The interface between the head of the window and the wall represents one of the largest thermal bridges of a building and one of the areas with the highest risk of surface condensation and mould growth. This study is concerned with the reliability and accuracy of assessing this thermal bridge heat loss and surface temperature at the junction of a window with a specific steel lintel where the window frame itself is excluded from the thermal model. Four cases were modelled, covering the evolution of the construction details of this junction, following changes in British legislation in regards to U-values. They were assessed with HEAT2D software under the standard (simplified) method and a more detailed approach. The outputs revealed that replacing the window frame with an adiabatic surface during the modelling process (as per standard) underestimates the risks of mould growth or surface condensation (as per Part LlA 2010), especially if the window has a high U-value. (C) 2014 Elsevier B.V. All rights reserved.