36" WF Pompeii Oven in Maryland

[bentedesco]

Your rebar lattice within the concrete top slab is providing a lot of structural "strength". Those weep holes will not weaken your slab in either pattern you suggest. And yes, you are putting weep holes only under the insulation board. Remembering to place your 1/2" tubing pieces into the form work also has the advantage of keeping you from hitting a piece of rebar during the "drilling option". Another option to David S suggestion above is to drill holes in your support board and push the tubing pieces just through that board. I would cut the tubing off just below the top projected level of the concrete. I know this may sound stupid, but take some bread wrapped in a bit of plastic wrap and make a taller "cork" for each tube that will stand above the finished concrete.

When the concrete is poured, leveled and just starting to stiffen, locate your "bread corks", make a slight depression around each tube that will collect water and drain directly in the tubes. Gulf put pieces of screen over the holes (both ends) to keep bugs from moving up/in and setting up residences. The trick with the bread is that it keeps the concrete out of the tube and is extremely easy to clean out later. I had used wood blocks for my ash slot form void and spent about 2 hours trying to remove the swollen wood after the slab had set.

David's ideas will work great and may be simpler...I just thought you'd like several options to consider. Hope all this makes sense and helps...good luck with your slab pour...don't forget to pound/tap the sides of the forms to release any air bubbles/voids and make the finish smooth(er)

Thanks for the reassurance about the slab, I've definitely got plenty of rebar in there right now (see image below). I was thinking of making the PVC about 1/2" taller than the slab and then cutting it flush with my angle grinder once the hearth cures... I like the bread idea MUCH better! Thanks for the tips!!

You can use some polystyrene plugs cut to length and insert as you lay the concrete. I also like to dome the slab up in the centre slightly so the centre is about 10 mm higher than the sides. This encourages water flow away from the dome and won’t pool under the floor. You can also raise the insulation off the surface of the supporting slab with something thin like 6 mm gal steel mesh or concrete pavers laid with gaps between them to provide a passageway for water/steam to the weepholes.

Awesome idea! I still think the pavers wont hurt, but the idea of doming the hearth is excellent! It looks like your screen is on the bottom of your durock, correct? (I assume held in with gorilla glue?)

[Gulf]

..... I was thinking of making the PVC about 1/2" taller than the slab and then cutting it flush with my angle grinder once the hearth cures.....

I would not do that. They will get in the way of the screeding. Set them about 1/4 to 1/8th inch below the top of the slab. You can easily rod them out from below later. Elevating the insulation is an alternative to "doming" the slab imo. That is is going to make setting the pavers level difficult if you intend to do both.

[david s]

The sloping of the supporting slab is really more important outboard of the perimeter of the outer shell so water runs away from the oven (assuming you are building an igloo style). In the case of raising the insulation to provide space for moisture movement some gal mesh provides more space but far less height than pavers.
in my build the mesh is glued to the topside of the supporting slab to prevent wasps and ants entering.

[Gulf]

in my build the mesh is glued to the topside of the supporting slab to prevent wasps and ants entering.

I guess that it is all in relation to where you live. In my neck of the woods "dirt daubers" would plug those holes the first season, if the wood storage opening is left open.

[Gulf]

Bentendesco,

The orientation of your bottom course of blocks?

[Gulf]

The sloping of the supporting slab is really more important outboard of the perimeter of the outer shell so water runs away from the oven (assuming you are building an igloo style

That is also the intent of the elevating the insulation. It just places the insulation much higher above the water that may collect on the hearth. The idea works for both an igloo and an enclosure. In the US, it is a basic concept built into all homes built on concrete. The dry level that we live on is placed at least 2.5" above the layer that brick are layed. Weep holes are provided on that lower level to allow water to escape to an even lower level to grade.

[mongota]

Bentendesco,

The orientation of your bottom course of blocks?

I think he was the builder who wanted to be able to move the oven with a forklift?

[bentedesco]

I think he was the builder who wanted to be able to move the oven with a forklift?

Correct (https://community.fornobravo.com/for...and#post406906), I went ahead and reinforced every cinderblock (as opposed to every over) and went overkill on the rebar. I know it probably still won't work to transport, however, given the family circumstances it's worth the shot.

[bentedesco]

Progress report:

I dodged the rain yesterday and was able to get the hearth poured. Thanks for the tips about the plugs! I actually just ended up using the paper scraps from the concrete bags and they worked great! The concrete is still quite green so I have her all covered up and I plan on starting my base over the weekend.

[bentedesco]

As I'm getting ready to start laying my ceramic board this weekend, I wanted to get some clarification regarding the entrance to my vault. I am currently planning on first laying down pavers to elevate my Ceramic Board off the hearth.

While I absolutely will be using Ceramic Board underneath my oven floor (+ soldiers), I'm wondering if I also should insulate underneath the flue?

So far I've thought of 2 possible options:

1. I'm torn here as while the flue is technically outside the dome and not in direct contact with "fire", I don't want to just put my flue right on the cement as I'm afraid it will act as a heat sink (**maybe** this will be mitigated by the pavers??)
   1. I know some people have mentioned a "heat break" and while I've seen them use a "Tee Angle Iron" to create a gap in the firebrick floor, I haven't seen any sort of barrier created between the dome arch and the chimney. So again... afraid that this option would result in the chimney acting as a heatsink from the dome.
2. My concern with insulating under the chimney is water exposure. Given that this is right at the entrance of the oven and beneath the flue, I'm afraid water will seep in and then spread to the rest of the Fb Board inside the oven.

Currently, I think I'm going to go with Option #1 and NOT insulate under the chimney, and just use pavers to hopefully give some insulation off the hearth to limit the "heat sink" effect. I'm debating about the value of adding a heat break on the floor without also adding some sort of break where the dome meets the chimney.

Thoughts?

[mongota]

I have 4" of board insulation under my entire fire brick build; under the oven dome and oven floor, the vent arch, and the vent floor.

I think having all of the substructure that any floor brick will sit upon (oven floor and entry floor) constructed the same way makes it easier to get a smooth top surface on your floor brick, helping to avoid lippage on the brick floor. I think it also lumps "everything firebrick" together and treats it all the same with regards to potential moisture abatement. And it also treats everything firebrick relatively the same thermally.

If you are building a true masonry chimney, a chimney 6' or 7' or 8' tall, all out of brick? Then I could see carrying that load directly to the hearth slab to minimize possible insulation compression. That can also depend on the footprint of your chimney. If it is well spread, with a large footprint due to buttressing, it could very well go on the insulation. If it's a "one brick wide" footprint, then it might be best to carry the chimney load directly to the slab. But I'd still recommend the vent arch floor brick being upon the same insulation as the oven floor brick.

Heat break:

1) Between my dome and my entry arch, the bricks are not mortared together. They are gapped just a bit, with the gap being filled with ceramic fiber rope and a couple beads of RTV silicon. I gapped them not just for heat transfer considerations, but also for differential expansion. I think it's important to isolate the vent arch structure from the dome structure just a bit, some for thermal, but mostly for differential movement. In retrospect I went overboard with my level of detail. Heck, even when I was building it I knew I was going overboard. But it is what it is, you know?

When all is said and done, I have continuous blanket insulation over my dome and over the vent arch. With the blanket doing its best to prevent heat loss from the structure as a whole, there's still going to be heat transfer from one structure to another. But...we do what we do.

2) Between my oven floor brick and my vent arch floor brick, right now my only separation between the two is a break in the brick floor pattern. My oven floor is herringbone, my vent arch floor is a running bond. Honestly, I see no need to get more technical than that, there is a temp drop at the break "as is". However, to take things further (lol...because that's what we do, you know?) when I get my insulated door on I'll take more temp readings. If I feel the need, I'll simply undercut the sides of each brick at that pattern break to create an inverted "V" gap between the two patterns, and either leave it as a void, or I'll probably tuck a wedge of board insulation in there. Again, who am I kidding. I'll add the wedge of insulation. Because that's what we do! lol

This is probably a very minor point...but even though my oven has only been cooking for a couple of weeks, I've cooked a few times inside the entry arch, with a pan on the entry arch floor brick. I've placed a door over the oven dome entry to keep high cooking temps in there, and another door over the face of the entry arch to maintain moderate temps inside the entry arch, with cooking pan placed on the entry arch floor. Having the entry floor brick insulated helped steady the entry arch temps during a long dehydrating "cook" of some tomatoes and basil, or whatever else we have happened to have in there.

[Gulf]

You can help minimize the possibility of water intrusion at the entrance by recessing a storm door.

[bentedesco]

I have 4" of board insulation under my entire fire brick build; under the oven dome and oven floor, the vent arch, and the vent floor.

I think having all of the substructure that any floor brick will sit upon (oven floor and entry floor) constructed the same way makes it easier to get a smooth top surface on your floor brick, helping to avoid lippage on the brick floor. I think it also lumps "everything firebrick" together and treats it all the same with regards to potential moisture abatement. And it also treats everything firebrick relatively the same thermally.

If you are building a true masonry chimney, a chimney 6' or 7' or 8' tall, all out of brick? Then I could see carrying that load directly to the hearth slab to minimize possible insulation compression. That can also depend on the footprint of your chimney. If it is well spread, with a large footprint due to buttressing, it could very well go on the insulation. If it's a "one brick wide" footprint, then it might be best to carry the chimney load directly to the slab. But I'd still recommend the vent arch floor brick being upon the same insulation as the oven floor brick.

Heat break:

1) Between my dome and my entry arch, the bricks are not mortared together. They are gapped just a bit, with the gap being filled with ceramic fiber rope and a couple beads of RTV silicon. I gapped them not just for heat transfer considerations, but also for differential expansion. I think it's important to isolate the vent arch structure from the dome structure just a bit, some for thermal, but mostly for differential movement. In retrospect I went overboard with my level of detail. Heck, even when I was building it I knew I was going overboard. But it is what it is, you know?

When all is said and done, I have continuous blanket insulation over my dome and over the vent arch. With the blanket doing its best to prevent heat loss from the structure as a whole, there's still going to be heat transfer from one structure to another. But...we do what we do.

2) Between my oven floor brick and my vent arch floor brick, right now my only separation between the two is a break in the brick floor pattern. My oven floor is herringbone, my vent arch floor is a running bond. Honestly, I see no need to get more technical than that, there is a temp drop at the break "as is". However, to take things further (lol...because that's what we do, you know?) when I get my insulated door on I'll take more temp readings. If I feel the need, I'll simply undercut the sides of each brick at that pattern break to create an inverted "V" gap between the two patterns, and either leave it as a void, or I'll probably tuck a wedge of board insulation in there. Again, who am I kidding. I'll add the wedge of insulation. Because that's what we do! lol

This is probably a very minor point...but even though my oven has only been cooking for a couple of weeks, I've cooked a few times inside the entry arch, with a pan on the entry arch floor brick. I've placed a door over the oven dome entry to keep high cooking temps in there, and another door over the face of the entry arch to maintain moderate temps inside the entry arch, with cooking pan placed on the entry arch floor. Having the entry floor brick insulated helped steady the entry arch temps during a long dehydrating "cook" of some tomatoes and basil, or whatever else we have happened to have in there.

mongota This is just what I needed, thank you!


So, I've all but given up the idea of the Masonry Chimney, given that I need to transport this, I think the best option for me will be to use a pipe for now and the once the oven has been moved, then I can consider adjusting it.

The pictures were very helpful, however I had a question as I noticed that you painted the exterior surfaces of your Fb Board (for waterproofing I assume), what product did you use to do this? <--- EDIT, I saw your post about RedGuard, thanks!! (https://community.fornobravo.com/for...399#post392399)

Your explanation of the heat break w/the ceramic fiber rope is perfect, do you remember the diameter rope that you used?

The idea of different patterns of brick for the entry arch floor intrigues me, however, I'm confused how this helps as a heat break? I think I'll also go with the inverted 'V' and stuff it full of Ceramic cord.

I know I'm probably overthinking, but as an engineer, it's hard to turn that part of my brain off. Thanks again for the input!

[bentedesco]

Updated, I just called the RedGard tech support line and asked them the operating temperature thresholds. It looks like RedGard will burn off over 165F. Given this, I'm not sure how long the RedGard will last.

[mongota]

Updated, I just called the RedGard tech support line and asked them the operating temperature thresholds. It looks like RedGard will burn off over 165F. Given this, I'm not sure how long the RedGard will last.

The RedGard has 4" of board insulation between it and the floor brick. Before I added the stucco shell over my dome, I had a couple fires and slid a thermocouple underneath the floor, between the 4" of board insulation and the hearth slab. The hearth slab has the RedGard painted on it. I had readings between 92 and 95 degrees as I moved around the circumference of the floor.

A cement-based waterproofing material like Thoroseal is always an option. I used that over the top of my stucco.