Energy Performance Certification: Misassessment due to assuming default heat losses

by Ahern, C. and Norton, B.


Energy Performance Certificates (EPCs), are issued when dwellings are constructed, sold or leased in the EU, are the foremost source of information on the energy performance of the EU’s building stock. Where the cost of obtaining the required data is prohibitive, EPC assessors use nationally applicable default-values. To avoid wrongly-higher EPC ratings for all existing dwellings, a standardised thermal bridging transmittance coefficient (Y-value) is typically adopted together with worst-case overall heat loss coefficients (U-values). These default U-values for roofs, walls and floors are drawn from building codes and regulations applicable at time of construction. Many older dwellings have undergone significant building fabric upgrades. Therefore, default U-values are considerably higher than the real U-values of those upgraded houses. This causes a systematic ‘default effect’ error in large national EPC datasets. For the dataset considered thermal default use overestimates potential primary energy savings from upgrading by 22% and by 70% in dwellings built before after and before thermal building regulations respectively. 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.

Published In: Energy and Buildings, Volume 224, 1 October 2020, 110229