<p>This study is devoted to the analysis of the heat transfer in single-chamber and double-chamber glass units installed in the outer and inner sashes of a composite window unit where the inter-glass space is filled with dehumidified air and inert gases. A mathematical model is constructed based on the solution of the heat transfer equation with constant coefficients in two-dimensional setting taking into account the layered structure and using boundary conditions of types III and IV. The problem is numerically simulated with the finite difference method on a uniform grid using the C++ programming language. The convective heat exchange through glass units is taken into account performing a series of numerical simulations in ANSYS Fluent software. It is shown that the convective heat loss in glass units can be reduced by increasing the thickness of the spacer frame and using inert gases with low thermal conductivity. The optimal thickness of the gas-filled chamber of a single-chamber glass unit is identified (when filled with air, dry air, argon, krypton, xenon) ensuring maximum thermal resistance.</p>