How much perlite below RMH cob bench?

About to build a 19cm diameter Peterberg batchbox rocket mass heater with a large U-shaped cob bench in a tipi. Since a tipi has no insulation, I want to store the heat as efficiently as possible in the bench by insulating the bottom and back with a perlite-clay slip mix. How thick should I make this layer?

Hi Philippe; Welcome to Permies!
To answer your question,  4" is the average layer of insulating cob under a mass.
Normally, to mix cob to be insulated, you would want to add cut pieces of straw / hay / horse poop to mix with your cob.  
Generally perlite is not used under the mass.   Straw is much cheaper than perlite.  No reason you can't though.

Using bricks and cement board to elevate your mass off of the floor is also a common practice.

thomas rubino wrote:Hi Philippe; Welcome to Permies!
To answer your question,  4" is the average layer of insulating cob under a mass.
Normally, to mix cob to be insulated, you would want to add cut pieces of straw / hay / horse poop to mix with your cob.  
Generally perlite is not used under the mass.   Straw is much cheaper than perlite.  No reason you can't though.

Using bricks and cement board to elevate your mass off of the floor is also a common practice.

Thanks for your reply, Thomas!
Hmm, so basically just cob, but with more straw? How much straw? I would assume that this doesn't insulate that well... am I wrong?

A good amount, cut to 3-4" or so.    It creates air pockets.
Is perlite better ?   Maybe so, if it is easily/cheaply  available.
A fast option could be a piece of rigid foam insulation slightly buried . Then build on top with standard cob.

Vermiculite mixed 6- 1 with Portland cement is a common semi structural mix.
You can mix perlite with cement but from my own experience it does not seem to be as strong.