Floor Construction questions Pompeii Oven Construction Discuss Floor Construction questions in the Pizza Oven Design and Installation forums; I want to build my floor using FB board insulation over my hearth pour. I would also like to build ...

[lhuhn]

I want to build my floor using FB board insulation over my hearth pour. I would also like to build the dome around the floor and not on it. Does the FB board need to extend beyond the floor perimeter by the thickness of the brick so the first course sits on the FB board, or can the FB board be under the floor only. If the first coarse sits on the FB board do you mortar to the FB board?

[GianniFocaccia]

Many builders construct their floor the way you have described, primarily to facilitate replacing a damaged floor brick if needed (rare). The insulation does need to extend past the floor by the thickness of the dome wall and you do not need to mortar the first course to the board. If you placed your dome onto the concrete support slab your dome bricks would bleed heat into the stand and never heat up to the desired temps.

[dmun]

The question is about insulation under the dome, instead of just the floor, if I'm correct. The answer is that, yes, absolutely, the bottom edge of the dome MUST be insulated. There's a huge thermal bridge that will leak heat into the support slab if you don't. In fact many builders cut the insulation board oversize to the edge of the insulation layer on top of the dome.

No, you don't mortar down the bottom of the first course of bricks. Some mortar will leak out from between the bricks, but this isn't a problem. The hot core of a brick oven is changing size all the time from thermal expansion, and you want to give it as much latitude as you can.

[lhuhn]

Thanks, I guess I will be ordering two FB boards.

[GianniFocaccia]

You might consider installing 3-4" of vermicrete under 2" of FB board. This is very effective and quite a bit more cost-effective than 4" of board.

[ThermoJax]

I thought that 2 inches of FB board on top of the 4 inch conrete top was enough.

[dmun]

I thought that 2 inches of FB board on top of the 4 inch conrete top was enough.

It is. More is not exactly overkill, but it's not really needed. Since heat rises, I think extra insulation on top is better than more under the floor. In any event the recommended amount of insulation will work fine once everything is dried out.