A woodstove thimble is the part of the woodstove pipe assembly that connects the internal and external parts of the chimney. They can be also called insulated wall thimbles, as they usually pass through a combustible wall and so must be insulated. Not all thimbles are insulated however, as in the case of passing through a masonary or brick wall, the chimney thimble need not be insulated in this case.

Here are the code requirements by the NFPA (National Fire Protection Association):

    A ventilated type 24-gauge metal thimble must be at least 12 inches larger in diameter than the stove pipe. (It can be made by a local sheet metal company or tinsmith.)

    A metal or burned fire-clay thimble must be surrounded by no less than eight inches of brickwork or equivalent fire-resistant material.

    Otherwise, all combustible material must be cut out of the wall to provide at least 18 inches of clearance on all sides of the pipe. Material for closing this opening must be noncombustible and insulating.

Of course, different installations may require different requirements depending on your location so always verify with your local ordinances.

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