The triangular 3-D shape of the attic enclosures and its exposure to both the indoor and outdoor environment make it different than other common building zones. A comprehensive heat transfer attic model has been developed and integrated to the building thermal model. In this paper, an integrated thermal attic/building model has been developed for the transientstate situation by including thermal capacitances of both indoor air and building envelope. The objective is not only to accurately take into account the characteristics of attics into building simulation programs, but also to study the effects of the attic temperature and ventilation rate on the energydemands and thermal comfort of the building.