The understanding of the thermal transport mechanism of foam glass is still lacking. The contribution of solid- and gas conduction to the total thermal conductivity remains to be reported. In many foam glasses, the solid phase consist of a mix of an amorphous and a crystalline part where foaming agents can be partially dissolved into the glass structure. We investigate the influence of incorporation of residues from foaming agents (MnO2 and Fe2O3) on the solid conductivity of cathode ray-tube (CRT) panel glass. We have prepared samples by sintering and melt-quenching technique to obtain samples containing glass and crystalline foaming agents and amorphous samples where the foaming agents are completely dissolved in the glass structure, respectively. Results show that the samples prepared by sintering have a higher thermal conductivity than the samples prepared by melt-quenching. The thermal conductivities of the sintered and the melt-quenched samples represent an upper and lower limit of the solid phase thermal conductivity of foam glasses prepared with these foaming agents. The content of foaming agents dissolved in the glass structure has a major impact on the solid thermal conductivity of foam glass. Hence, the solid thermal conductivity of foam glass can be optimized by altering the foaming agent and its content.