Single wall vent clearance from duct board

I think I’ve asked this before, but here goes again (as I couldn’t find it searching): Does the 5" or 6" (can’t remember which) clearance apply when the material is duct board?
Thx in advance.

Single wall vent pipe needs 6" to combustibles.

I know, I stated that ;-). Is duct board combustible and/or is this set up incorrect due to proper clearances not being in place?

It depends on the ASTM rating of the duct board.

Chances are, it isn’t rated as such.


Is that duct board supply or return? If it is supply duct it should have a fire rating.

Fire-Rated Assemblies
Mat-Faced Micro-Aire, SuperDuct RC or Micro-Aire Type LP duct board
1" (25 mm) thick is accepted in lieu of sheet metal duct in many fire-rated
assemblies. See UL Label for design numbers.