I wondered about that, Burra, but the Finnish contraflow heaters have an initial rise of the heat and then a drop back to the level of the fire (at least that far) and they are all masonry and seem to work fine. I suppose someone needs to try it with a rocket stove and see how it works. But then, I thought I'd seen pictures of more than one rocket stove that was completely covered with cob (except the top), and as far as I know they worked fine.
Burra Maluca wrote: The problem with cobbing round the entire barrel is that the 'siphon' effect depends on the hot air leaving the riser losing heat and sinking down the barrel.
Cob all around the barrel might stop the siphoning effect, so the warm air won't flow through the rest of the tubing in the 'mass'. It might even stop the whole rocket effect and you'd end up with flames shooting up the feeder tube!
Except... if you can build a barrel from brick (4.5inches thick) and it still works.... how is cob any different? I have seen pictures of barrels covered with cob till only the flat top of the barrel shows (as a cooktop), but I have never seen anyone going "oh no, I covered my barrel and now it doesn't work!"The only reason I can see not to cover even the top flat part, is cracking. This is always a problem when mixing clay and other materials. People who do good masonry work use a mortar that is as close to the brick they are using for this reason. Steel expands much faster than clay.
Aside from that, All of the RMH I have seen pictures/videos of, have the bottom half of the barrel covered in cob.... often 8 inches think or more, at least on the bench side. This is the part of the barrel that needs to be cooler for things to work. Cob conducts heat better than air. Maybe too good for the top part?
I think the key work is 'might' - it's just something to be aware of, and a reason to build a mock-up outside to play with and test to make sure it all works the way you expect it to before everything gets 'set in stone'.
If your design siphons well you might get away with it, but if it only just works putting cob around the barrel might just stop it working. But then, if it's right that cob conducts better than air, I'd be happy to be wrong on this one - cob does look prettier than a plain barrel!