The aim of this work is to consider the issue of heat transfer through the building envelope. The function of the building envelope is to prevent the penetration of water and water vapor. It also prevents the infiltration of air, which is crucial for energy consumption control, condensation avoidance, etc. In order for all components of the building envelope systems to fulfill their function, they must include a solid structure, a drainage plane, thermal barrier, air barrier, and vapor barrier. The thermal envelope is a part of the building envelope. This paper presents three cases of thermal flow through the wall: a case without additional thermal insulation, with 5 cm thick thermal insulation (polystyrene), and with 10 cm thick thermal insulation (polystyrene). The existing condition of the building involves an additional 5 cm thick polystyrene thermal insulation. From the results, it can be seen that a significant reduction in heat exchange is achieved by adding 5 cm thick insulation (polystyrene), as the savings compared to the same building without insulation amount to 61.6%. Meanwhile, the savings between additional insulation with 10 cm thickness compared to 5 cm thickness is 28%.