Insulating Hearth Question

[Double Trouble]

I plan to start fabrication of my pompei oven this week. In the Forno Bravo plans, it seems to shows the insulating hearth (perlite and cement) extending to the sides of the concrete hearth. In the plans, they mention shortening the insulating hearth dimension near the front of the oven, but there is no mention of shortening it on the other sides.

I see on this forum that some folks make an insulating hearth which is smaller than the concrete hearth in all dimensions....while some build up the concrete hearth on the perimeter...etc.

Is there a preferred method? I was thinking that perhaps some folks used concrete on the perimeter for strength in building walls...or perhaps to reduce cost?

Thanks in advance for your help.

Dean

[JAG]

Re: Insulating Hearth Question

Dean,

Either way will work for insulating.

There may be a few very minor thermal differences, but nothing that should effect the thermal profile of your oven. I think it comes down to the question of, do you want to build another form on top of your concrete hearth for the perlcrete after it is cured and use less percrete, or do you want to use the form you have built for your concrete hearth and save some construction, but use more perlcrete. Of course if you use the same frame make sure it is deep enough to have 4" of insulation plus the thickness of the concrete hearth.

I did an 8" deep frame for my hearth. I did 4" of concrete and then after curing I did 4" of perlcrete the same size as my concrete hearth. A big reason I did it this way was because of the vision I had for the finished oven. Having the floor of the oven sit 6.5" (4" ins.+2.5" brick) up was to high for my plan.

Say, where are you in Ohio, I'm Columbus area, Pickerington.

John

[dmun]

Re: Insulating Hearth Question

The advantage of a smaller area of perlcrete is that the stuff is crumbly, and doesn't hold wall anchors if you are doing an enclosure, and if you aren't, you have the problem of how to finish it. You can't leave it exposed, because it will get wet.

On the whole, I don't see any reason not to limit the under-oven insulation to the size of the dome insulation.

[DrakeRemoray]

Re: Insulating Hearth Question

What dmun says is correct as usual. To work around the problem, I sunk j bolts down into the hearth slab when I poured it and they stuck up through the perlite insulating layer. Then I attached a steel stud footer to those for my house type enclosure. If you are doing an igloo, I think you would have additional incentive to limit the insulating layer to under the oven...

[Double Trouble]

Re: Insulating Hearth Question

Thanks to both of you for your amswers/help. Having never made or worked with perlite, I didn't know if it would support anchors and the elements. Thanks again !
Dean