water.

‘SAP is a measure of the economic performance of a building and not a direct measure of energy efficiency. It uses energy cost as a surrogate for energy consumption and/or carbon emissions, making it better suited to policies aimed at reducing fuel poverty than at reducing energy or carbon’

Whilst many of the voluntary low carbon construction standards focus on a target for kgCO2/m2, the EIR rating is not simply transferable, as the banding is judged by how it compares to a reference building for that building type and configuration.  This means that there is no specific kgCO2/m2 for the EER A-G bandings. However Figure 7 illustrates a typical range of kgCO2/m2 performance for the higher EER bandings, based on some typical values.

Figure  Estimated DERs for EPC EER Bandings D to A, adapted from

An important consideration in these simplified metrics is that on site renewable energy, usually in the form of solar PV or solar hot water has the potential to significantly improve the SAP and EPC rating, whilst resulting in minimal improvements in energy conservation, via insulation measures etc.

Fabric Energy Efficiency

In the latest revision of the building regulations a Fabric Energy Efficiency (FEES) Standard has been introduced. The aim has been to promote measures that target the fabric performance of buildings as the initial means to deliver energy demands and CO2 emissions. This relates to a growing recognition of the importance of energy conservation and thermal detailing in delivering low energy buildings.

For existing buildings that are marketed for sale or rent, an EPC must also be produced. This is based on a more simplified version of the SAP methodology; RdSAP 2012. Whilst the basic metrics are the same, RdSAP makes more heuristic assumptions regarding thermal performance and places no requirements for air testing or commissioning certificates.