DuraPlus 8″ Firestop Radiation Shield The DuraPlus 8″ Class A Chimney Pipe Firestop Radiation Shield is used in ceiling/floor penetrations when a ceiling support is not used. Not required when passing through the roof. Fits on underside of ceiling and adjusts to cover framing. Features: Type: Radiation Shield. Class A Chimney Pipe Component. Use in Ceiling/Floor Penetrations when Ceiling Support is not Used. UL Listed. Specifications: Overall Dimensions: 16.5″ (L) x 16.5″ (W) x 16″ (H). Inner Diameter: 12 3/16″. Outer Diameter: 12-7/8″.