/ Field knowledge · IMC 805.8
Insulation day can bury a clearance that already passed inspection.
A factory-built chimney can have an open clearance zone at mechanical inspection and lose it when blown insulation is added. IMC 805.8 is the handoff between the chimney trade and the insulation trade. Distinct from the Type L / listed-vent shield (802.8).
Educational 2024 International Mechanical Code (IMC) 805.8 from Daily Code Talk. Confirm the adopted edition. Distinct from factory-built listing (805). This is not a shield design.
- Two paths: a code-built steel shield at least 0.0187 inch thick (No. 26 gage), keeping insulation at least the manufacturer’s required clearance to combustibles; or a shield that is part of the listed chimney system, installed per the manufacturer. Do not swap a required listed component for a generic detail.
- In an attic, the shield extends at least 2 inches above the insulation and is secured against displacement. A later insulation-depth increase requires the 2-inch relationship to be reviewed again.
- The shield does not authorize reducing the chimney’s required clearance. Insulation shield, firestop, radiation shield, support, and weather flashing are different functions — one piece is not assumed to do all of them.
On the set: chimney outside dimension, required separation, shield configuration, final insulation depth, and top of shield in section. Coordinate with architectural assemblies and energy/insulation specifications.
Do not pack the air space
Do not pack loose insulation, foam, or other material into a required air space on the assumption that more insulation improves fire protection. Inspect before insulation (identity, clearance, support) and after (final surface, 2-inch extension, nothing in the clearance zone). An attic insulation upgrade is a trigger for another review even when the chimney itself is untouched.
Source: Daily Code Talk — IMC 805.8 · vent shields: 802.8 · listing: 805 · offsets: 805.4–805.5.
What to send
The chimney listing, attic / insulated-assembly section, planned insulation depth, and whether a listed shield or a code-built 26-gage shield is assumed.
If the chimney goes through the attic
Send the section. Masterbuild will say whether 805.8 is a missing shield — or the insulation trade is about to bury a clearance that already passed mechanical inspection.
Send Project Background+1 (786) 398-6940 · osmany.portal@masterbuildconsulting.com
Masterbuild provides engineering services under a written scope. Nothing on this page is project-specific engineering advice, a code interpretation for your project, or a representation about permit approval. Confirm the adopted edition. No. 26 gage (0.0187 in) and the 2-inch attic extension are 2024 IMC 805.8 education from Daily Code Talk — not a shield selection. This page is not legal advice.