This paper studies the key parameters governing the process of thermal storage by external thermal mass in buildings such as walls and roofs, expose directly to ambient temperature variation with a constant ventilation flow rate. As the solar radiation gain in a building is not a constant, a non-constant heat source is considered. We model the total heat source in the building as a semi-periodic one, including heat gain from people, lighting, other heat-generating equipment and the direct solar gain. In addition, we also consider that the outdoor air temperature has a phase shift delay with respect to solar radiation as the earth acts as heavy thermal mass. It is found that when the ventilation rate is constant, both the phase shift and fluctuating amplitudes of indoor air temperature are determined by the time constant of the system, two dimensionless surface convective heat transfer numbers, the Fourier time constant, the fluctuating amplitudes of internal heat source and the outdoor air temperature considering a periodic internal heat source. Simple formulae and some numerical solutions for predicting phase shift and fluctuating amplitudes of indoor air temperature are also presented.