Abstract: A window assembly having improvements for thermal efficiency is described. Dead air spaces are created in and around the frame of the window assembly through the addition of nonconductive members. The nonconductive members, and the dead air spaces thereby created, reduce thermal conduction by the frame and improve thermal efficiency of the window. The nonconductive members may be adapted to cover air pockets within the frame, such as between opposing support legs. Other nonconductive members may be adapted to cap exposed portions of the frame. Other nonconductive members may be adapted to hold a panel against the frame.