Floor insulation

[RickH]

Hello,
I am starting to build a 42" Pompeii oven, I have my base built and I am about to start the floor insulating process. It is my understanding that the ceramic board retains heat better than the perlite/concrete mix.
I was originally going to do 4" of the mix, but I am now thinking of doing 3" of mix and a 1" ceramic board. I would like my oven floor to retain heat longer for baking the next day.
However, the ceramic board is quite expensive. does anyone have any thoughts on this? would it be worth it? TIA

[SableSprings]

Welcome to the forum community Rick! A couple of important considerations on your build. Perlcrete provides approximately half of the insulation value of ceramic board (and yes, ceramic board is expensive!) So, to equal 2" of ceramic board you would need 4" of 5:1 perlite/vermiculite:cement as your base insulation. Many builders have chosen to use the insulating concrete (IC) mix layer because of the costs involved. It's critical to keep insulation dry for it to be effective.

There are ceramic board options now that are non-hydrophilic (won't absorb water), but IC will. We currently recommend drilling (or pre-pour setting) 5 or so 1/2" weep holes through the supporting hearth slab to allow moisture to escape from under the oven insulation floor. In addition, we recommend putting ceramic tiles in place between the hearth slab and the bottom insulation. Mosaic tile sheets placed upside down (mesh side up) create pathways for moisture to "find" and escape through the weep holes. If this doesn't make sense to you, let me know & I'll give you a link to a build that shows this pretty well.

Back to your original question, our normal minimum recommendation for floor insulation is 2" of ceramic board or 4" of IC. I have 4" IC under my full size firebrick floor and can easily bake lots of bread after pizza the previous day. To retain heat, you will definitely want to plan on having an insulated door as well...

Water/humidity creeping into your top & bottom insulation is going to be a major concern for a WFO in Florida. I'd strongly recommend building an enclosure to protect the oven (and you) from the weather. Again, welcome to our community!

[UtahBeehiver]

Mike pretty much covered the major options, attached is a table showing the p/vcrete ratio and associated insulation properties. In the scheme of overall cost of the oven, even though CaSi is expensive, it is relatively nominal in cost relative to the total project cost.

[david s]

Thanks Russell, That table is most useful. While the 5:1 ratio, recomended for under floor insulation in order to provide sufficient strength, does require double the thickness of cal sil to achieve the same result, a 10:1 mix is about the same insulation value as insulating blanket used over the dome. Because in that position it is not required to support a lot of weight, the weaker mix is an ideal insulative substrate over which a hard shell can be rendered.

[majikmatt]

"Mosaic tile sheets placed upside down (mesh side up) create pathways for moisture to "find" and escape through the weep holes. If this doesn't make sense to you, let me know & I'll give you a link to a build that shows this pretty well."

Hey Mike, I'd like to see that link... I'm in a similar spot as Rick. Unfortunately I think I may have to wait another season as I'm in CO and Winter is Coming...

[Gulf]

"Mosaic tile sheets placed upside down (mesh side up) create pathways for moisture to "find" and escape through the weep holes. If this doesn't make sense to you, let me know & I'll give you a link to a build that shows this pretty well."

Hey Mike, I'd like to see that link... I'm in a similar spot as Rick. Unfortunately I think I may have to wait another season as I'm in CO and Winter is Coming...

Semmital Farm 36"

[SableSprings]

Here's another build that illustrates use of the "upside down" mosaic tile sheets (post #21, but whole thread is a good read).

https://community.fornobravo.com/for...ge2#post407535

[david s]

Here's a pic showing under floor tiles supporting cal sil insulation. It's a bit hard to see but it also shows one of the four drain vent holes which lines up with the dome insulation space. (Also two in the centre of the floor)

[RickH]

Thanks Mike, I think I will go with 4" of perlite mix, However the holes I would like to avoid. my top slab is 5 1/2" with a bunch of steel in it. I understand the moisture problem, I was thinking of using the ceramic tile upside down as shown but in between each 1' square install a 1/4" rope from end to end so the moisture can wick out. think that would work well enough?

[SableSprings]

I'm a little concerned about using a wicking rope ( wicking can go both ways ). If you are using the perlcrete option for base insulation, remember that you want it to extend out slightly beyond the perimeter of the actual planned firebrick cooking floor/dome walls. This insures a more complete isolation of the WFO from heat loss at the edges. Following this thought...extend the tile sections as well. This will allow moisture to escape to the outside oven perimeter as you start your curing/drying fires. Are you going to build an enclosure over the WFO so you can cook/bake/entertain under cover? I hope you are as that will greatly reduce the amount of moisture that your insulation may pick up from rainfall through the outer render (cracks) and seeping in from the slab sides. With an enclosure, just the tile sheet "channels" will probably be more than enough to let the perlcrete dry out in a reasonable amount of time. FYI: David ( DavidS ) has noted that also providing moisture exit paths along the dome insulation perimeter through the slab is another (additional) great preventative measure for soggy insulation...but it too involves weep holes.

If you are doing the enclosure, just providing an easier path for moisture to escape initially may be all you ever need it for...no rope required

[david s]

Because of the large spaces between the steel reinforcing mesh, striking it is unlikely. Estimating the mesh pattern will als help to figure out where it’s placed. There’s not much use providing pathways to non-existent drain vent holes. Simply drill pilot holes first (1/4”) and if you do strike steel move the hole 2” diagonally and you should avoid striking more.

[RickH]

I am going to use perlite, what is the recipe for under the floor? is it 5 parts perlite to 1 part Portland cement? am I supposed to add lime as well?

[SableSprings]

Yes Rick, 5:1 (perlite or vermiculite:cement) by volume is the proper mix for floor insulation (no lime needed). If you have some clay/fire clay, throw a handfull in as it well to help working the mix. Do mix slowly by hand and be sparing with the water. This mix seems impossibly crumbly but it will set up with enough compressive strength to support the top structure. Remember that putting down a tile layer (fabric/mesh topside) between the perlcrete and hearth slab (with weep holes) is our recommendation. Also you are going for 4-5" thickness of the perlcrete extending beyond the projected dome footprint for adequate floor insulation.

[RickH]

Thanks Mike for your response, I hope to be pouring this before the weekend, (I will post some photos) my next question is what to use to set the floor brick? I saw that a dry mixture of sand and fire clay works well, however I am having trouble finding fire clay, are there alternatives?

[SableSprings]

The sand is just used for final leveling/bedding of the cooking floor bricks. The addition of a little bit of clay can be helpful in the leveling & final bed prep but is not required. I also used perlcrete for my floor insulation and made an effort to make that perlcrete layer as flat/level as I could. You'll find that there can be some variation in firebricks edges (and dimensions). Try to pick out your best bricks by laying them out on a smooth, flat surface (most garage floors work well for this if you don't have big, stable work table). I did these two steps and then only needed a thin layer of coarse, builders sand on top to provide a nice bedding for the cooking floor. I spread the sand out (and had to remove a few bigger stones), used a plant mister to dampen the sand, and used a notched trowel for the final prep. Set the bricks in, lightly tapping them down and constantly checking the overall level. I have a couple brick edges that have raised up slightly over the years along one back, perimeter area, easily avoided. Pretty happy with how well this oven has performed over the +24 years it's been in operation.

Good luck and don't forget about the top slab weep holes (and layer of tiles) to give moisture/water an escape route from your bottom insulation layer.