This article is devoted to the method of determining the maximum possible limit of R-value increase of buildings enclosures based on the payback conditions. The R-value affected by the climatic conditions of the construction region, the cost of work and materials and its installation that required to improve the resistance to heat transfer, as well as the annual percentage rate. Payback conditions of any measure are formulated. The value of investments to reduce the coefficient of heat transfer of enclosing structure per unit is limited by a characteristic of the construction region, which is denoted ω. The calculations of the ω-value for some cities in China with a severe climate are performed and calculations for selection of window units are made.