The use of wood, an eco-friendly building material, is increasing to achieve carbon neutrality and improve energy consumption efficiency in buildings. Cross-laminated timber (CLT), which addresses the shortcomings of traditional glued-laminated timber (GLT), has gained attention as a building material. Research into plywood-core CLT (Ply-core CLT) is ongoing, which substitutes the core layer of CLT with plywood to better utilize domestic wood resources. The purpose of this study is to evaluate the energy performance of buildings through green remodeling for reinforced concrete walls, the main structure of old buildings in Korea that have been used for more than 30 years. As a result of retrofitting a reinforced concrete structure with wood panels, CLT panels showed energy savings of 65.95% for heating and 17.56% for cooling. In comparison, Ply-core CLT panels achieved energy savings of 66.28% for heating and 17.92% for cooling. Specifically, since the target building is a library with high heat generation, cooling energy savings were more significant than heating energy savings. Furthermore, due to the characteristics of Ply-core CLT, the energy savings were higher than those of CLT panels. Ply-core CLT, which uses plywood with low thermal conductivity as the core layer, further improved the thermal performance of the walls and contributed to enhanced energy efficiency.