In devising viable energy efficiency policies that can reduce the greenhouse gas emissions of existing dwellings (e.g. UK's Green Deal), data are required on current insulation levels and its influences. One such data source is the seldom used UK Energy Saving Trust's Homes Energy Efficiency Database (HEED), which this paper investigates using Norfolk UK local authorities as a case study. The HEED's reactive and longitudinal data collection strategies contribute to underlying biases, which is the likely reasoning for its differences with the English Housing Survey and UK 2001 Census. These differences had a cascading effect in that they manifested themselves in the indicative financial and carbon assessments undertaken. Similarly, sampling concerns also implicated correlations surrounding influences of current dwelling insulation levels. Providing one is transparent about potential biases and data concerns, the HEED can play a substantial role in guiding policy decisions and understanding dwelling stock characteristics (e.g. what makes dwellings 'Hard to Treat'). In particular, its vast (national) geographic coverage yet high resolution enables local context to be explored: a factor that this study shows to significantly shape insulation levels. (C) 2014 Elsevier Ltd. All rights reserved.