The new ibc international building code that most places have adopted says that combustible framing must be 2 clear of the firebox or chimney.
Chimney framing clearance.
Combustible sheathing can touch the masonry to close the gap in an exterior wall for example if it is at least 12 from the nearest point in the firebox or flue.
It appears the original installation allows about 1 5 if clearance in a framed pass through the ceiling in other words the current fiberglass insulation is about 1 5 away leaving the lower chimney chase able to breath or release heat.
Framing the chimney chase.
Typically uses 2x4 structural members sheathed with osb or plywood exterior building sheathing.
Stovepipe and chimney connectors must also be a certain distance from both combustible walls and ceilings.
The nfpa calls for an 18 clearance to combustible walls for single wall stove pipe.
Double wall stovepipe is used when you don t have 18 or more clearance from combustibles.
Properly framed and constructed the portion of the chimney chase that extends above the roof surface at a height to meet the roof clearance distances will not require extra bracing.