insulated brick + insulation = bad?

Thought I read somewhere that if you use insulated brick that extra insulation is not good. True? If so why?

I'm building my first system and deciding between insulated brick and 100 year old clay brick.
Planning a 6in system with a 55gal drum. I can't find metal drums smaller. I would need the heat riser to have extra insulation just to fill up space inside the drum to give the proper gas exit side space.

I am basing my design off "the book" but the build description there assumes you're building with old clay brick

Don't worry about that. I think it's a joke.


Tho, i you have larger top gap and side gap than in the book, it's works better, i preffer 3 inches all over. So don't put too much insulation. And as usual i'll say it, make your transition from barrel to flue a good two to three times the CSA.

Dan Woloz : There is an entire book that can be written to cover your 7 short simple sentences. The Rocket Mass Heater that Ianto Evans evolved
out of the earlier rocket stove has little changed from its original models, (its future is wide open and exciting ) It was, and is in its creators hands
a highly crafted space heater constructed out of dirt (sand and clay, COB) Dirt will not burn, it will not rot, insects wont eat it, and yes, it is as

''Cheap as dirt'', most of the rest of the materials can be found in the waste stream. This series of lucky co-incidences is something everyone
embraced, its creator most of all. 'The Book' reflects this philosophy,which is after all inline with the ideals of permaculture and good for us and
the Planet ! Re-cycling is a Great thing, there are many who have crafted their own RMH spending well under $100.oo. Truly, Bragging Rights !

This has had an unfortunate effect of nearly freezing the development of the RMH, and as it is a primary goal of fellow members like myself
to help New members build Successful RMHs we add some confusion by sending our newbies back to 'The book', this is still the very best advice
we can give !

A source of possible error has crept into the plans as some guidelines meant to be minimum or maximum dimensions were interpreted to be rigid
laws of physics, mostly those later errors have been caught and exposed as such !

So, insulation is good, be very generous in the Feed Tube and Burn Tunnel, here a major goal is freaky high temperatures quickly to promote the
highly efficient, clean burn that creates the heat energy that does its 'work' at the Heat Riser/Barrel interface to pump its hot exhaust gases 30'
horizontally!

At the Heat Riser the insulation performs a special function helping to separate and maintain the marked temperature differences between the hot
rapidly rising gas stream within the Heat Riser and the gases in contact with the inside surface of the barrel, which radiates off much of its heat,
cooling and becoming denser and falling down through the transitional area to turn and flow out through the horizontal pipes of the Thermal Mass

Without the insulation to promote the marked temperature difference in the two gas streams, the temps would equalize and the heat pump effect
would stall out! The Two inch gap at the top of the Heat Riser,and along sides of the barrel are minimum distances, that can be exceeded, indeed
the smooth unimpeded flow of the hot exhaust gasses to, down through, and out of the Transitional Area make the 2'' gap absolute minimums.

We ARE NOTtrying to fill up the space in the drum with insulation !

Many if not most of the earliest RMH Builds were made with the dead soft red/orange red 100+ yr old building bricks, and the world wide adoption
of the RMH has seen this trend continued overseas with a generous sprinkling of homemade 'Firebrick' I personally would use the old bricks and
save my money for the final vertical Chimney !

The last two years especially, has seen a lot of improvements in RMH design, My best and strongest suggestion would be to create your 1st rocket
out of doors following the more conventional pattern, and expect that this will not be the last RMH you ever Build ! For Good of the Craft. BIG AL

Thanks for the clarification ya'll. I did read 2-3in space for the sides in the book and think (mistakenly) that was rigid requirement

So, extra insulation on the back of lightweight insulated bricks, fine, but advisable? Worthwhile to add extra insulation to ins.brick?

I do plan to do my build outside. No doubt I will screw it up somehow and I'd prefer to do that out of doors

Thanks again

Dan W. : I think 'The Book' does a good job of showing you the shape of the base layer for the Combustion core, the problems begin when you
build over a Wooden floor or over concrete , some of the best insulation out there is vermiculite or perlite, 2''-3'' can make a very good heat shield but
needs to be re-enforced with a clay slip to give it adequate strength to carry much weight, where weight-bearing is important we add enough Clay slip
into the mix to make a base ball sized/shaped ball that holds together but will 'pop' apart when squeeze between finger and thumb !

This material is less insulate and needs 2Xs the thickness of loose fill Perlite, but can be packed into a frame of bricks that will support the weight of the
other layers ! Failure to use insulation will make it both harder for the 'Combustion core' to come up to a clean burning temperature and take much
longer to reach that steady-state! Even more so when your whole Rocket is incased in tons of wet Cob !

Hope this helps and is timely ! For the Good of the Craft ! BIG AL

I realize the importance of insulation but wondering specifically about lightweight kiln/fire insulative/refractory bricks. Do they need extra insulation on the outside of them in the burn tunnel and heat riser? Would it hurt to do so? Would it be a waste?

Dan : Nothing like direct questions to keep a man honest ! Yes additional insulation is much better than no insulation which irreguardless of brick type will depress the
burn temps initially and for sometime after you start your fire!

About the heat riser / barrel gap, as most of the heat energy radiated off of the 55 gal drum happens in the upper 1/3rd any way, some people do pinch this gap to
raise the working temperature on the top of the drum for cooking! If The barrel has had it's original paint carefully removed, the entire barrel can be oiled just like
'seasoning' a cast iron pan and the entire top used as a cooking surface.

Tall people rarely have problems with the height of their barrel top unless they are building over a wood floor, short people can slightly reduce the height of the heat
riser* to get a lower Barrel top !

Hope this is useful and timely, for the Crafts! BIG AL

* there is a 1 : 2 : 4 Ratio to the dimensions of the feed tube, burn tunnel and heat riser, again some measurements are minimums, some are suggested Maximums!
A.L.