Domestic building fabric performance : closing the gap between the in situ measured and modelled performance

There is a growing body of evidence available to indicate that there is often a discrepancy between the in situ measured thermal performance of a building fabric and the steady-state predicted performance of that fabric, even when the building fabric has been modelled based upon what was actually built. However, much of the work that has been published to date does not fully investigate the validity of the assumptions within the model and whether they fully characterise the building. To investigate this issue, a typical pre-1920’s UK house is modelled in Designbuilder in order to recognise and reduce the gap between modelled and measured energy performance. A model was first built to the specifications of a measured survey of the Salford Energy House, a facility which is housed in a climate controlled chamber. Electric coheating tests were performed to calculate the building’s heat transfer coefficient; a difference of 18.5% was demonstrated between the modelled and measured data, indicating a significant ‘prediction gap’. Accurate measurements of air permeability and U-value were made in-situ; these were found to differ considerably from the standard values used in the initial model. The standard values in the model were modified to reflect these in-situ measurements, resulting in a reduction of the performance gap to 2.4%. This suggests that a better alignment between the modelling and measurement research communities could lead to more accurate models and a better understanding of performance gap issues