A wall thimble is an important component for venting stoves and other appliances through combustible material. Wall thimbles act as insulation between the pipe and the wall. They usually incorporate a steel casing to protect drywall, wood frames and other materials from high temperatures and risk of fire. You should never place insulation into the area within the thimble's cylinder. But you should pack insulation snugly against the outside frame of the thimble between the thimble and the wall.
Installing a stove pipe safely through a wall requires a wall thimble and adequate insulation between the thimble and the wall, according to the Michigan State University Extension website. When using an Underwriter's Laboratory all-fuel thimble, make a hole 4 inches larger than the thimble and place fiberglass or rock wool between the thimble and wood framing. To avoid fire hazards, place 6 inches of fiberglass insulation directly against the outside of the thimble between the thimble and wall materials.
It may seem counterintuitive to place insulation against the wall thimble instead of placing noncombustible material there. The reason for this precaution is because the insulation prevents heat from radiating to conductive surfaces like brick and concrete. These heat conductors, though they cannot catch fire, could start fires by directing heat to adjacent combustible materials. You should never place insulation directly within the wall thimble pipe. The pipe is made of thick steel that protects outer surfaces from the heat of the stove pipe within it. Placing insulation within the thimble could easily lead to fire.
Wall thimbles are used whenever a stove pipe passes through a wall, ceiling or other combustible material. This occurs when you need to vent a wood- or gas-burning stove to an exterior wall for a chimney connection. The insulation and clearance requirements of a heating device can vary by model and manufacturer. Consult these guidelines and your local building codes before installing.
Other Wall Passages
When using a fire clay thimble to connect the stove pipe to a chimney flue through a wall, provide 8 inches of insulation around it. This can be brick masonry, rock wool or other noncombustible insulation. Do not install a clay thimble through a concrete or similar masonry wall as these materials could crack. Heat could rise through the cracks, causing a fire in building's wood frame. A through-the-wall vent system with no thimble requires the removal of combustibles within 18 inches on all sides of the pipe. Close the opening with insulating, noncombustible material.
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