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2. ENGINEERING AND TECHNOLOGY
Energy Performance Certificates (EPCs) are issued when dwellings are constructed, sold or leased in the EU. Where the cost of obtaining the required data is prohibitive, EPC assessors use nationally applicable default-values. To ensure that dwellings are not assigned a wrongly-higher EPC rating, a standardised thermal bridging transmittance coefficient (Y-value) is typically adopted for all existing dwellings while worst-case overall heat loss coefficients (U-values) are used. Default U-values are applied to a specific building element type (roof, wall, floor etc.) based on building codes and regulations applicable at time of construction. Due to significant building fabric upgrades, default U-values are considerably higher than real U-values. This constitutes a systematic 'default effect' error typical of large national EPC datasets. For the dataset considered thermal default use overestimates potential primary energy savings from upgrading by 22% in dwellings constructed when thermal building regulation applied and by 70% in dwellings built before thermal building regulations. A methodology has been developed that derives from an EPC dataset, a method for calculating a realistic energy-improvement payback when use of pessimistic default U-values is unavoidable.
Energy Policy, JEP-S-19-04059, under review