The complex assessment of a thermal performance of the envelope on the basis of natural measurements and numerical modeling of the heat and moisture transport in building components was executed. On the basis of natural measurements it was shown that thermal performance qualities of a maintained building do not meet standard requirements. For the purpose of establishment of the reason of low level of a thermal performance numerical modeling of the heat and moisture transport in an external wall according to the computer program developed by the author, by two options — on the basis of the actual and design data was executed. It was shown that results of numerical modeling on the basis of actual data will well be coordinated with natural data that confirms reliability of the results received on the computer program. On the basis of numerical modeling with use of design data it was established that the design solution of external walls corresponded to standard requirements for a thermal performance, and, therefore, deterioration of a thermal performance can be caused by the violations allowed during construction and reconstruction of the building.