Icon Technologies's Skylight Inner Dome for a 21" x 13 1/4" Skylight opening. An Inner Dome provides a layer of insulation and reduces condensation forming on the outer skylight. • Molded of white ABS plastic material with a clear center insert. • Fits all standard 21" x 13 1/4" skylight openings. • Opening Dimension: 21" x 13 1/4" • Outer Dimension: 23 1/8" x 15 1/2" x 3 1/4" Finish off your replacement with an ICON skylight | Icon Technologies Skylight Inner Dome, for SL1321 in White | ABS Plastic