Sustainable development focused on energy and material savings is driving attempts to decrease the thermal conductivity of heat insulation materials. Foamed glass is one of the most sustainable thermal insulation materials and has unique, load-bearing properties. In this study, contributions to the effective thermal conductivity were quantified for an open-porous-type foamed glass. Foamed samples were prepared by heating a mixture of waste flat glass, the foaming additives carbon and Mn 3O 4, and the nucleation agent TiO 2. The density of the foamed glass was 117 kg m −3 (total porosity 95.8 vol%) with a closed porosity of only 5.8 vol%. The latter was minimized by promoting partial crystallization. The thermal conductivity was 54.3 mW m −1 K −1 at atmospheric pressure. Fitting the pressure dependence of the thermal conductivity gave a base thermal conductivity of 26.7 ± 0.4 mW m −1 K −1, and a gaseous contribution of 27.7 ± 0.5 mW m −1 K −1 with a pore size of 0.77 ± 0.07 mm. Thus, the base thermal conductivity, which is attributed mainly to the thermal conduction in the solid phase, is much higher than in the case of fibrous glass insulation products.