Rocket mass heater foundation

Another question about RMH. I have a concrete slab that I will be building on for my RMH. The concrete has not been sealed. Do I need to seal the floor before I install the RMH? As a cob builder I avoid concrete because concrete wicks moisture but won’t be able to seal my floor until later this month and I would like to install the heater before it gets too cold. Thanks in advance!

Hi Diane;  The important question) is  the floor insulated from the ground ? Under your rmh there will be no extra moisture for the concrete to steal.

So, in answer to your question. No, the floor does not need sealing.

On to my question , is this concrete insulated ? If your answer is no ... then you will want / need  4" of insulated cob under your core and mass.The concrete will soak up your heat and transfer it directly into the ground.  You can't run your stove long enough to heat the earth ....
 
If the answer is yes, it is insulated well... then your golden ! Build away!

Sorry, this reply is late, I've been hibernating all winter and now back at building my RMH. My foundation is 6 or 8" thick. The floor is not insulated only sealed with a sealant for concrete.

Right now the concrete slab is uneven and I plan to level it out with 2" of clay stabilized perlite that will be under the heater core. I plan to use brick around the perimeter of the bench which will be cob but unsure what to put between the slab and concrete. Someone mentioned tar paper to lay down on top of the slab and then start layering the cob for the bench. But I've also seen just broken up pieces of clay bricks (the ones with two hole in them) and I've also seen just some regular gravel as the bottom layer.

When you say insulated cob do you mean straw heavy?

The square you see--the rocket mass heater will go in there but those bricks will be laying down instead of on their side. I will level the inside with clay stabilized perlite. The L portion will be the cob bench.

Hi Diane; Welcome back!     Yes, insulated cob is straw heavy. Be sure to cut your straw into 2-4" pieces.    Rather than 2" I recommend 4"on the floor and  you won't need any thing else.
After you have your insulated cob down . Set your pipes and carefully cover them with cob only (no straw) After that ,rock and cob in layers. The more rock the better.  No air spaces.

thomas rubino wrote:Hi Diane; Welcome back!     Yes, insulated cob is straw heavy. Be sure to cut your straw into 2-4" pieces.    Rather than 2" I recommend 4"on the floor and  you won't need any thing else.
After you have your insulated cob down . Set your pipes and carefully cover them with cob only (no straw) After that ,rock and cob in layers. The more rock the better.  No air spaces.

Do mean insulated cob just under the bench where the pipes will go? Or do you suggest putting insulated cob under the heater core instead of the clay stablized perlite?

Just to clarify I don't need to put down broken up rock as the first layer for the bench? I can put down insulated cob? I just found out that my concrete sweats so I'm a little worried that mold will grow under the bench if there is too much moisture from the concrete. I don't need to lift the cob off the concrete?  

Thanks in advance

Hi Diane;
Definitely use the perlite fire clay, under the core.  
Under the mass. If your seeing moisture, and we know its not insulated underneath. Then you might consider putting 2" of rigid foam  (eps) down first.
Then just an inch of straw cob to bed your pipes on.
No need for rock under your pipes. That heat is going to rise.
After your pipes are  protected by cob , then use as much rock as you can for the mass. It holds heat much better than cob and every one means that much less cob you have to make.

EDIT) I should add you can use concrete board and bricks to raise your mass off the floor. This is more commonly done with a wooden floor but no reason it couldn't be done over a slab.