Which perlite grain size to choose

[K3vlar]

Hello, im building my first oven. Ive got everything planned out, base already built.

I plan on doing 12 cm (4,5 inches) of perlite-concrete bottom insulation. I cant find any vermiculite for sale, only perlite, that comes in either 2mm or 1-4mm grain size (125L bags for around 20€).

I plan to use it for bottom and outer insulation aswell. Which one should i go with ?

[david s]

Just as a mix of grain sizes (sand and heavy aggregate) in standard concrete produces a stronger result, so too a mix of perlite aggregate size will do the same. I use a 50/50 mix of fine vermiculite and medium perlite and find this works well, but should think a mix of the two perlite sizes should do the same. The finer grade requires more water in the mix which means more to remove, but it helps make the mix more workable.

[UtahBeehiver]

Also, floor insulation ratio should be 5 to 1. See attached