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You need insulation under your oven to minimise the escape of heat through your slab and over the top of the dome to maintain the heat within the thermal mass created by the material in your dome and hearth.
It also stops others being burnt by extreme heat on the outside of the dome bricks.
Think of it as trying to isolate the hearth and dome. Regardless of how hot it is outside it will be a lot hotter inside that oven, if you are using it.
Insulation is about keeping as much of the heat you generate from your fire wood as you can. Strictly speaking, you don't need it, in the sense that you can cook a pizza in an uninsulated oven.
That's the simple answer, here comes the complicated one.
If you want to bake bread in your oven tomorrow after a pizza party tonight, without lighting the fire again, you need insulation to help retain all that heat you pumped into the bricks.
If you want to make 90 second margheritas with crispy crusts, you need insulation to help you attain the very high temps involved.
The better the insulation, the quicker your oven will reach pizza temps, and the less wood you will need to do it.
I've built an oven with lesser quality insulation and a thin shell and it works well. I have a pizza party, wait about 3 to 4 hours with the fire removed and the door open, then bake a batch of bread.
I helped my mate build an oven with 6 inches of aerated concrete all round - under the floor and over the top, with any voids between his round dome and square enclosure filled with loose perlite.It's brilliant for having a pizza party tonight, bacon and eggs tomorrow morning, followed by a batch of bread.
My advice is, even if you doubt the need, put in insulation, especially under the floor.
If you build the oven then decide you want to insulate it, you can do the dome easy. If you haven't put it under the floor before you build, it's near enough to impossible to put in later.
I see you live in Hawaii.
Would scoria or pumice be easy to obtain? At the least, mix the concrete slab that goes under your floor bricks yourself, using scoria or pumice for the large aggregate (gravel) portion of your concrete mix. That will at least insulate the floor to some degree.
If you can also substitute fine scoria or pumice for the sand used in a normal concrete mix, that too will improve the insulation qualities.
I am in the tropical area of Hawaii. REMOTE. Why do I need insulation.?
We live in the tropics too and I suspect that the heat loss would be marginally less in high ambient temperatures. But your ambient temperature is nowhere near your internal 400 C and as heat wants to travel from hot to cold, then you need insulation.