In the EU, Energy Performance Certificates (EPCs) are issued for dwellings whenever they are constructed, sold or leased. Where requiring data would be prohibitively costly, nationally applicable default-values for the thermal transmittance coefficients of the building envelope are employed. Use of such worst case default U-values ensure that a poor dwelling does not attain a better energy rating than is merited. In the absence of empirical data in Ireland thermal-default U-values, as in many other EU member states, are determined by the type and date of construction and then prevailing building codes. Using 463,582 dwellings representing 32% of the total Irish dwelling stock, this work assesses the relevance of current default U-values. Significant levels of retrofits have been found to lead to the default U-values used now being higher that is typical in reality, thus decreasing the accuracy, and hence credibility, of an EPC. Lack of certification accuracy also inhibits investment in energy efficiency. (C) 2016 Elsevier B.V. All rights reserved.