A pre-formed insert with stable dimensions for mechanical insertion into a hollow member and that reduces the Nusselt number and convection across the hollow member. The insert may be formed of a low heat conducting material like PVC and have extensions and internal voids that impede convection in the hollow and conduction through the insert. The inserts may be used in heads and sills of aluminum windows, doors and frames. In one embodiment, an insert is received in an open hollow and may cooperate with an insert in a frame hollow to decrease convection at the head end of a sliding window or door. An insert may be placed within the hollow of a window or door beside a roller assembly.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH
 The development of the disclosed subject matter was supported by funds from the U.S. Department of Energy Award No. DE-EE0004012. The U.S. government has rights in the invention.