Thermal Time Constant (TTC) indicates the time required for change of 63.2% of temperature difference. This work develops useful application of TTC for determination of temperature change in buildings. Higher TTC results in slower change of room temperature, contributing to thermal comfort, to quality of the building and to energy conservation. The procedure developed in the work is for both purposes: to find the temperature change in the period of time and to find the period of time of specific temperature change.