Clay flue pipe for the riser

Hi everyone,
I am assembling the materials to build a rmh.  All I need at his point is the pouter barrel and some perlite.  I've watched a lot of videos, and I have a question about the riser.  Is there any reason I can't use clay flue pipe?  Would it need any other insulation around it?

Hi Damon; Welcome to Permies ! Clay flue liners can not take the quick temperature changes and will crack .  You can cut your liner in two (length wise) use stainless steel bands to hold it together and then it must be wrapped with insulation (rock wool) . Consider building a perlite & fire clay riser instead.  Use a 16 gal grease barrel as an outer form and a round "sonotube" concrete form as a burnout inner form. Pack in between with a perlite heavy mixture . This makes a super insulated very durable riser.

I watched a video where either Ernie or Erica said that the metal barrel is important because it loses heat quickly, which helps the hot air to fall to the bottom of the barrel.  If you build the outer barrel from masonry, does that change the dynamics, somehow?  Like, will it radiate less heat from the barrel (hive? bell?)?  Would that, in turn, mean that hotter air would make it to the ducting and mass, and therefore your exhaust would be hotter, unless you have enough mass to soak up the heat?

If the temperature in the riser and the barrel match .... you will lose your draw and stall or worse start a reverse flow.   I highly recommend buying the Builders guide , it will help keep you building a proper rocket.

I've ordered the book.

Is there any reason you can't use cement in the bench mass?  I have a lot of broken up concrete from a previous project.  Once you prove your system, you can encase the bench ducting, permanently, right?  Is cement better or worse than cob for banking and radiating heat?

Congratulations on purchasing the builders guide, it will help clarify most of your questions.   Yes , after your transition area  you can have a mass made with concrete, its expensive , hard to place and very permanent. Most people use cobb for availability and cost reasons, also so that their mass  "could " be removed at some future date. (By a seriously uninformed person) I believe heat retention would be comparable between either.