This paper presents an approach to evaluate the sensitivity of a roof design to condensation problems, given the uncertainty to achieve continuity of airtightness in practice. The approach consists of a repeated number of simulations with a 2D heat, air and vapour transfer model to predict the variation in roof moisture performance due to various discontinuities in roof geometry. The set of discontinuities is calibrated by comparing measuring data of roof airtightness to simulation results. On the basis of the methodology, the paper explores the effectiveness of different measures to reduce the sensitivity of cavity insulated roofs to condensation problems. The results show that even when a roof design complies with condensation control standards, a lightweight system remains sensitive to condensation problems as a result of air leakage through the discontinuities, joints and perforations, common to most existing construction methods. The sensitivity of a roof to interstitial condensation due to air leakage essentially depends on the heat and vapour transfer properties and design of the layers outside of the thermal insulation (roofing and underlay)