Insulation below or above concrete base for Oven Hearth

[tdibratt]

Hi, I was away for a week on vacation and took with me the Pompei Oven plans and read through them. Something I never realized in searching the forums lately is that sometime between 2005 and now, the recommendation for the vermiculite insulation has switched from being poured below the conrete to above the concrete just below the oven hearth fire bricks.

As I recall the last oven I built the vermiculite mixture below the concrete is hard but appears more crumbly than regular concrete. Does this introduce any complications in setting fire brick level or in finishing the exterior facade? I imagine most people would want to completely cover the vermiculite even around the edges.

Thanks

[Dutchoven]

Yes you will cover it...it remains quite porous and crumbly...it works much more efficiently being on top of the structural layer. You can screed a thin layer of fireclay on top of the vermicrete to make it easier to level the floor bricks or just a dry layer of sand and fireclay. Should cause any real complication
Best
Dutch

[nissanneill]

There is no way that I would put the vermiculite below the concrete pad as it would add to the termal mass.
I agree with Dutch that the vermiculite cement is crumbly, I would say mor like a cork consitancy than crumbly though.
I found in my installation, the it was quite easy to get nice and smooth and flat but iI did have some suitable rectangular aluminium screeding lengths and topped it off with a steel cement trowel. It was almost shiney, flat and smoot, ideal for laying the hearth bricks directly on. I did however have to lift the ocassional one uo dur to minor thickness variations with the seconhand bricks that I used.
You will have to address the problem of rendering the edges or tiloing the edged of your base but that is an easy venture with the adhesives that are available today.

Neill

[Dutchoven]

Cork is a great description...couldn't find the right word! Thanks Neill!
Dutch

[tdibratt]

What about putting the vermiculite mixture below the concrete and then separating the firebrick hearth from the concrete with the insulation board. I presume that would also work although not sure the vermiculite is offering any benefit and could just as well be excluded?

Thx

[SpringJim]

Originally Posted by tdibratt

What about putting the vermiculite mixture below the concrete and then separating the firebrick hearth from the concrete with the insulation board. I presume that would also work although not sure the vermiculite is offering any benefit and could just as well be excluded?

Thx

What are you trying to accomplish with that setup? What is the function of the vermiculite layer then?

...two different separated insulation layers? That sandwiched concrete layer would act as a long term thermal mass/heat sink

Yes, exclude the vermic layer or put it above the hearth with the board for better insulation!