HeatMod6 is web-based user friendly application that allows calculating and analyzing building total heat balance, taking into account such essential thermo physical processes as heat transfer by transmission through building structures, ventilation, diverse internal source of heat gains, solar heat gains and other factors. Web-based approach offers such benefits: - make calculations on-line without need of saving any data to local storage; - access to your calculations from any computer with internet access; - no need to install any application; - always up-to-date mathematical model and classifiers; - operation system independent; - standard form certificate (PDF printout). Heatmod6 is adapted to Latvian climatic conditions and complies with the Latvian legal documents [1] (based on EN ISO 13790). There has been made practical probation of the application and its mathematical model by comparing the results with: - measurement data and determining the specific buildings energy efficiency; - such popular applications as EFA, EFA2, HeatMod5, PHPP.