Before I pour...hearth question

[Pizza Freak]

Hello All,

I posted earlier about a Casa 90 I'm planning on installing. I'm using an existing retaining wall in lieu of a "stand", or, more exactly, the earth behind the wall. I've excavated for an 8" perimeter footing (the interior will be filled with gravel) upon which I'll pour the hearth. So, essentially, the hearth is on the ground and I'm using the elevation provided by the retaining wall as my "stand".

I'm at the point where I'm about ready to pour the footing, but I'd like to make sure I'm not making a mistake with my hearth plans. My original plan for the hearth was to pour a 3.5" reinforced slab of standard concrete (supported by the footings), and then use two layers of FB board (4" total) and a layer of firebrick "splits" for the oven floor. The Casa 90 itself will go on top of this sandwich. I plan to finish the whole thing off with refractory mortar, 3" of FB blanket, and vermiculite inside a gable house structure.

My concern with this hearth plan is...
If I want to use the WFO not just for pizza, etc., but also for bread, might it be better to go back to the "traditional" hearth of structural cement and vermicrete on top? Might I get better heat retention that way, or is the thick layer of FB board just as good? And if I use the FB board, do I need to find some way to seal the seams between the individual boards?

Also, one question about where the firebrick and Casa 90 meet. Is it customary to put a strip of refractory mortar at that joint, as you do on the seams of the oven?

Thanks A Lot!
PF

[dmun]

Re: Before I pour...hearth question

The cal-sil board is a much better insulator than vermiculite concrete. The only reason not to use it is if you can't afford it. At 4" you have twice as much as specification, no need for any more insulation underneath.

No need to try to fill the joints of the Cal-Sil board. It's weird stuff, and will suck the liquid right out of the mortar.

Just a question: Are you using the firebrick splits instead of the Casa 90 floor, or in addition to it? If it's just the one inch of firebrick, you might not have a thick enough floor.

[Dutchoven]

Re: Before I pour...hearth question

Just a question: Are you using the firebrick splits instead of the Casa 90 floor, or in addition to it? If it's just the one inch of firebrick, you might not have a thick enough floor.

I would agree...if you are not using the Casa floor...1.25 inches of firebrick will not be enough for retained heat cooking...
My $0.02
Dutch

[Dutchoven]

Re: Before I pour...hearth question

Hello All,
Also, one question about where the firebrick and Casa 90 meet. Is it customary to put a strip of refractory mortar at that joint, as you do on the seams of the oven?

Thanks A Lot!
PF

PF
Very imiportant answer here...you should only put the mortar on the joint between the dome pieces on the OUTSIDE...none at all on the inside...and there is no need to mortar the bottom joint of the dome...the dome and the firebrick will very likely expand and contract differently so that will allow some slip between them...but please...I cannot stress it enough...no mortar on the inside...and in lieu of just sealing the joints I would recommend a thin coat of the refractory mortar over the entire surface...
Best
Dutch

[nissanneill]

Re: Before I pour...hearth question

Hi PF,
I did exactly what you are doing and building your oven in the ground behind and using an existing retaining wall for the height/base.
See my initial posting at:

http://www.fornobravo.com/forum/f8/n...-1-a-2005.html

which will lead you through my total construction but using vermiculite cement as an insulator rather than the board for my 40" Pompeii.
My oven works fine and is used for all types of cooking, but I need to improve my bread making skills.

All the best with your build and don't be afraid to ask people questions.

Neill

[Pizza Freak]

Re: Before I pour...hearth question

Hi Guys,
Thanks a lot for the responses. Valuable information.

I was going to use the firebrick splits because they are available locally (very local...3 miles away). I wasn't planning on using the Casa 90 oven floor over them (but I certainly could). Or would it be better to just use a layer of "full size" firebrick underneath the dome (maybe with one layer of FB board underneath), or just use the regular Casa 90 oven floor over the double FB board layer? I'm trying to keep the height of the oven floor down a bit...of course I need to keep the concept of "insulation" paramount, but I'm trying to come up with the best combination of ingredients here to make sure I have a well functioning oven.

I suppose mentally, I'm having a problem envisioning the FB boards as functioning as a "heat retention" component of the hearth (if you wanted to bake bread, for example). It seems like from your answers that they are more heat reflecting, which is intuitive. Vermicrete or firebrick would seem to retain more heat, but should I cut back on a layer of insulation board and add a layer of full sized brick, or is the supplied oven floor over the boards sufficient. This was sort of the crux of my original question.

And thanks, Dutchoven, for clearing up the issue of mortaring the joint at the base of the oven. NO MORTAR!

This forum is a great resource. Thanks for all your help!
PF

[Dutchoven]

Re: Before I pour...hearth question

Hi Guys,
I suppose mentally, I'm having a problem envisioning the FB boards as functioning as a "heat retention" component of the hearth (if you wanted to bake bread, for example). It seems like from your answers that they are more heat reflecting, which is intuitive. Vermicrete or firebrick would seem to retain more heat, but should I cut back on a layer of insulation board and add a layer of full sized brick, or is the supplied oven floor over the boards sufficient. This was sort of the crux of my original question.

And thanks, Dutchoven, for clearing up the issue of mortaring the joint at the base of the oven. NO MORTAR!

This forum is a great resource. Thanks for all your help!
PF

PF
The FB boards are insulators and will not retain heat but , they will keep the heat from seeping out into the concrete hearth slab below. I would not recomend lessening insulation and would recommend using either a full firebrick...or two layers of splits...or a layer of splits and the casa floor...the difference in total height is about three inches...and I wish I would've built our oven 3 inches higher...less bending to see the back of the oven...as far as retained heat...it is only in the firebrick/masonry part of the oven...imagine a sponge...bigger sponge more water can be held...thicker firebrick greater thermal mass and longer cooking times(without the fire in the oven)...for instance in our oven floor is 4.5 inches thick, walls and dome are 4.5 inches thick plus 2 inches of refractory mortar for a total of 6 inches...over 350F oven temps for about 10 hours without a fire burning in the oven
Hope this clears up some confusion...Oh and no mortar inside the casa at all...it will only flake off into the food
Best
Dutch

[dmun]

Re: Before I pour...hearth question

If you've bought the fabulous, expensive, carefully engineered Casa 90 floor, why wouldn't you want to use it? If you need to reduce height, use high-tech insulation instead of vermiculite - it has twice the insulation value.

To review: The following are insulators:

Forno Bravo insulation board or other Calcium-silicate board.
Insblock19, or other mineral fiber board.
Mineral wool blankets.
Vermiculite or perlite, loose or solidified with portland cement.
Some castable refractory products.
Insulating firebricks. (the light ones)

These products will keep heat on the inside of the oven. They won't absorb and reflect any heat to speak of.

the following are solid refractory products:

Heavy fire brick.
The sort of vitrified refractory products like the forno bravo ovens are made of. Very similar to firebrick, just different shapes.
Some forms of castable refractory products.

These products will absorb heat, and reflect the heat back on your food.

The heavy oven liner, and the light insulation surrounding it are two entirely different things.