A chimney can have an open clearance zone at mechanical inspection and lose it when blown insulation is added. The important handoff is between the chimney installation and the insulation trade. Show a physical shield and its relationship to final insulation depth so the required separation survives the next phase of work.
Two Installation Paths
IMC 805.8 addresses factory-built chimneys passing through insulated assemblies. For the code-built shield path, it specifies steel at least 0.0187 inch thick, identified as No. 26 gage, maintaining separation from insulation at least equal to the manufacturer's required clearance to combustibles. In an attic, the shield extends at least 2 inches above the insulation and is secured against displacement. Shields provided as part of a listed chimney system follow the manufacturer's installation instructions.
Identify which path applies; do not replace a required listed component with a generic detail without verifying the permitted installation.
Dimension the Space That Must Remain Open
The shield does not authorize reducing the chimney's required clearance. Show the actual chimney outside dimension, required separation, shield configuration, final insulation depth and top of shield in section. For a concentric round shield, the preliminary inside diameter must accommodate the chimney outside diameter plus the required clearance on both sides. The listed component geometry and instructions still control the actual assembly.
A Simple Attic Coordination Example
Suppose the selected code-built detail uses a common reference plane and the planned insulation surface is 14 inches above that plane. Its shield top must be at least 16 inches above the same plane to provide the 2-inch extension. This illustrates the vertical relationship only: it does not select the chimney clearance, shield diameter or listed accessory. A later insulation-depth increase requires this relationship to be reviewed again.
Drawing and Trade Coordination
Coordinate mechanical penetration details with architectural assembly sections and energy/insulation specifications. Identify material thickness for a code-built shield or the exact listed shield for a manufacturer-system installation. Show attachment and any manufacturer-required collar or related component. Distinguish insulation shield, firestop, radiation shield, support and weather flashing functions; one component should not be assumed to provide all of them.
Inspect at the Right Stage
Before insulation, verify component identity, clearance geometry, support and attachment while visible. After insulation, verify the final insulation surface, shield extension, displacement resistance and absence of material inside the required clearance zone. Record accessible conditions and photographs. Do not pack loose insulation, foam or other material into a required air space on the assumption that more insulation improves fire protection. If an existing assembly is concealed or its system cannot be identified, record that limitation and obtain a qualified evaluation.
Owner and Maintenance Value
Add the installed shield and chimney identification to the closeout information. Tell the owner and future insulation or roofing contractors that changes around the chimney require the selected-system instructions to be checked. An attic insulation upgrade is a relevant trigger for another clearance review, even when the chimney itself is untouched.
Project Boundary and Reference
This is 2024 IMC model-code education with a labeled coordination example. Confirm adopted edition, amendments, listed-system instructions and AHJ requirements. 2024 IMC Chapter 8, International Code Council.
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