The problems related to restoring the fenestration of old buildings while preserving their historical elements and achieving modern acceptable conditions and parameters of the buildings’ microclimate were studied. A visual and instrumental examination of existing fenestrations was conducted assessing their influence on the microclimate of the premises. Development of possible variants of modernization while ensuring the maximum preservation of the historical details and elements of the buildings was carried out, including the modelling and calculation of these variants by certified software. The most optimal variants were selected, and recommendations for the restoration of windows and skylights, ensuring the fulfillment of modern requirements, were developed. The results of the work will be used in the restoration of the main building of the Museum of Fine Arts, which in 2025, will make up part of the town museum on Volkhonka Street, Moscow.