I have just joined this very informative forum and I am ready for my new build. Having researched different threads, I have put together what I think might work. I am going for a 850mm dome over a fitness ball with a 5;1 Perlite/Ciment Fondu mix. I was going to make this about 80mm thick, is this enough? I will then cover this with thermal insulating fibre and a finishing coat of heatproof render. I amu sing a 125mm chimney with an opening that is 280mm high at the top. I am going to seat this onto a base made from the 5:1 mix (100mm thick) seated on paving slabs. Will I need a further insulating barrier (sand?) between the two to store the heat in the bottom better? I am going to finish the floor off with heatproof render for a smooth finish. Does this all sound okay? Thanks in anticipation.
The floor of your wfo will be subject to both impact and abrasion so I don’t think a rendered floor would be much use. Try and locate some storage heater blocks because they are often cheap, or free and make an excellent base when laid in loose sand. For your underfloor insulation you could try and find thermolite building blocks blocks (aka breeze blocks) to use as a base. Instead of your 5:1 mix; these will give you a better surface to work from.
Welcome to the forum and good luck. I have used thermolite blocks cut in half lengthways to give 4.5" of insulation on the base. I'm trying to build as cheap as possible and they were free. I used a 50/50 mix of fireclay and kiln dried sand to lay the firebrick on top of the thermolite blocks, as my firebricks are chamfered at about 4mm difference.