Abstract: One or more side by side cylinders of light transmitting material are mounted for rotation about their axes within a building structure wall opening or the like. The cylinders each bear internally a narrow panel of thermal insulation material such as polystyrene which extends diametrically across the cylinder from one side to the other. The cylinders are mounted for rotation about their axes such that the thermal insulation panels may be aligned and in the plane of the opening to prevent both light and heat transmission from the exterior of the wall of the interior of the building structure. A half cylinder of slightly larger size may surround each cylinder, is fixedly mounted to the building wall to provide at least exteriorly a double glazed wall for increased protection against thermal loss, particularly when the cylinders are rotated such that the insulation panels are at some angle to the plane of the wall opening.