UK 42" Low vault oven build. - Perlcrete or insulating fire bricks

[Thenightbreed]

Hi, firstly I would just like to say thank you in advance for anyone who can answer my question and to all those questions/answers by others before, as this forum has helped me change and evolve my build several times already and I'm still basically just getting started.
I'm hopefully going to construct a 42" low vault oven. I have already built the base and poured a 3 1/2" concrete slab. I have the perlite to construct the second insulating layer, but also have 350 insulating fire bricks as I was able to get these for next to nothing.
I have way more than enough insulating bricks to complete the insulation layer of the outer oven (they are the 9"x 4" full size bricks), but was wondering if they would be better than perlcrete to lay underneath the medium density fire bricks on the oven floor? Or could I do a mixture of the two and lay them on, beneath or encased in a layer of perlcrete???
Your help would be greatly appreciated.
Thanks you

[david s]

Free is the best price so definitely use them for under the floor laid unmortared. They have the advantage of being dry so unlike vermicrete don't require prolonged drying. They are also really easy to cut. They are not so suitable for over the dome insulation though, you'd be better off using blanket covered by a layer of vermicrete. Both of these materials will conform to the curve way better than insulating bricks.

[Thenightbreed]

Brilliant, thank you for your advice.