A so-called fictitious ambient temperature is derived from real ambient temperatures and the thermal behaviour of a building envelope. By replacing the real ambient temperature in steady-state heat transmission calculations with the fictitious one, the thermal storage effects of a building envelope are taken into account. The fictitious temperature is derived by regarding the building envelope as a lumped capacitance or lumped mass. As, in practice, a building envelope differs from true lumped masses, an active thermal mass is defined using a correction coefficient. The simulation program DOE-2, version 2.1D, has been used for determination of the value of correction factor for a number of basic wall and roof designs, and for a number of validations of different building designs and locations. The comparison between the model described in the article and DOE-2 shows a good agreement.