This paper studies the processing, structural, and property relationships needed for highly expanded open-cell polycarbonate (PC) foams. Acoustic insulation, thermal conductivity, and mechanical properties of the foams were characterized. Using foam injection molding, samples expansion ratios ranging from 1 to 8 and open-cell content up to 85% were prepared. Relative to their solid counterparts, the sound wave transmission loss was increased up to 2.5-fold while their thermal conductivity was decreased up to fivefold. Thus, lightweight rigid PC foams, useful in both sound and thermal insulation applications, were developed.