Rocket Mass Heater New Construction Question

My wife and I are building a 40x40 Earth-sheltered house on our homestead. The centerpiece for the living area will be a rocket mass heater that we will be building ourselves.

My question is should I install a layer of XPS polystyrene between the concrete floor and the heater's mass so that the floor will not absorb all of the heat. While this might be good at first glance but the burn time would be greatly extended if I had to heat the entire floor along with the heater's mass too. There will be a layer of XPS between the concrete and the ground.

Thanks,

Brian
www.hopehomestead.com

Good idea, but maybe the wrong insulation?
A perlite/cement mixture is often recommended, as it is not easily compressed and it can with stand higher temperatures without degradation.

The XPS can withstand 30 psi, but I am not sure about how it would stand up to heat. I'm not sure how much heat would be at the bottom of the stove. Wouldn't the perlite and cement mix still conduct heat?

Hi Brian;
The mixture William was referring to is  perlite /clay with cut straw added as an insulator. No cement normally used near a RMH.

Your  slab is already insulated from the earth, that is good.

What style RMH were you thinking of building?  A standard J tube in 8" ?  Or are you thinking the new batchbox in 6"?
Were you wanting a vertical brick bell ? Or a piped mass with bench?

If you build a brick bell then no need for extra insulation on the floor (it doesn't get hot enough to matter)  
If you go with a piped mass / bench then you Do want to put down 4" min, straw / clay / perlite mixture before putting your pipes down.

EDIT)  It is possible the XPS could off gas when heated if not covered up.

I was planning on going with the older bench style. But I guess I need to fully research the other one too.

Hi Brian;    
Yes, the brick bell has some advantages over the piped mass.
1) Space,  bells go vertical and take up unused space.
2) Weight, bells are much lighter than a piped mass. Less extra floor support.
3) If built as a batch box ,much less time required feeding fire.  Most insulated homes only burn in the morning and evening.

Piped mass has its advantages as well.
1) They hold heat longer and you can sit on it.
2) A batch box can be used with a piped mass. You can sit on it.
3) Heat released is at knee level. You can sit on it.
4) If building as a J tube,  an inexperienced person can easily build one  (Note, a bell can be built as a J tube as well)

Batch box's have a few metal pieces that need fabricating.
Build information on J tubes is readily available.
Build information on Batch box's is also available but not yet formatted in a "builder guide"

We here at the RMH forum are attempting to collect that info and make it more user friendly.
We have plenty of helpful rocket scientists happy to help guide you thru any questions that you may have.