An insulation panel includes a foam core with a reinforced and vapor-impervious facing disposed on one or both sides of the core. A portion of the facing extends beyond the core and has an adhesive so that the overhanging portion can be secured to an abutting panel to cover a seam between the abutting panels. A fixture for forming the panel includes two heated platens, preferably provided with a textured surface, held apart by a spacer and between which the facing is mechanically suspended. An installation of the insulation panel includes panel-securing elements attached to frame elements of the building and including first and second flanges spaced apart by the thickness of the panel. A portion of the insulation panel is disposed between the first and second flanges, the first flange is secured to the frame element, and a cover panel disposed over the insulation panel is secured to the second flange.