posted 8 years ago
I believe you're referring to double wall metal insulated / Class-A chimney sections. Those will not hold up very long as a heat riser.
Probably the easiest heat riser to make yourself is one stacked up with IFB (insulated fire brick) on edge, for a 2.5" riser wall thickness. Some folks stick the IFB together with clay slip. Mine has the IFB mortared in place, mortar made of a 1:1 mix of fire-clay and sand.