This paper analyses the relevance of an approximate method, using factors such as time-lag and decrement factor, in the prediction of heat flow of an unconditioned room. The approximate method, for conditioned environment, suggested by Koenigsberger et al (Ref. 8) has been modified suitable for the unconditioned environment and utilised in this analysis. The theoretical analysis is supported by a case study, which is a room in the top floor a multistorey building. The results show that the approximate method is better in predicting the values of heat flow, but involves less effort than the admittance method.