Modifying concrete with polymers offers numerous benefits, leading to more efficient, durable, and sustainable building practices, as well as energy savings for heating and cooling purposes. When selecting a polymer or polymer-modified concrete for a specific application, it is crucial to consider the desired properties and performance characteristics to achieve optimal results. In this research, the effect of adding rock wool and sand to a polyester-cement mixture on compressive strength and thermal insulation was studied by creating four different samples. The first sample consisted of 70% cement and 30% polyester. The second sample included 60% cement, 20% polyester, and 20% sand, without rock wool. The third sample comprised 60% cement, 19% polyester, 19% sand, and 2% rock wool. The fourth sample contained 58% cement, 19% polyester, 19% sand, and 4% rock wool. The results indicated that the basic sample (70% cement and 30% polyester) had a compressive strength of 74.233 MPa and the highest thermal conductivity of 0.7079 W/m·K. Among the three modified samples, the second sample (20% sand without rock wool) achieved the highest compressive strength of 89.34 MPa. Conversely, the fourth sample, with 4% rock wool, demonstrated the lowest thermal conductivity values, making it the best sample for thermal insulation. This superior insulation performance is attributed to the inclusion of rock wool, which has excellent thermal insulation properties, helping maintain stable internal temperatures and reduce energy costs in buildings.