This paper presents a numerical investigation of dew formation on the roofs of buildings, and its influence on building thermal and energy performance. A new semi-analytical model was developed to evaluate the effects of rooftop dew, including the release/absorption of latent heat due to condensation/evaporation, the variation of apparent thermal emittance of the roof upper surface, and the dew runoff characterised based on laboratory experiments. A set of case study simulations was carried out by implementing the rooftop dew model in the building performance simulation (BPS) software EnergyPlus. Through its impact on roof top surface temperature, dew was found to significantly influence predictions of benefits and penalties of a ‘cool roof’ versus a bare metal roof. When dew effects were neglected in the simulations, the electricity savings attributed to the cool roof were over-predicted by between 39 % and 204 %, however, the gas heating ‘penalty’ of using a cool roof was over-predicted by 266 % to 362 %. Results from this study demonstrate that the thermal effects of dew on roofs can be important in BPS studies, and the model that is presented provides a relatively simple means by which to include such effects in simulations.