Homebrew Cast Oven Help

[SableSprings]

Just to be clear, the hearth is the support slab of reinforced concrete upon which the oven is built. Current best practice is to provide several weep holes (for trapped moisture to drain/escape) through the hearth slab beneath the oven. Next, add a layer of porcelain files (surplus, small mosaic file sheets are easiest) with the gaps arranged so water has a clear path to the weep holes. On top of the tiles you can lay the 2" ceramic board. Two inches of board is enough insulation beneath the cooking floor bricks. If you want more insulation, you can put a layer of perlcrete or vermicrete over the tiles, then lay the board on that insulating concrete layer. Using a 50/50 mix of perlite and vermiculite for the 5:1 cement mix works well as the insulating concrete. In my opinion, the two inches of ceramic board is adequate insulation for the oven base, but many add the additional insulation "just because"... Make sure that you plan for the outer dome bricks or casting to rest on the insulating base...your goal is to isolate the entire oven with good insulation (board for the base, batting for the dome).

Hope that helps. Looking forward to your build.

[david s]

If you employ the trick with the weep holes and tiles, then you really don’t want to be filling those spaces with a vermicrete mix. It would be better to cast a complete vermicrete slab on some ac sheet orply, then lift it in place. Alternatively place the calsil on the tiles and cover it with the vermicrete layer.

[watchthenanacrossing]

Here's a sketch from my understanding of how to set up the vermicrete slab. The weeping holes will be drilled all the way through the supporting slab around 6" apart. Is there a pattern for the weeping holes that is better to follow? I found 8" square glazed ceramic tiles in my basement lately. Would those be okay to use?

I was originally planning on putting two layers of insulation below the firebricks (2" each layer, 4" total), but the 2" vermicrete slab would be enough? For my cooking, I plan on a lot where the heat needs to be retained, such as overnight cooking.

Back to my earlier question, can I use only perlite instead of vermiculite for the slab? Right now I have easier access to perlite than vermiculite, so that's why I ask.

[SableSprings]

I used perlite in my 5:1 base insulation mix and it's fine. If you are going to use perlcrete, then you need to do at least a 4" layer. As David pointed out, you don't want to pour the perlcrete over the space out tiles...it covers up the channels to the weep holes. The tiles you have are perfect, but I'd recommend you go with 2" of the ceramic board minimum if you are skipping your original plan and intending to do retained heat cooking/baking. The issue is that an insulating concrete (5:1) needs to be twice as thick as a corresponding ceramic board for equivalent insulation value. So 2" of ceramic board is roughly equivalent to 4" of insulating concrete. Yes, it is expensive...but well worth the price!

Here's some options if you still want to include a 2" perlcrete layer and 2" ceramic board layer. (For my money, I'd make the lower layer perlcrete and put the ceramic board on top.)
1) As David suggested to avoid clogging the "channels", make 2" deep forms and cast the perlcrete sections separately. (I'd suggest several pieces here because the 5:1 mix is pretty crumbly to handle.) This method has the advantage of allowing you to unmold and dry out the sections a bit faster before placing on top of the tile pattern.
2) An alternate method would be to cover your tiles with the garden/driveway fabric that lets water through but not the dirt (i.e. insulating cement ). Then by putting the 5:1 mix over the fabric covered tiles, it would be less likely to drop down into the channels and block 'em. The tiles I recommend are the smaller mosaic pieces set on a fabric mesh sold in most big box stores. You simply turn the 1' square sheet of tiles over so the mesh is up and then the mesh prevents the insulating concrete mix from dropping down. Often you can find a friend or even in the stores where they have several squares of mosaic tiles that are extras, slightly damaged, or discontinued at dirt cheap prices...but since you have tiles that are perfect, use them!

I simply noted earlier that David S has found that a 50/50 mix of perlite & vermiculite with a handful of clay thrown in mixes better and is easier to work with. Hope this long winded explanation helps!

[david s]

Dry loose perlite is a better insulator than dry loose vermiculite. However, as soon as you add cement and water there’s no difference in insulating value. Even at 10:1 there is more mass in the cement content than either vermiculite or perlite in an insulating mix.

[watchthenanacrossing]

Yea this is definitely helping out. Well, I can say there will be more pizza and high heat cooking than retained heat cooking. So, would 2" of calsil be adequate? My original plan was 4" of calsil.

[Gulf]

Are you going to use those IFB spliits from your first build for anything?

[Neil.B]

I personally would build based on high & retained heat cooking. You never know how you might want to cook in the future and at least it will give you options.

[watchthenanacrossing]

I am trying to decide whether to use the IFBs I have. It scares me seeing how a few of them warped, so I'm not sure how they would perform as IFBs. I imagine as isulating bricks they would be okay to use.

[watchthenanacrossing]

Gulf Do you think those bricks I have would be okay to use? So it would be 2" calsil, then the IFBs I have, then the FB floor.

[Gulf]

I do. But, I would place the IFB on top of the open spaced ceramic tile for elevation and drainage. I would place place the CalSil on top of the IFB then the dense firebrick on top of the CalSil.

[Gulf]

You always want the best insulation closest to the dense firebrick chamber.

[Gulf]

I'm back at my lap top now. I don't like the "hunt and peck" on the iPhone. Hence the two short replies above. The IFB, you have, (that I think we determined but not verified from your firtst thread ) are pumice. Since they are only splits, they are not going to be much extra insulation but they will be some. Any amount of extra insulation that you can get for the money will help toward retained heat cooking. You should be able to determine any high spots that you may have in the IFB floor, They should be fairly easy to abrade and level before you install the CalSil over it.

[watchthenanacrossing]

Alright, all sounds good. I think I have my route for making the insulation under the floor. Calsil boards are hard to locate so hopefully I can acquire some. I'll be checking back in once I get the materials and start putting it together.

[Gulf]

I did a search for "industrial insulation near Central City, NE". I found ICS Products inc. in Lincoln, NE. I called them to question what they had available. They don't stock the CalSil board but can get it for you from thier distributor in Omaha, NE. They do stock the ceramic fiber blanket that you may want later in the build. ICS's number in Lincon is (402) 466-3233. Ask for Andy. You may want to call the distributor in Omaha if you don't think that is too far to drive. There may be other sources nearer you.

Knauf Insulation near Centreal City NE