I'll chime in with a few points I've learned and a discovery last night of a major problem that was most definitely causing serious issues with the system performance over the last week. Hope it helps others
#1 - we used the clay slip method for making our
cob - it's been cold and our nearly pure clay was coming in as chunks of nearly pure ice for some of the build, so using some boiling
hot water really helped get things going. The problem here was that the water content in the cob is way higher than traditional cob and on top of this, due to frozen hand syndrome, we went even wetter than recommended. The wetter the cob is, the longer it takes to get things running optimally, so fire early, fire hot, and fire long. Fire for 24 hours at a time if you have enough
coffee on hand

And, for the love of God, build your
RMH during warmer, drier weather!
#2 - even quarter-inch thick saplings take quite some time to season in a wet climate like central Maine. 3 months wasn't enough and much of it is still green and springy once it thaws out, still full of moisture that you can hear hissing out as your
RMH chokes on the steam. Our dead standing
trees are even loaded with moisture in this climate. Dry wood is VERY important - more important than I ever thought when I ran a
wood stove.
#3 - in a wet climate, with wet soil, building your RMH directly on the ground inside a polytunnel is going to just add to your battles with water. If I rebuild next year, a bed of gravel is a VERY worthy investment and building before the plastic goes on the polytunnel is just plain smart.
And for the feature of this post, last night Jerry the RMH was not lighting up - no draft. I checked first for an
ash buildup in the back of the burn tunnel (had just cleaned it the day before so didn't expect much) and instead found a quickly growing pile of loose clay and perlite under the heat riser!
15 minutes later, I had a
bucket of cob chunks ready to reconstitute, the barrel off by the door, a mountain of dry crumbly perlite chunks and a seriously deformed 8" galvanized sheet metal that used to resemble ducting. Evidently, the ducting, from the last few weeks of extreme heating and
cooling, had deformed dramatically at the base of the heat riser, folding inward at the seam nearly 4" and completely choking off the heat riser!
This deformation also caused/allowed the perlite to fall out into the burn tunnel - serious hole in the bean bag had started happening. With the clay slip/perlite, I would have expected the perlite to have held its own structurally, but it was pretty much all loose and dirty looking. There were some chunks of still bound up perlite, but these were very fragile and crumbled with any amount of pressure (couldn't even pick them up), so I'd imagine the thermal expansion/contraction is what worked the majority of the perlite loose.
I rebuilt the heat riser by first inserting the round ducting into the square hole in the top of the burn tunnel, ensuring this deformation wont happen again. The perlite obviously didn't have nearly enough slip to give it structure, so I added more and repacked it into the 14" surround. I got the barrel back on and Jerry fired up. He ran like a champ for about 45 minutes...but then no draft! That wet perlite was allowing the stagnation Erica mentioned so every 15 minutes or so, draft was gone again and the fire was out.
So, #4 - don't choose galvanized ducting for the inside of your heat riser if you have a choice of more solid material, and make sure you use enough clay slip on your perlite if you're going that route. Use as much insulation as you can, but expect to have issues while it dries.