GT Lab Program is suited for conductance and transmittance of calculations using a thermal flux-meter. Progressive Averages Method is used in conductance calculation where conductance is obtained from the total readings performed on which the average value is calculated and not by instantaneous values for temperature and thermal flow. Surface coefficients of heat exchange are selected for transmittance calculation, which combine the effects of the above phenomena and are tabulated in UNI 7357/76 Regulations and subsequent adaptations depend on geometrical situation (e.g. vertical, horizontal structure, etc.), and in the UNI regulations accompanying Law 10/91 (e.g. in the UNI 10345 regulations for window elements).