Question: Can I mix insulated flue and single wall.Ê If I use insulated pipe until I get out and away from combustibles, can I then use single wall to the cap. Answer: No, once you go to class A you must stay there until the termination. A few reasons for this: 1. The pipes are not designed to mate correctly in this way. 2. The single wall will get too cold, ruining the draft and causing creosote. 3. The single wall could get too hot and ignite birds nests, etc that might be near or built on it.