Insulation on a vent connector

Single wall furnace vent connector is insulated with foil faced fiberglass- no paper. Just the glass and foil (the foil is the strange foil / Mylar material we often see on dryer vents) Allowed?

Doesn’t sound right.

Picture?

Sorry- no photo. This is an out of state friend- not an inspection. I’ve seen commercial boilers with insulated vent connectors, but it is always rock wool or ceramic insulation with metal skrim backing.