The thesis constitutes an extensive study of heat flow and air permeability of building enclosures with an aim of ensuring optimal indoor microclimate. The thesis consists of three main parts: research of literature and a computational model regarding the effect of external wall structures on indoor microclimate, an experimental study, and the development of a computational model. The first part of the thesis examines earlier international studies regarding building enclosures and calculation methodology, and the existing moisture migration and heat flow calculation models for external wall structures, as well as the characteristics of building enclosure materials. In addition, a critical review of the regulatory documentation was conducted, and the concept of optimal microclimate was defined. The tasks for achieving the objective of the work were concretized in accordance with the researched information. Based on the results of the research conducted during the work and taking into account the experimentally obtained results and the known calculation methods of thermal engineering processes, rooms were modeled with the help of finite element computational fluid dynamics software, selecting optimal indoor microclimate as the main parameter. The following results were achieved in the course of the work: • A model for calculation of heat flow processes in building enclosures has been developed and experimentally substantiated. • Proposals have been developed for changes in the Latvian Construction Standard LBN 002-01 “Thermal Engineering of Building Envelopes” and LBN 231 “Heating and ventilation of buildings”. • Substantiated application of building enclosure materials in various types of buildings has been developed and proven with the help of dynamic simulation software. The objective of the thesis is to obtain results with scientific and practical application, as well as to provide answers to current questions related to ensuring optimal microclimate by taking into consideration the characteristics of building enclosures. The basis of the research is the scientific and technical hypothesis regarding the possibilities of increasing building construction and usage quality by taking advantage of the solidity characteristics of the structures.