Glass foam is a cellular material with excellent thermal insulating ability. The thermal conductivity (λ) of cellular materials is complex to understand because it is determined by the composition of the solid and gas phases, pore size, solid microstructure (amorphous or crystalline, struts, and walls), and radiation. In this paper, we focus on the influence of gas composition on λ of glass foams. Glass foams were prepared by a physical foaming approach, where powder pellets of cathode ray tube (CRT) panel glass were sintered at 640 °C in Ar or N 2 atmosphere at elevated pressure (5–25 MPa), and then cooled down to room temperature. When heating the sintered samples to the viscous state (approx. 10 8 –10 6 Pa s corresponding to 650–740 °C), the high internal gas pressure in the closed pores expands the glass melt. We heat-treated each sintered sample multiple times in air at atmospheric pressure to gradually decrease the density, and thereby to obtain multiple λ values of each sample at different densities, while the chemistry of the sample remained almost identical. Gas chromatography revealed binary Ar-CO 2 and N 2 -CO 2 gas compositions in the Ar- and N 2 -sintered samples, respectively. The thermal conductivities of these gas mixtures (λ gas,mix ) were calculated to be 16.4 and 23.1 mW m −1 K −1 , presenting 22 and 31% of the effective λ for the Ar- and N 2 -sintered samples, respectively. The lower λ gas,mix resulted in a lower λ of the Ar-sintered samples compared to the N 2 -sintered ones at low density (<0.6 g cm −3 ). Therefore, it is crucial to control the gas composition for tailoring λ of low density glass foams.