Time to continue but still undecided

[P3 Stoaker]

G'day All,
Hope all is well living in these crazy times. I posted a few questions a few months ago in preparation to the hearth and insulation steps but it got too hot to lay concrete. With the cooler weather now I can get going again. I am still undecided on which way to go and would appreciate any feedback.
I have 7 inch available for hearth and insulation. The oven floor and dome bricks will then be on the insulation, therefore planning around 3 and half inch for the hearth and around 3 and half inch for insulation layer. I'm thinking perlite and/or calcium silicate board for insulation. Full perlite would be easy enough but LOTS of posts say more insulation is best and whilst I don't plan on cooking bread I'll make the best oven I can within the 7 inch. Alternatively I thought of having 3 by 1 inch layers of silicate board directly on the hearth but I have a few concerns about going this way, numerous comments about not having oven directly on the board and I'm concerned and led to believe if the board did get wet it could collapse then, ALL bad. However will perlite do the same thing?
Has anyone used silicate board under perlite, in this case I would have 1 inch board under 2 and half inch perlite? I'd be happy with that amount of insulation, but would this work?
Any thoughts/recommendations appreciated.
Kind Regards.

[SableSprings]

As you have read, the major advice is do as much insulation as possible. The reason we don't recommend putting the insulating board directly on the concrete hearth is water transfer to the board. It literally wicks water like a sponge! My advice would be to drill some 1/2" holes through your concrete or use short pieces of PVC embedded in your hearth pour. The idea is to create an avenue for water to "drip" out if it finds its way under your oven. To keep the water from reaching your insulating board, I'd recommend using porcelain or glass mosaic tile sheets laid upside down on the hearth and set so the gaps between tiles "feed" to the drain holes. (Remember, these tile sheet squares don't need to be pretty or matching...lots of places have odd squares laying around that are left over from jobs, broken cases, or samples....i.e. inexpensive!) After laying the tile sheets on the hearth, you just lay the insulating boards on top and you've got a pretty well isolated (from moisture) layer on which to lay your cooking floor bricks.

Here's a link to a good illustration of this physical, tile sheet water barrier.

https://community.fornobravo.com/for...andpoint-idaho

Incidentally, the perlite/cement/clay insulation mix will absorb and hold water pretty easily but won't get mushy. It's just that you figure with a 5:1 (perlite:concrete + handful of fire clay) you need twice the thickness of what you get with the insulation boards used in todays ovens (so your three inches of silicate board would take at least 6-7" of a perlite/concrete insulation layer for a comparable amount of insulation. My oven used only a 4-5" layer of perlite:concrete as the base insulation and my cooking bricks are laid directly on top of it. I've been using the oven for over 10 years and put a lot of bread through it without any issues at all. (In fact, my biggest problem with bread baking is getting the oven too hot and having to put dough on flipped sheet pans so they don't burn on the bottom......because it's difficult to lower the floor temp in a short amount of time.)

Hope that helps!

[P3 Stoaker]

G'day Mike,
Many thanks for your time to reply. I certainly think the tiles are a good option where moisture could get in. I'm not 100% sure I understand. Could you please confirm;
1 Are we talking about moisture that gets in from above and/or beside, or are we talking about moisture that wicks up through the complete thickness of the slab?
2 I have 3 and half inch available for insulation and thought rather than just perlite/vermiculite, one inch of silicate board would greatly increase the overall insulation meaning I would have 2 and half inch of perlite and the 1 inch of board being equal to around 2 and half inch perlite resulting in equivalent of about 5 inch perlite, noting you have 4-5 inch.
3 Advice was not to put the dome and oven floor bricks directly on the silicate board.
4 Mike, do you think I should have the tiles under the board if the board sits on the hearth (at this stage I'm assuming the moisture doesn't actually work it's was through the hearth) and is completely covered and surrounded by perlite, effectively sealing the silicate board within the perlite. I would carefully wrap the board in foil to minimise the board wicking up moisture while the perlite sets. In this case I would still fit weep pipes in the hearth, so as to let the moisture out while perlite dries.

So I guess it all comes down to my theory, if constructed as in point 4 above, the only way moisture could enter is by seeping/wicking through the hearth, or perlite layers? Could this happen?

Thoughts and comments appreciated

Stay safe in this 'new world'

[SableSprings]

It would help us all to reply and give you "tailored" answers if you would go to your User Settings, Account, & about halfway down you'll see Conversation Detail Options with Show Signatures, Show Avatar, & Edit Signature. Choose the Edit Signature and put in at least your general location. That way, your general location info is always showing on-screen with your posts. Then if someone has local knowledge they can more effectively get that info to you....it really helps all of us to help you by knowing your area's climate and your possible suppliers.

#1 Yes, both! Most often the water will either come through cracks in the dome and move down to hearth where it pools. Pooling also happens on the "wings & lips" of the hearth platform. With water around the perimeter of the dome...it will seep underneath following the more impermeable hearth surface (hence the drain holes). David S and several others have advocated not only the weep holes on the hearth, but making the hearth surface slightly domed so water flows to the outside edges and doesn't pool on top. Moisture also will wick up concrete...it may not seem significant...but...

#2 Having the equivalent of 2" of ceramic board as bottom insulation is plenty for just pizza. If you want to do some extended cooking (several days) then using insulating board for filling your entire 3" would be recommended (p.s. If you use the tiles, you'll be losing some of that 3.5" available space.) The advantages of the board are that you can lay the cooking floor directly on it and you don't need to have a long cure & drying period for the perlcrete/vermicrete layer. I did want to make sure you are not thinking of using just perlite or vermiculite as both materials will compress & shift without the Portland cement component.

#3 Don't know where your advice not to lay the cooking floor came from...virtually all the ovens here have successfully been laying cooking floor bricks directly on the board. If the board is a bit uneven or bricks have slightly varying thicknesses, sand/fireclay is often put between the ceramic board and the cooking floor bricks simply for leveling. Note: There are some insulating boards that are of a lower grade and don't do as well with the normal higher temps seen in pizza firings...max recommended temps should be in the board specs.

#4 The tile squares are laid upside down (connecting mesh on top) directly on the hearth surface. Again, aligning gaps between tiles so weep holes are not covered. The mesh on top method simply gives much better support for the board that will rest on top of the tiles and allows free flow of any liquid that does get underneath the oven...water is amazing stuff! Perlite is porous and will grab and hold onto water pretty well (although not as good as ceramic board). Your thought of wrapping the ceramic board (adding a top and bottom perlite layer) with aluminum foil is a lot of extra work for no advantage (if water does get in your "sandwich", it won't have a way out...again, water is amazing stuff).

Ceramic board was too expensive and not widely available when I built my oven. If I was to build another now, I would drill/mold weep holes in my hearth, lay down tile sheets, lay down 3-4" of insulating board, and build my dome on top of the board (with the cooking floor inside the dome's perimeter instead of supporting the dome bricks). I firmly believe the only way to (mostly) eliminate water getting into my insulation "wrap" of my oven is to build an enclosure that provides a good roof over its head (and mine ).

Hope I answered your questions adequately...you will love having an oven to work with...but this planning time and thinking about how you want to use the oven are critical...so keep asking questions and reading on the forum. (and thanks, I am trying to stay safe...told my neighbors that my baking for the neighborhood will continue, but I'm not risking anyone's health right now. )

[P3 Stoaker]

G'day SableSprings,
WOW, many, many thanks for your detailed response. The info you provided is a great help. I've also had terrific help from mpmckenzie, chach and david s on this forum, many thanks to all, it's certainly great detail to go with the basic instructions.
SableSprings, I see what you saying and agree water is amazing. My base is around 80 inches square for the actual oven as well as side areas. My thoughts now are to have 3 by 1 inch layers of calsil board vertically, ( 3 inch thick) cut a bit larger than the outer edge of the dome bricks, sitting on the tiles as you describe, then as you say build the dome on that. I will need to fill the remainder of the area with something to provide an overall flush finish right accross the top, level with the casil board. I will use perlite as I need to fill with something and figure perlite is as good as any. For info, I will be laying 20x40 inch slabs of granite, 1 and quarter inch thick around next to the dome to cover the working surface all around the oven. The dome insulation will then cover the cuts and neaten the finish. So whilst I will seal the granite joints, like you say, water will get in one way or another.
The alternative option would be to have hearth, tiles, 1 inch calsil, and 2 inch perlite completely covering, and surrounding the calsil which in theory "shouldn't" allow water in but I agree and understand you thoughts are that water will get in anyway, hence the benefit of the tiles, and my thoughts are If going to the trouble of tiles and weep holes ( not all that difficult anyway) I might as well have 3 inch calsil, over the alternate combination, the result being a lot better overall insulation.
Again, many thanks for your feedback. I'm close to pouring the hearth and just need to finalise the plan as thickness is critical with a desired lower level ( lintels in place ) and a fixed upper edge or height, for the oven floor/dome bricks to sit on, flush with top of bricks. Not sure if you saw my earlier posts, I'll include another photo of the brick base.
Kind Regards

[UtahBeehiver]

Nice brick work. CaSi is your best insulation, also the most expensive. Raising the CaSi off the hearth with tiles and channeling water to weep holes has been done by several builders with good success. Wet insulation is one of the most common problem with oven performance. Water has what I call "sneakosity", it seems to find it's way in no matter how well you waterproof.