The UL Listed Saf-T Thimble allows single-wall stovepipe to pass through a combustible wall. It attaches to the chimney therefore avoiding a dangerous breach should the chimney settle and start to pull away from the house. The Saf-T thimble features a solid-pack insulation that telescopes inward toward the wall, allowing for easy adjustment for wall thicknesses of 4" - 8 1/2". Choose from 6" or 8" insulated wall thimble sizes. Both need a 16" wide by 17" high opening for installation. Instructions are included.