Using vegetated facade systems (VFS) as a sustainable solution for existing and new buildings and evaluating thermal performance of these sytems are not a new concept. However, there is a gap in literature about measuring thermal performance of VFS applied on an insulated wall. Also, in the research literature, there are few studies measuring thermal performance of felt type VFS in temperate climates, and data about the thermal performance of VFS during winter periods is still scarce. Thus, the aim of the present study is to measure the thermal performance of a felt type VFS applied on a thermal insulated existing wall that us located in Kocaeli, Turkey, under Csa climate conditions during heating and cooling periods. Test results indicate that the felt type VFS acts as a shading device and has a positive contribution to the thermal performance of building walls during a cooling period. In daytime when there is a high amount of solar radiation, felt type VFS decreased exterior surface temperatures of the insulated existing wall by a maximum of 24.4°C, 32.2°C and 37.2°C, in spring, summer and fall periods, respectively. Additionally, indoor air temperatures of the vegetated facade were lower than indoor air temperatures of the reference facade with the maximum difference of 1.8°C during the cooling period. Also, test results indicate that the vegetated facade never dropped to below 0°C while exterior surface temperatures of the reference facade dropped below 0°C at nighttime in the winter period. Thus, it can be claimed that the felt type VFS behaves as a thermal buffer and enhances the thermal performance of the exterior wall of the existing building during heating periods at nighttime. As a conclusion, although differences between exterior surface temperatures of vegetated and reference walls were high, differences between interior surface temperatures of vegetated and reference walls were not meaningful. That is due to the fact that the existing building exterior wall assembly includes 5 cm thickness thermal insulation material which enhance the thermal performance of the brick wall. Finally, according to solar reflectance results, it can be claimed that vegetated facade systems have a positive effect on reducing urban heat island effect.