Abstract: To improve thermal and acoustic insulation between the outside (O) and the inside (I) of a structure having windows or doors therethrough, stationary frames (10, 12; 52, 54) are secured to the building, retaining movable window or door frames (2). The stationary frames include two metal rails (10, 12; 52, 54), one (10, 52) of the frames being located at the outside (O) of the building and the other (12, 54) at the inside (I) of the building. The inner and outer rails are separated from each other by a double-walled chambered strip (14) having first and second elongated strip portions connected by cross ribs (21, 22, 23) which, therebetween, define chambers (17-20). The chambers are serially located in a direction of thermal gradient between the outside and the inside of the building and arranged in stepped or Z-shaped configuration.