Insulating slab with Super Isol

[Pedro_2004]

Thanks for the welcome James
I did actually use quite a bit of your site as a resource for building my oven so its kind of a hybrid design !
Most of the folks around here are mad for 700 degree f. pizza baking, and use their ovens only on, say, weekends. It's brutally difficult, and time and wood consuming, to heat up a high thermal mass oven to that temperature.
That I do agree with ! ..once there its great but it does take some doing
I do see your point so I defer to the Italians great experience in this ..

[james]

Hi Pedro,

Welcome to the group. I'm sure you are going to enjoy your oven.

There are numerous postings on the forum that discuss the trade offs between the various oven design styles and hearth desigin philosophies. Take a look around -- it is interesting to read. For people just getting started, we strongly recommend the Italian oven design and hearth with the insulation on top. It is basically the oven and hearth design that you see in all of the pizzerias and backyard brick ovens in Italy. I have seen instructions for the insulation directly below the cooking floor in nice 3D drawings, simple line drawings, and hand sketched instructions from Italian builders and designers. There is a great deal of history and experience behind the design regarding heat up time, high heat retention, oven efficiency and pizza cooking.

I have to admit that I get a kick out of an Australian telling folks how to make a pizza oven. It's kind of like getting your NFL gear and rulebook from an Italian. Who knows what you'd get. There are scores of thousands of people involved in the pizza oven and pizzeria industry in Italy, with multiple trade shows, magazines and professional associations -- and I don't think that they are all wrong.

James

[dmun]

The reasoning behind putting the insulation layer below the concrete layer is that you want to increase the thermal mass of the oven ...you actually need the concrete slab to heat up to allow more heat to be stored in the oven and then radiated back for cooking .

If you want enough thermal mass to bake multiple batches of bread, and keep the oven hot from day to day for everyday use, then you want as much thermal mass as you can heat up.

Most of the folks around here are mad for 700 degree f. pizza baking, and use their ovens only on, say, weekends. It's brutally difficult, and time and wood consuming, to heat up a high thermal mass oven to that temperature. But if you are a regular baker, rather than a weekend pizzaolo, then the massive bread oven is the way to go.

[Pedro_2004]

I've just built my barrel shaped oven .....using the method advocated by Rado ... firstly we poured 4" vermiculite concrete as the insulation layer for the hearth then 4 " of concrete ....surely the reasoning behind putting the insulation layer below the concrete layer is that you want to increase the thermal mass of the oven ...you actually need the concrete slab to heat up to allow more heat to be stored in the oven and then radiated back for cooking ....sticking insulation above the concrete IMHO is not the correct way to go...also again IMHO it doesnt really matter if the aluminium slip plane does eventually corrode all you are trying to do is prevent the concrete slab chemically bonding to the support blocks whilst its going off....once its cured then it will be able to move at will

[dmun]

slip plane

Question for anyone....why did the plans change from utilizing the slip plane?

Not only was it decided it was not needed, but someone with experience in masonry pointed out that the aluminum would be corroded by the lime in the concrete.

[maver]

James addressed this above, there just is not enough thermal expansion (with insulation above the concrete structural layer of the hearth) for it to be necessary. I built mine with a slip plane. If it really is unneeded, consider extending rebar from the block stand up into the hearth to lock them together - I believe that idea has been discussed earlier on this forum.

[dalucca2003]

Maver,

Sorry for the confusion.....for now I am only filling the cells with the rebar. I thought filling the other ones with the hearth pour would be overkill. But now that I think about it, if I go get a yard of cement from a rental location, I will need to dispose of the extra concrete one way or another. Good advice.

Question for anyone....why did the plans change from utilizing the slip plane?

[maver]

I don't think the cells need to be filled, but if you do get a yard of concrete you might as well because I think otherwise you will likely have extra concrete - you need to put it somewhere . I don't think I'd bother filling them prior to pouring the hearth, the every other rebar/concrete filled cells will keep things from moving with the pour.

I used 3x5, which on the inside dimension of the block stand was just under 2 sheets. I duct taped the one seam from above to stop concrete from pushing through. You do need a bit of reinforcement, but no more than you would need for a larger sheet, just good vertical reinforcement for the slab weight, I braced 2x6 laid horizontally on the edges and several through the middle with about 10 vertical 2x6.

[dalucca2003]

So if no slip plane needed then you might consider extending the concrete board over the block to block the empty cells - I don't think you need every core filled with concrete structurally, and if you are mixing your own concrete it will save you a bit of labor.

A yard should be sufficient to pour the hearth to six inches and fill every other cell (calculate this for your particular hearth size to be sure).

That was my plan to go get a yard of cement which should be plenty. Question Maver.....first paragraph you state no need to fill empty cells but last sentence you mention filling other cell. Are you talking about the ones which require rebar? I have already filled most of mine and plan to finish every other one prior to pouring the hearth.

Additional question on concrete board....I went to Home Depot and all they have is 3' x 5'. Do they make it bigger and where to go to obtain. Otherwise I see a lot of work reinforcing the board from underneath.

[maver]

So if no slip plane needed then you might consider extending the concrete board over the block to block the empty cells - I don't think you need every core filled with concrete structurally, and if you are mixing your own concrete it will save you a bit of labor.

However, if you are doing super isol (so no need to mix your own perlite or vermiculite concrete) I might suggest you look into getting your concrete premixed. For a job this small you probably would not rent a truck and a pump, but getting a BIG truck -1/4 pound or more such as Ford F250 - with a heavy duty hitch (I borrowed from a neighbor) you may be able to find a rental company like I did who will rent you a big (1 yard capacity) mixer on a trailer, fill the mixer with the concrete and water for you, then you just tow to your house and pour into buckets and dump it into your form. A yard should be sufficient to pour the hearth to six inches and fill every other cell (calculate this for your particular hearth size to be sure).

[mrpbjnance]

My Durolock coverd up the holes. Or you can cover them with something else. No need to use all the extra concrete to fill up the holes.

[dalucca2003]

So if no slip plane is used, do we just fill the remaining cinder block holes with cement from the hearth pour?

[james]

Skip the slip plane

The aluminum flashing was an experiement that we tried early on, and with more experience, we have removed that step from the instructions. It does not get hot enough where the hearth meets the concrete blocks to worry about needing a slip plane. You can save the time and effort.
James

[maver]

Either may be ok, I think

Just be sure to use flashing as a slip plane (probably under the board). I'm unsure of any reason to cover the concrete blocks with the cement board though. I just placed mine inside the block stand over the frame. Is there any reason you would want to extend the board over the block?

[mrpbjnance]

I had the framing sitting on the block and the durolock inside that also sitting on the block. But do not that leaves an awkward ledge around your block when you remove your forms for the hearth, that I have not figured out how to finish yet. Personnaly I build a pier using blocks in the center that I left in place to support my hearth. It does not take a lot of space but is not needed and the 2x4s alone work well. You can also use the blocks to support the durolock during the pour then remove them when the hearth is dried.