Thermally insulated windows and similar panel structure exposed to differential temperature conditions on opposite sides may comprise a fixed metal frame and, insulated therefrom, a movable metal frame supporting a window glass or similar panel. The movable frame is displaced into and out of venting association with and thermally insulated relationship to the fixed frame. Thermal insulation serving as weatherstripping between the frames provides a thermal barrier or break. It may be in connecting attachment to either the venting sash or the fixed frame, and in overlying relationship to the other in closed position.