The ultimate goal of this work is to establish a prediction method based on Power Balance Model for prediction of flow rate through cross-ventilated building. For the establishment of Power Balance Model, the lost power across stream tube sections must be determined in advance. However, the loss of power in the stream tube was not well studied by other researchers but this concerned critical step forms the focus of the present CFD study in which transported power in stream tubes formed at two selected models: (i) a suspended rectangular model, and (ii) a pitched roof single-storey house model standing on a flat ground surface, was documented. For the development of a new method applying to predict the lost power, decrease of the transported power across the stream tubes through both types of models will finally be shown in this paper.