Exemplary implementations of a thermally-efficient slidable fenestration assembly are glass window systems or glass door systems having one or more sliding glass panels. The fenestration assemblies are adapted to be mounted in an architectural structure such as a building or house. Accessory channels in the fenestration framework may be provided to facilitate nail-fin, retro-fit or screen adaptors as means to attach the assembly to the surrounding architecture. Stiles, tracks and rails of the assembly are specifically configured to reduce heat transfer across the fenestration assembly, while simultaneously maintaining the structural integrity and durability of the overall assembly. Certain stile, track and rail components may comprise materials of relatively low conductivities. Preferred stile configurations include interlock elements arranged to reduce the assembly's vulnerability to tampering from a position outside of the fenestration.