Most of the time people spend inside, so it is important to ensure pleasant stay and protect human from the influence of the external environment. Building envelopes are built-in parts of the building which separate interior from external environment and unconditioned space. Building envelope is mainly consisted of external walls of building. For evaluation of thermal performance of external walls, it is necessary to observe hydrothermal behavior of building envelope of building under what it depends on transfer of heat, air and moisture. In these thesis elementary terms are described and heat, air and moisture transfer principles. Influences on external walls are elaborated as well as requirements which are being set up for external walls to provide the right physical process of transferring heat, air and moisture without appearance of damage, so they could be resistant. Important requirements for construction products are described which are used for thermal insulation of external walls as well as requirements for them and way of their installation. For few materials and products that are part of system for thermal insulation of external walls, and also on different types of buildings, there is discussion about their advantages and disadvantages. Also, there are described assessment systems and stability check of performances for construction materials for thermal insulation.