Question about hearth and dome.

[serlbacher]

I have just finished pouring my 3 inch thick concrete slab (no vermiculite just plain concrete) and I am ready to move on to the insulation for the hearth. I have downloaded the FB instructions as well. I am planning on a 36 inch oven. I have 7 Thermo 12 Gold pieces of insulation measuring 12 inches x 36 inches x 2 inches thick that I am going to use. My questions are as follows:

1. Should I double up on the insulation and make it 4 inches thick? If so, will I have enough boards?

2. Does the first row of fire brick rest on the insulation itself or can it rest on the concrete slab around the insulation? Or is either correct?

Thank you in advance.

[RandyJ]

I am almost done with my 40" oven. I did go with 4" of ceramic board insulation. I used 6 24"x36"x2" pieces to do the job. Other than the corners I couldn't have gone with much less than what I used. It was nice to not have to deal with the vermicreete, but your buget Wil dictate whether that is do able. From my quick math you would need almost twice as much. But if you angel the corners you might be able to save a little. From my early useage I am very happy. I have no door and was 375F 24 hours later and 175F 40 hours later. I have to think the floor has at least a little to do with that.

Randy

[fremen325]

I would put the first course on the insulation layer (whether board or vermiculite concrete ). Otherwise the heat will flow into your structural concrete layer which is bad, not for the concrete but for trying to get up to cooking temperature.

[RandyJ]

Good catch freman. I missed that. Yes you need to be on the insulation or it is pretty much useless and the whole stand will act as a heat sink.

Randy

[serlbacher]

Journeyman,
I notice you have concrete pavers under your 4 inches of ceramic board. Can you tell me why you decided to add them? I have some laying around, if needed.
Susan

[Gulf]

Journeyman,
I notice you have concrete pavers under your 4 inches of ceramic board. Can you tell me why you decided to add them? I have some laying around, if needed.
Susan

Susan,

I can't answer for "Randy" but, it is an excellent idea! Getting the insulation high and dry above any place that water may pool is a great idea! I've preached moisture barriers between the floor insulation and the structural hearth for a while. Now, I will preach "elevation" and "moisture barriers" .

Joe

[serlbacher]

Sounds great.
1. Any need to glue those pavers down or will they be okay without?
2. Give me an example of a moisture barrier that I can use. And will the moisture barrier go between the concrete pavers and the Therma 12 Gold?

[Gulf]

If the structural hearth is flat, they should just lay there just fine. No need to glue. If they are rocking, a little sand with a notched trowel should be all that is needed to stabilize a level. (But, I'll let Randy weigh in on what he did). A piece of polyethylene plastic, a vinyl floor covering remnant, scrap pool liner etc. or painting with a commercial water barrier will do. The barrier should be placed directly under the insulation imo.

[RandyJ]

Sorry I missed the question. There is a simple answer for the pavers. I wanted to have my floor at a certain hight, and I'd did not want to poor another 1.5" of concrete as my support slab is already like 6" thick. So the cheapest and easiest way to get the height I wanted was paver stones. The biggest added plus is that if water gets in the insulation does not get wet and just sit in the water. I am glad that people like my idea,.

Randy

[Wozza]

Everyone I've asked about the insulation has said 'more is best'. I guess you can't put too much on. I used 50mm thick Hebel blocks laid directly on the hearth and then 75mm insulating tiles on top of that. No sand or sand/clay mix - just laid them straight on top of each other. I cut them roughly with an angle grinder so they match the external diameter of the dome itself. The pizza oven floor tiles went on top of that trimmed to match the insulation and I'm building directly on the cooking floor. The floor tiles are pretty flat but I need to grind down a millimeter or so in a couple of places where a raised lip may catch the peel.

The recommendations as to whether to fit the floor inside the dome and build on the insulation or let the floor extend past the edge of the dome and build on that seem to run 50:50. Neatly trimming expensive floor tiles to fit inside - well, I put that in the too-hard basket.

The outside edges of the of the insulation and floor look as rough as guts but the insulation and render will cover all that.

[fremen325]

It looks like you are making good progress. Are you going to butt your dome against the arch in the photo? Or are you going to start angling in from the arch you have?

[serlbacher]

Thanks for all of the input--
I think I will play it safe and pour 4 inches of the vermiculite. I wont have time to get more Thermo 12 Gold.

1. I noticed in the Forno Bravo plans that they poured the 3 inches of concrete and the 4 inches of Vermiculite in one pour. Is it going to be a problem having to pour the vermiculite as a completely separate pour?
2. The vermiculite I bought was from Menards. It is called " Micaflake Loose Fill Attic Insulation 3 cubic feet" with an "R" value of 9.2 at 4 inches thick. Is this the right stuff?

[sharptailhunter]

Hi, I can't answer whether or not the vermiculite you picked up is the right stuff, but I bet it probably is. There's no problem pouring your perlcrete/vermicrete on your already poured hearth slab. Do you still have your ceramic fiber board? Didn't you say You had enough for a one inch thick layer? If so, and if it were me, I would pour your perlcrete three inches thick and after it has cured or dried enough, I would place your one inch thick layer of ceramic board on top of that and then build the floor and dome on top of all that.

[serlbacher]

Yes, I still have my 2 inch ceramic fiber board. 7 pieces. I just poured 4 inches of Vermicrete. 6 buckets to 1 bucket of Portland Cement. I hope that was right. I added just enough water to make it look like oatmeal as directed. This is some weird stuff. I am hoping it doesn't crumble when I remove the forms. Sure looks like it could. I think I am ready to cut the fiber board. Going to find something for waterproofing between the the fiber board and the vermicrete layer next. Everything sound good so far???

[Gulf]

Leave the forms on as long as you can. The vcrete should be allowed to dry as long as it can before covering with the board insulation. Also, I don't think I would waterproof between the insulation layers. The water will just be trapped. Any waterproofing or liner should be between the vecrete and the hearth slab. If you don't have any under there don't worry, neither do I.