J tube

We are building the rocket mass heater core with fire brick. The bricks will be mudded with clay or furnace cement. What is the best way to insulate the J tube? Should we allow for heat expansion between the J tube and the cob? If so would ceramic fiber blanket suffice for both?
Thanks to all for their guidance
Bob

Ceramic fiber blanket is ideal for insulating the J-tube and heat riser. That would as you mention avoid any expansion issues as well. You would want some sort of insulating material in any case, and they tend to be softer or at least weaker and more forgiving.
I'm not sure space between the firebricks and a cob layer would be necessary, though if the cob were thinnish and served as the finished surface a tiny gap would reduce shrinkage and heat expansion cracks.