Thoughts on perlite as insulation

[P3 Stoaker]

G'day All,
If anyone has thoughts/recommendations/experience good/bad in relation to the following please let me know.
Building the base now and designing the structural/insulation layers. I'm somewhat limited as I have about 6 and 3/4 inch thickness to work with for structural and insulation layers. It's been difficult to determine if the fibre or silicate boards are suitable to have the dome and floor bricks laid directly on them. I cant really sink the board into the structural layer and cover with high temp mortar as that would reduce the structural layer thickness too much. Bottom line is I intend to use a little under 4 inch of structural concrete and a little over 3 inch of perlite on top of that and lay the oven dome and floor directly on the perlite. I have 2 relatives that very very successfully cook loads of pizza's in their ovens and they don't even have an insulation layer.
Any advice welcomed, have you used perlite?
Kind Regards

[david s]

For adequate strength for under floor insulation you need to make the mix 5:1. It is essential to add the correct amount of water as too much results in the water washing the cement off the grains of perlite leaving an inconsistent mix.
Mix 5 parts perlite with 1 part cement, by volume, in a barrow so you can inspect it as you mix. Then add 3 parts water slowly, mixing as you add. Break up any lumps with the back of your spade.
Place the mix, trowel the surface nice and flat and allow to dry for at least 2 weeks if possible. The underfloor insulation is the hardest to get dry so resist building straight over it. Also a few holes through the supporting slab helps eliminate the water once you start using fire to drive it out.

[P3 Stoaker]

OK thanks for the great advice David, appreciate it.