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Short answer: yes, you can use them under the floor, but not as a structural layer.
Insulating (isolation) firebricks are great for thermal isolation, but they are too soft to support the oven and cooking floor directly. They’ll compress over time and can lead to cracking or an uneven hearth.
The proven setup is:
Structural slab or block stand
Dense firebrick or concrete hearth slab
Insulating layer (your insulation bricks or calcium silicate)
Cooking floor (dense firebrick)
So your insulation bricks belong under the hearth floor, not as the base you build the oven on. Used that way, they’re excellent and will help the oven heat faster and hold temp longer — perfect for a Neapolitan oven.
If you already have 100 of them, that’s a big win. Just make sure the load is carried by something dense and strong above them.
Good luck with the new build — sounds like it’ll be a great one
Insulating (isolation) firebricks are great for thermal isolation, but they are too soft to support the oven and cooking floor directly. They’ll compress over time and can lead to cracking or an uneven hearth.
The proven setup is:
Structural slab or block stand
Dense firebrick or concrete hearth slab
Insulating layer (your insulation bricks or calcium silicate)
Cooking floor (dense firebrick)
So your insulation bricks belong under the hearth floor, not as the base you build the oven on. Used that way, they’re excellent and will help the oven heat faster and hold temp longer — perfect for a Neapolitan oven.
If you already have 100 of them, that’s a big win. Just make sure the load is carried by something dense and strong above them.
Good luck with the new build — sounds like it’ll be a great one
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