Y'all are AMAZING. I've been weeks reading everything I can find here on the subject, and I have an idea on how to proceed with my build on a beefed-up (plenty strong) suspended wood floor. I may tweak my dimensions, but let's assume an 8" system in a day bed, 3.5' x 10' with three runs of pipe buried low in the cob. I'll be building with red brick exclusively. It was free and I'm poor.

I'm planning to space rows of brick on the narrow edge about 6" apart to create 4" air channels under and behind the stove. I'm thinking to mortar these together on the edge to allow them to conduct heat between them. My logic is that that will help avoid really hot bricks and spread what heat they absorb to dissapate more quickly.

Above the bricks - foil, shiny side down, facing the airspace, then one layer of durock.

I'll mortar a solid form of 4" bricks along the edges of the durock to contain my indulation: a mix of perlite, clay slip and a bit of Portland cement for strength. This is where I'm less sure. I want to put another layer of durock over my insulation to build on, and I think I need to support that cement board with something much stronger than loose perlite or even clay/perlite. I'd thought to add cement to add bearing strength throughout without hugely reducing insulation value.

On top of the insulation - another layer of durock, making a contained insulation sandwich with a perfectly flat surface to build on. Beyond that, I'll follow the destructions in the book exactly. Where does this plan suck?

Thanks, y'all!

Oh. Not red brick exclusively. I lied. Just the burn chamber and heat riser. I'll use cob/urbanite for the mass.

Welcome!
To be honest I have not built a RMH yet. It's still under construction. I have however used various perlite based insulation mixes over the years to build a furnace for metal casting and a forge for blade smithing. I do not believe you need to add the Portland cement to your mix. If anything I believe it could cause problems in high heat areas do to its decomposition temperature.  In my furnace I used a mix of perlite and Rapid Set Mortar Mix It worked extremely well. The cement in Rapid Set is Calcium Sulfoaluminate with the remainder of the product being silica sand. Decomp temp is 2460 F. The last rebuild of my forge was a mix of fire clay slip and perlite and it seems to be holding up just as well as the furnace. For reference, I had to rebuild my forge because I used a sodium silicate and perlite mix on the original build and melted it. The forge got so hot it melted the perlite. The clay slip and perlite mix has held up much better.

I do not believe you need that second layer of durock. Although it seems like the perlite and clay slip mix isn't very strong, it holds up well under a distributed load. Bricks are not perfectly uniform and flat either, so i wouldn't worry about trying to have an utterly flat surface to build on. Level, yes. If you don't have it yet, ernie and erica's book on building RMH's is well worth the investment and I think will help you with some of your questions.

Once you get build be sure to post updates on how it goes and lot's of photos. We love photos!

Hi Rachel;  Welcome to Permies!
Building over a wood floor is easy.   Red clay bricks  laid flat , but not touching, you want air to be able to circulate and you want the brick to radiate any heat it has picked up. Use as many as you think you need for support.
Concrete board (durock) laid on top.  If you want to face the durock with foil that is fine. No need for your double up idea.
A perimeter of brick around the  edge at least 4" tall, 6" would be better , fill with fire clay / perlite to within 1" of the top of the perimeter wall. 
build your brick core base here, top up the last inch with fire clay / perlite mix to stabilize your base.  You can now continue your core build and mass as you go, you are safely isolated from your wood floor.

Well, remember that these RMH are heavy.

In any house, i think you can safely assume it will take 150KG per square Meter. 33 pound per square foot. That's the euro norm for a bedroom.  More than that, if on the bottom floor, in any house, in the lounge and kitchen, usually, over here they count 300kg  m² So if you go over those numbers. You better be careful, and support the joists from bellow.  For public access buildings, you double that again. And for heavy storage, a metric ton per square meter.

And it's is a good idea, to make a load spreader above too.

Small I beams aren't that expensive secondhand. 4 inch ones are sufficient to spread the load over several joists.

I don't know about US norms, but they might well be in the same range.

In any house, i think you can safely assume it will take 150KG per square Meter.

So there are people who shouldn't stand in upstairs bedrooms...or have I got that wrong?
This has blown my mind

natasha todd wrote:
In any house, i think you can safely assume it will take 150KG per square Meter.

So there are people who shouldn't stand in upstairs bedrooms...or have I got that wrong?
This has blown my mind

Well, it's a norm, for "uniformly distributed load"  So, in a 20 m² room, you can put 3 metric tons. Not too bad. And this not the breaking load, but safely accepted load. The breaking load might be X5 that figure. And that takes creep into account.

https://www.designingbuildings.co.uk/wiki/Uniformly_Distributed_Load

Tables 3.2 and 3.4 here

https://www.huduser.gov/Publications/pdf/res2000_2.pdf

It's even smaller than i thought in the US.


Then, you have the point load. Thankfully the floor spreads the load. Because some people would have hard time walking on it without cracking.


I'm not that heavy, but can assure you, that when i walk on the OSB temporary floor in my flat, it squeaks.