Hi All,
I was stacking whole dense firebricks the other day and put together a core and riser entirely of brick.
Now I’ve decided I don’t want to deal with that heavy old mass in my combustion unit so I’m setting out to purchase fiber board and blanket to make my rmh more efficient and easier to use.

( 1. ) I gathered that at the minimum the 8 lb blanket was necessary in order to be in direct flame contact. Should I get the half inch and wrap the inside of my pipe twice or will the inch stuff joined together flush at the seam be better?

( 2. ) Do I need the 2600 degree stuff or is 2300 sufficient? Wondering on both the board and blanket.. I feel like I should know this and I was planning on 26 for good measure but.. maybe I could save a few dollars. (Edit: Thomas, I just saw your response from a week ago in a related thread outlining the products you had purchased and planned to use so I guess 2300 works..)

( 3. ) Regarding the board I think I can make a tunnel to spec. and put some split bricks for the feed so I don’t know what to ask but any advise would be appreciated. Is there a preferred method of fastening the board together at right angles or should I be thinking of my brick / fiber board core in a different configuration. Maybe board for just the ceiling and rear of tunnel/base of riser area. I know someone has done this before me. Which part of the Feed/tunnel can be fiber board?

thanks I’ve been dreaming this into existence for five years and now I think it’s going to finally come together.
I live at 5040ft east of Ashland Oregon. The winters are long. My cabin is up on pier and post so I can’t build mass inside at least not yet. I’ll be working on my foundation soon hopefully but for other reasons. It’s shifted over the last 30 years. I’m putting my first Rmh in a greenhouse. It’s a 12ft diameter dome. I’d like to heat the whole floor. Any ideas for a short wide circular bell that can be walked on?

You guys rock thanks for paving the way

Hi Jesse;  Welcome to permies!
Yes, 2300 F is fine.  You want at least 1" boards. It was suggested that 2" for the roof would be extra nice but not required.   For the blanket go with the 1", I,m planning on the #8 but was told by pinhead that he used #6 on his original 5 minute riser and it is still working today.  Peter van den Berg also thought that #6 would be fine.
I asked Matt Walker how to join ceramic boards . He uses aluminum foil around them and uses red clay brick as a backer.  This is the item # at ebay for the board i have( 222715872389)  it is 39.4" x 19.7" x 1" thick $95.60 delivered.  This along with split brick for the feed tube is enough for an 8" J tube core.  Sadly the blanket comes in 25'-50' rolls for about $95 delivered
About your greenhouse.  If you want to heat the floor you will have to insulate from the ground . Most people use a raised bed .  You still need to insulate from the ground but your not burying insulation underground.

Ok so I’m about to order the 2” board in the nearly 20x40“ dimension you described getting in the 1” thickness.
In regard to the blanket, I can get the 2300* in 8# or 6# the denser is 5$ more but I don’t want it to be dense I was thinking. And since the lighter one should hold up as it has from others experience there’s no need for me to think that the 8# one would be better. My conclusion is that the 6# will actually perform better. Just wanted a second opinion before I pass up the 8# for five more dollars.
Also the blanket comes in a 48” width and was considering getting more and having the ability to make a 12” heat riser or a 4’ long one of any diameter. I’m guessing most people don’t end up needing to make it longer than two feet?

Guys, careful with two inches and heavy weight superwool.

Here's what Peter said recently.

http://donkey32.proboards.com/post/30096/thread

I bought the 2” x 19.7” x 39.4” ceramic fiber board at 2600* because it was only ten dollars more than the 2300. the two inch wasn’t a whole lot more than the one inch either, and I read it was preferable to have the double thickness at least in the tunnel ceiling so why not throughout?
I also bought the 1” x 24” x 25’ of 2300* 6# blanket since it will be less dense and apparently is easier to bend and still holds up. I hope my fires not hotter than 2300*.

Any reason not to do a ten inch system? I can see that 8” is going to be easiest to construct from the board that I got and I already have some 3’ lengths of 10” ss duct so maybe I’ll try bigger another time once I succeed with this.

Hi Jesse;   2" ceramic boards will be super and what the heck 2600 F will be good as well. #6 blanket is fine. Your riser temps will not reach 2300 F .
As far as riser height , hardly any would be 24" , mine is a total of 51" counting the 16" drop at the core.  
My new riser is planned on being 64" counting the core
Not to many 10" cores out there.  I almost designed my new one around a 10" but decided to sick with 8" for the availability of pieces.

I bought myself five gallons of sand for two dollars so I can make some test bricks. I’ve never made cob before not sure why. Probably since I didn’t have any sand. I’m going to do a soil settling jar test to see how much silt might be in my clay soil. It seems very clay rich. In a week I’ll be able to put my core together. I’m going to cut an 8” x ~20” for the base. 10” x 20” walls and 12” x 20” ceiling which I will cut the rear part off of for the back wall. I guess I will make the feed from split bricks and put some of the blanket around those. I know a source of 2” soapstone slabs if I can get them I’ll build my bell bench/raised bed from stacked soapstone so it won’t take long to dry.

Jesse, any construction supplier should be able to sell you a ton of sand for 20 bucks. Plus transport.

Jesse Baker wrote: I know a source of 2” soapstone slabs if I can get them I’ll build my bell bench/raised bed from stacked soapstone so it won’t take long to dry.

I'm very envious of you Jesse... Soapstone is a fabulous storer of heat. I obtained some chunky blocks of it several years ago and used to place them on the top of our box stoves to act as thermal storage. They worked very well, still being warm many hours after the fire in the stoves had gone out.

I just put together all 3 sizes of five minute risers I had pipe for. I made a 4” x 2’ riser that I tested on a little burn tunnel I stacked out of whole dense fire bricks. I know they need to be insulated in order to heat up properly. I’m going to build a 4” rocket stove to fire my gas water heater. I made the 6” and 8” x 3’ risers for some real RMHs that I will build the 8” very soon. My 2” fiber board should be in the mail soon but I discovered that I’ll likely have an easier time with my heater if I build the whole feed and tunnel from split dense brick other than the ceiling so that my draft will last through the end of the burn.

Hi Jesse;
 Looks like your gathering up all your materials.
I'm curious, what made you think your tunnel other than the roof, needs to be brick?
Your feed tube definitely should be built using split bricks,your tunnel... I believe you would be better off using ceramic boards throughout your tunnel.

Once your stove is up and running, the heat stored in your mass or bell will keep the draft going till your wood is all gone.
Using brick in your tunnel is stealing heat until it comes up to temp.  While it is coming up to temp you are not burning nearly as efficiently as it is when those bricks finally stop stealing heat and allow it to go to your mass.
Using ceramic fiber boards in your tunnel and blanket in your riser allows ALL the heat to instantly head to your mass /bell.  As soon as you have a fire going all the heat,  is not heating your tunnel up instead it is heating your mass...
Experimenting is what brought rocket mass heaters this far.  Working with clay and cob it easy to do a rebuild. Try several different configurations and see what YOU think is best. Your doing a great job.

Hey Thomas,
Yeah I read a post yesterday from Erica Wisner that I could not locate just now.
She had provided the opinion that with a fully insulative core/riser with just a brick feed tube could lead to a hot stack effect in the feed at the end of the burn and possibly smoke back. In the case of the split brick through the whole tunnel the heat is equally stored under the riser side. I felt like the full burn tunnel of split brick was really not much mass especially compared to a full core/riser from whole brick. We shall see. Luckily I already have split dense brick 8 7/8” x 4 3/4”. X 1 1/4”. Uncle Mud’s Cottage Rocket has a nice split brick feed and tunnel but I’ll have to work out the CSA.
Thanks for the encouragement and input!

I received my Ceramic Fibre Board and now I’m faced with the task of cutting it properly.
I drew out a plan to help me visualize and do it right the first time. It seems to be exactly enough.
I’m thinking of using a razor blade to cut the CFB. I think Matt Walker said he’s used a serrated knife. I’m not sure what to do.

Cool. I got my tunnel cut out. I may get some ss screw or just wire to pin it together. I need to cut the bricks for the feed a little and maybe cut some blanket to wrap around them. I want to encase the core in aluminum. I have a piece big enough for a tray at the bottom and some straps to go around the side and over the top. I wish I had a press brake or something to bend the edges of my tray. I’m sure I can rig something up.

Looking Good Jesse!
Keep the pictures coming!
What did you end up using to cut your cfb?

I cut with a combination of the box cutter, razor in vice grips and the serrated thin knife. When I had gotten most of the way through I turned it over and followed the places it had cut all the way. My riser is 41”. 35” in the pipe.

I also altered my cutting pattern from the drawing.

Another thing is that my riser with the one inch blanket inside the ten inch ss pipe measures about 7.5” ID. Should I try to compress it and widen the cross section..

Hey, that looks like a "snorkel" hot tub in the background!
Actually 7.5 x 7.5 is the suggested id on an 8" rmh. So I would not worry about it.
I like your cutting design, did you find it on another post or design it yourself.
Are you wrapping the core with foil ? Using stainless tie wire to hold it ?
A hammer , a straight edge table , couple of vice grips and you have a funky sheet metal brake.  

Hm I thought it would have been 7.5”x7.5” square for an 8” round and mine is inverse of that.

Yeah it is the snorkel tub. I picked it up second hand tub preassembled stove not installed. They never put the stove in because the tub never sealed and first time I heated it, it stopped leaking.

The pattern I just made up on that graph paper and then changed a little so the narrow edge on both sides of the riser would be more connected.
I might use ss screws or wire pins to hold it a bit. I’ll put some earth/sand mortar in the seams before I put it together and place it in its aluminum box.
Right that’s about what I was thinking I guess it doesn’t need to be pretty, just trying to make the core as relocatable as possible.