A trim assembly for use in concealing open spaces created by siding materials, wherein the assembly comprises a trim member configured to cover and conceal the terminal edges of the siding while simultaneously creating an expansion channel which allows the siding to expand or contract without causing premature bulging, bowing, splintering, cracking, and the like. The assembly may further comprise an insert wherein the insert is configured to readily engage with the trim member, and where the insert in combination with the trim member creates the look of butted cedar siding. The insert is configured to allow the siding to expand and contract in both a horizontal and a vertical direction.