Although aerogels exhibit excellent thermal insulating properties, their high costs and poor mechanical properties have hindered their application. Semi-rigid polyurethane foams (SRPUFs) have been widely used in daily life as thermal and acoustic insulators owing to their low cost and good mechanical properties despite their poorer thermal insulating performance compared with aerogels. To utilize both aerogels and SRPUFs, in this study, we prepared a SRPUF/aerogel composite using an economical and practical approach for the first time. Specifically, a sol state of polymethylsilsesquioxane (PMSQ) was prepared via the hydrolysis of methyltrimethoxysilane under acidic conditions, followed by the gelation of PMSQ on the surface of SRPUF under basic conditions to yield the SRPUF/aerogl composite. The fabricated SRPUF/aerogel composite with 63 phr (parts per hundred resin) of PMSQ aerogel exhibits promising properties, including a decline in the thermal conductivity from 41.0 to 36.0 mW/m·K and an improvement in the average sound absorption coefficient at higher frequency range from 0.57 to 0.67 compared with neat SRPUF. Moreover, the composite exhibits enhanced hydrophobicity, which is crucial for preserving the structural, thermal, and acoustic features of the composite in use. Hence, we expect that this study will set a precedent for the utilization of the excellent thermal properties of aerogels for various applications.