The effective temperature, a linear combination of a building’s enclosure temperature, mean radiant temperature, and the ambient temperature, is a measure of the thermal comfort inside a building. Results from an analytical study of the relationship between the effective temperature, glazing areas, and thermal energy storage in passive solar buildings are presented in this paper. Spectral analysis is used to find the effective temperature response of a mixed-direct and indirect-gain passive solar building to Typical Meteorological Year (TMY) weather data. The response is given by the complex product of the building transfer functions and the weather inputs. Results are presented for the sensitivities of the maximum, minimum, and average daily effective temperature to changes in the direct and indirect gain glazing area and thermal energy storage.