rocket mass heater riser - ceramic fiber board

Mud and I are working on a book.  This is the part about ceramic fiber board risers.  We strong encourage everybody to share their experiences.  We hope that this thread will grow to ten pages of information about ceramic fiber board risers.


This is typically four pieces of ceramic fiber board that are wrapped with a metal case or a metal frame.  Note that ceramic fiber board is rather delicate when you first work with it, and it becomes hard and brittle once it has been heated.  

It might be wise to wrap the riser with a layer (or two) of high temp wool.  

The cost of materials to make one riser is about $80 to $120.  Add $30 for a wrap.

This material certainly tolerates high temps well, but I have to say that I have my suspicions about its ability to insulate.  

Mud says

Mitering the corners of the ceramic boards where they come together lets them hold each other in place. An inch of ceramic board wrapped in cheap sheet metal makes a decent riser, but keep in mind that ceramic fiber board has an R value of about 1 per inch (about half the R value of ceramic wool) and costs about 5 times as much as the ceramic wool.

Here you can see an animation from davin hoyt who made the plans for the rocket oven that is in our rocket oven movie.  This has a ceramic fiber board riser.

Another method for the corner joints is a rabbet joint. 1/2 the board thickness wide, and 1/2 the board thickness deep, in the mating edges of both boards.
This "step" in the edge of the boards keeps them located on each other, and also eliminates a straight path out through the joint between the boards, if the cuts/fit isn't perfect.
This way, your metal case won't see the combustion through any cracks (which is the beauty of CFB, to have fewer joints than bricks).

Kenneth Elwell wrote:Another method for the corner joints is a rabbet joint. 1/2 the board thickness wide, and 1/2 the board thickness deep, in the mating edges of both boards.
This "step" in the edge of the boards keeps them located on each other, and also eliminates a straight path out through the joint between the boards, if the cuts/fit isn't perfect.
This way, your metal case won't see the combustion through any cracks (which is the beauty of CFB, to have fewer joints than bricks).

maybe a simple drawing?

Here's a sketch of rabbet joints.
If the panel is to be a lid/roof of a construction, I would make it so that the full thickness of the roof sits on top of the walls. (another way to think of it, is the roof completely covers the tops of the walls)
In the lower right-hand view, what appears to be a floor, would also be how a roof would work.

To reduce exposure to ceramic dust from cutting, a rabbit joint could be fabricated by building a riser box inside another riser (two layers) and alternating/off-setting the joints. The butt end connection (corners) could be pinned along the joint seam with small metal rods. The assemble can be held together with an angle iron frame.

This joint would assure tight fitting (square) surfaces at the corners and would not compromise the board strength by cutting a rabbit reducing wall thickness.