Determining realistic U-values to substitute default U-values in EPC database to make more representative; a case-study in Ireland

Energy Performance Certificates (EPCs) are the foremost source of information on the energy performance of the EU's building stock. Inherent in all EPC methodologies are trade-offs between reproducibility, accuracy, assessor expertise and costs. During an assessment, where accurate building data acquisition would be excessively invasive or costly, nationally specified default values are used. Default values are necessarily pessimistic to; avoid a better-than-merited rating, enable homeowners to know the advantage of energetic refurbishment, encourage homeowners to record upgrades informing EPCs, and propel assessors to seek-out information to provide an accurate rating. This work reviews default U-value use across Europe and the UK before focusing on Ireland's EPC methodology, finding 1 in 3 entries in the Irish EPC dataset to be characterised on default U-values in 2020, leading to the dataset presenting an overly pessimistic view of the stock, thus lacking validity. To mitigate the thermal energy performance gap between theoretical rated energy consumption and actual or likely energy consumption arising from the selection of unrealistic default values for parameters; this work reviews the literature to identify and catalogue predominant/prevalent construction characteristics over time and calculates associated U-values that can be substituted for unrealistic defaults, thereby making the EPC dataset more representative and resultant national building energy stock models more robust. A total of 38 wall (8 predominant) and 4 predominant roof types were characterised finding differences between default and realistic U-values to be as high as 187 %. A generalisable find-and-replace methodology for the Irish EPC database is also proposed.