Under tightening requirements for the energy efficiency of buildings, the development of building materials with reduced thermal conductivity and with the use of secondary resources is gaining particular relevance. A promising area is the use of cullet in the production technology of silicate bricks as a wall building material. The aim of the study was to investigate the influence of dispersity and the content of glass powder obtained from sheet glass waste on the physicomechanical and thermophysical properties of autoclaved silicate bricks. Glass powder was examined as an active silica-containing additive and a source of alkaline Na⁺ ions that affect hydrothermal phase formation. The samples were molded by semi-dry pressing at a pressure of 20 MPa and 10–12% humidity of the mixture, after which the bricks were autoclaved according to the industrial regime of the silicate brick factory of West Kazakhstan Corporation of Building Materials, JSC (174–175°C, 0.8 MPa). The introduction of 5–15% fine glass powder was found to decrease the average density and thermal conductivity coefficient of the product while maintaining strength at the level of grades M150–M200. Microstructural and phase analysis showed a predominance of an amorphous and semi-crystalline C–S–H phase with the suppression of tobermorite formation due to the alkaline effect of glass powder. The study determined that the modification of silicate bricks with glass powder allows obtaining more energy-efficient wall materials without changing the industrial production technology. The findings testify to the prospects of recycling cullet in the silicate brick production technology with the aims of increasing the energy efficiency of the enclosing structures.