Many types of foams are used for thermal insulation in building, frozen food industries and LNG containment systems. Low thermal conductivity, low density and low flammability are required for thermal insulation. Among many foams, phenolic foams are preferred for thermal insulation due to its lower flammability and lower gas generation than any other polymer insulation foams. However, it takes long time to manufacture large size phenolic foams and the environmental regulation limits the use of blowing agents. Urethane foams and polystyrene foams are widely used in spite of their high flammability and toxic gas generation because conventional phenolic foams usually have higher thermal conductivity than expected. In this work, a foaming method for the resole-type phenolic foams was developed using microwave and air instead of blowing agents, and its thermal and mechanical properties were measured. From the measured results, it was found that the phenolic foams developed had low thermal conductivity and low density suitable for insulation foams.