2024 Neapolitan oven build

[daidensacha]

Insulation and Heat Break.

I’ve been struggling to understand exactly what it is and how/ where to incorporate it. I’ve read many explanations, but i can’t grasp the gap. In Insulation and heat break I found a little clarity.

Some put the heat break between the oven arch and the front arch, which David says means it will be warmer. David places his between the front decorative arch and the flu vent I think if I understand correctly, which is cooler.

I this plan, I made my vent 230mm, 1 brick in depth, whereas formerly I had it at 114mm, or half a brick, and wasn’t sure it was enough. As it is now, I could incorporate a gap between the inner arch and the outer arch, filling that gap with insulation to prevent heat loss. Is this correct?

Also, is the size I have planned for my vent overkill, and could I get away with half a brick depth? I want to eliminate as much as possible smoke venting from the front of outer arch.

What to fill the break with? 8:1 vermiculite mix with clay or lime instead of cement?

[david s]

Creating a flue gallery in brick seems to be popular, but has some big drawbacks. Firstly it is really heavy and it therefore tends to act as a heating, drawing stored heat in the dome then lost up the flue and from any of its surfaces that are not insulated. This is the reason some builders isolate it from the dome in order to reduce heat transfer by conduction.
Secondly constructing curves in small brick units is difficult and introduces many weak points.
using sheet metal, preferably stainless is a good alternative because it can be made relatively easily and results in much lower thermal mass. Overcoming the massive heat sink problem.
I place a gap between the outer decorative arch and the flue gallery because my cast gallery is quite light (only around 10kg or so which means not too much thermal mass to act as a heat sink. By doing this it allows the inner oven and gallery to heat and expand as one unit, but still having room to move inside the cooler outer shell which has the outer decorative arch integrated to it. I’ve seen many ovens with cracked front arches caused by the inner oven parts expanding against the outer and cooler decorative arch..I use blanket in the gap, sealed
off with some 6:1
Vermicrete.

[daidensacha]

david s In the image above, if I would reduce the depth of the outer arch to 1 brick - 230mm, and and the vent to half brick - 114mm, the vent would then be 373mm wide and 114 mm deep. I hadn’t accounted on the measurement to add a decorative arch, so this would then allow me 114 mm at the front of the outer arch to have a heat sink (What size gap is recommended?) and then a decorative arch which would protrude from the final mortar coat.

This leads to my next question, with what do you cast the vent? I asked the supplier I am buying my materials from if I need refractory concrete, and he replied while he’s not an engineer, he thinks normal concrete would suffice.

In my plan I have an inner flu made from same material as firebrick, with a 25mm gap filled with fire blanket, then bricks and mortar. Between the arch and the flu I’m building formwork to cast my own. This will be covered with mortar, with bricks in the front. It will take the weight of the chimney, and flu. Is concrete a good material, or would vermicrete at say 5:1 be good?

[david s]

Any gap to reduce heat transfer by conduction does little to prevent heat transfer by radiation, but a gap of around 8mm does work to reduce heat transfer significantly.
I cast my flue gallery using castable refractory. But as the flue gallery sees much lower temperatures than the oven dome or floor there are alternatives. I don't think standard concrete is adequate, but using the homebrew as a castable does stand up to the service temperatures we use ok. Creating a mould, then making a casting from it that will release properly is difficult and not really viable for a one off cast. However a hybrid solution is far easier to make using a sand hump mould covered in cling wrap, reduces thermal mass and also allows for a much shallower entry which makes working the oven far easier. The drawing should tell the story. It show a butchered anchor plate set into the wet casting for later attachment of the flue pipe. The second drawing shows how a more funnelled gallery performs better.

[david s]

I was looking through some old drawings and came across these ones that you might find helpful. Regarding the steel bracing of a low dome you may be interested in this method which does not require pinning the band to the supporting slab either side of the entry. Obviously you can't have the band across the front of the entry.

[daidensacha]

I was looking through some old drawings and came across these ones that you might find helpful. Regarding the steel bracing of a low dome you may be interested in this method which does not require pinning the band to the supporting slab either side of the entry. Obviously you can't have the band across the front of the entry.

There is so much I don’t know, but I’m never shy about asking questions when I don’t. I spent the whole day yesterday reading and researching. In searching the forum for more info about heat breaks, I came across Napolitian style 106cm build inside a hobbit house with images showing exactly as you illustrate. His brickwork is really a sight to see.

I have options, that doesn’t look hard to do, the angle has threaded rod connecting the two oven the arch. He has two uprights to give support to any pull from the dome. Stainless band is available here and not expensive. What’s not clear yet is how he created tension in the strap. I was looking at metal pallet band last night, and how they fasten that. Yesterday I also looked at yachting stuff, for tensioning ropes to see if I could adapt that to this application. What he uses in his images I have seen before, but I’m not sure what it’s called, and I’m still googling to find out.

I attached this image to show how i did a quick mock up with the angle at the side so the arch. On the left I did another mockup with a smaller opening for the flu, and a 10mm heat break between what would be the decorative arch in red and the outer arch. I have scraps of “Siebdruckplatte”, high quality ply with waterproof coating on it, which is great for making non stick formwork. Perfect for the oven vent.

[daidensacha]

Any gap to reduce heat transfer by conduction does little to prevent heat transfer by radiation, but a gap of around 8mm does work to reduce heat transfer significantly.
I cast my flue gallery using castable refractory. But as the flue gallery sees much lower temperatures than the oven dome or floor there are alternatives. I don't think standard concrete is adequate, but using the homebrew as a castable does stand up to the service temperatures we use ok. Creating a mould, then making a casting from it that will release properly is difficult and not really viable for a one off cast. However a hybrid solution is far easier to make using a sand hump mould covered in cling wrap, reduces thermal mass and also allows for a much shallower entry which makes working the oven far easier. The drawing should tell the story. It show a butchered anchor plate set into the wet casting for later attachment of the flue pipe. The second drawing shows how a more funnelled gallery performs better.

There is a guy on Youtube with a channel only about concrete, he knows hi stuff. So many different playlists, but the one I that drew me to his channel was about vermicrete. You might be interested in his playlist of lightweight aggregates, He makes and tests the strength and weight of vermiculite and perlite at different ratios. Another aggregate he mentioned was pumice. My ears pricked up because I have 10kg of 2-8mm lava pumice I bought for bonsai years ago. I’m going to test a mix of vermiculite and pumice to see if it would be a viable mix for the vent. In the end of the day it might just be worth coughing up 50 bucks and getting a 25kg bag of castable refractory cement.

On the image above, with the tow different size vent openings. I’ve seen a variety builds but little advice about the vent opening size and what is recommended. On the right it seems overkill, and almost like putting an open window in the oven dome to cool the house down. On the left, I went wide for when the fire is initially lit and smoke is pouring out, but it is really depended on the pull of the flu that will create the vacuum to draw the smoke up and out. That with an inward flow from the arch should create a good outlet. I’m working on my vent shape, where it meets the flu per your drawings, to improve it.

[daidensacha]

Mortar: Waterproof or Vapor permeable water resistant?

I wonder if someone has some knowledge that would help me choose the best option?

My final order is almost ready to be put in, but I’m having a dialog with the supplier who has 2 types of mortar, and is recommending the vapor permeable water resistant one. Here is a breakdown of the two products.

1. Weber.dur 136 oven plaster - light plaster mineral - grain size 0-1mm - 30kg

• water-repellent
• water vapour permeable
• offers optimal adaptation to heat-insulating masonry

2. Weber.dur 120 oven plaster - cement plaster - grain size - 0-3mm - 30kg

• water-repellent
• weather and frost resistant
• mechanically highly resilient
• offers optimal adaptation to heat-insulating masonry

Note: I am in Germany and the German supplier knows and sells to the German market. Most ovens I have seen here have a permanent fixed door that seals closed for heating and usually stays closed.

The supplier recommends the Weber.dur 136, and says that the water permeability is important to allow any moisture in the oven to escape. I was planning on waterproofing the oven in the end. While it will be under a terrace it will also catch rain and snow in heavier weather conditions. My thought is if water can get out, water can get in. I’m accutely aware of the importance of ensuring the oven remains try during the build, so will keep it covered. Also ensuring adequate drying time between vermicrete coats. I thought that when cooking with the oven at 500C, any moisture from the cooking will vaporise, and be vented though the oven opening and flu vent.

Did I misunderstand something? Is it better to waterproof, or not? Is water permeability more important than water proofing with this style of oven?

Any feedback really appreciated.

UPDATE: Read through thread Waterproofing dome and it seems this topic is well covered. Vapor permeable is the way to go.

[david s]

Where are you intending to use this product? Is it as a mortar for the inner brick dome?
Or Is it a cement render for the exterior?
What temperature are you expecting it to withstand in the area you intend to use it?

[daidensacha]

I was exploring using it to cast a light weight vent between the top of the arch and the flu. See image dotted lines. I just found out I get a 5% rebate for signing up to emails from the supplier I’m getting my materials, so I might just spend half that rebate on 25kg of castable refractory.

I’m not sure what temp it gets in the vent, but far from the temp of the dome. I think it would stand the heat there. The structure of the chimney will be built around it. The harshest treatment it might get will be when it gets cleaned.

[david s]

The products you referenced are mortars and are not likely to be suitable. Get a bag of dense castable refractoryy. It contains calcium aluminate cement which is more heat tolerant than ordinary Portland cement OPC, high temperature aggregates and burnout fibres which reduce the risk of explosive steam spalling.

"I’m not sure what temp it gets in the vent," Put it this way, the black soot in my flue gallery never burns off and that happens at around 300C. OPC starts to degrade north of 300C, but to be safe castable refractory would be a safer choice for your flue gallery

[daidensacha]

Neapolitan dome reinforcement

David kindly brought this to my attention, that because of the accentuated curve of the Neapolitan style oven, it requires additional reinforcement of the lower soldier course. I have a clear idea of what needs to be done, see images posted by David a few posts before this one. See also in the images provided for oven build in Napolitian style 106cm build inside a hobbit house.

This sent me down a rabbit hole. I’ve spent countless hours researching, googling trying to find similar applications of this type of reinforcement to see if I can buy some stainless steel bands. I came across 2 example where it is used. Reinforcing walls of old water tanks, and hot tub hoops. In the US, it might be easier to get hot tub hoops made to order. Here in Germany not, but I did find a hot tub complany in Estonia that would gladly make and send them to me in Germany.

I‘m no expert with steel, but I found out in Germany V2A Stainless steel (also known as 1.4301), or V4A stainless steel (the newer version) is rustproof, heat resistant to 600C, and easily welded using all electrical methods. It is relatively inexpensive too when I contacted a supplier to ask about the price of 2 x V2A stainless steel bands 3000 x 50 x 1.5mm.

I‘m making brackets similar to what is used in the Napolitian style 106cm build inside a hobbit house. I will connect the band to the brackets with solid stainless steel rivets. The Bands will be secured to the front of the oven with M10 bolts anchored to angle at the arch.

I‘m just documenting this in case someone else also needs information about bands and what to use, or where to get it. I‘m going this route as it is by far the most inexpensive, and will give a good result. Hot tub hoops would also work, if your somewhere where you can get them.

[daidensacha]

Oven Dome Temperature and reinforcing band material

In theory I understand my goal with the oven is to heat and cook at 500C. This is purely conceptual for me as I have no experience yet heating an oven with wood, and cooking in one at those temperatures.

I also understand the quantity of wood required, and therefore heat up time is relative to the oven size and thermal mass that needs to heat up to temperature. That said, in choosing the correct material for reinforcing bands for my oven, what I need to consider is the temperature the metal bands will be exposed to. V2A, or 1.4301 stainless is heat resistant up to 600C. WIth its anti corrosion properties it would also be resistant to rust from any accumulation of moisture in the dome.

Bear in mind placement of the reinforcing bands around the outside of the brick dome. I wonder if someone with experiential knowledge can share with me what temperature they would be exposed to sandwiched between the firebrick some and the insulation blanket? I’m not sure if I need to look for something with resistance to higher temperature.

[rsandler]

I can't speak for the exact temperature on the outside, as I've always had my ovens insulated before going to full cooking temperatures, but I have never gotten the lower dome walls on either of my ovens (both hemispheres) above even 500C on the inside. The inside will generally be hotter than the outside, so surely we're talking lower still for outside temperatures. The peak of the dome will definitely get above 500C--can't say exactly how high as my IR thermometer only goes to 550C--but the lower walls are always cooler. I'd defer to others like David S if they think differently, but would think that a material resistant to 600C would be just fine banded around the lower part of the dome walls.

[daidensacha]

rsandler Thanks, I’m moving forward with my plan to build the brace reinforcement out of V2A Stainless steel.

I picked up 4 x 100mm lengths of x50x3mm flat V2A stainless steel, and 4 x 40mm lengths of V2A 25x4mm stainless tube yesterday. Called into a local engineering workshop today and they were more than happy to weld them together for me.


All tthe other pieces are on the way.
1 meter M16 A2 stainless threaded rod, with washers and nuts. ( I will cut this into 4 x 250mm lengths, or shorter, once I know what’s needed.)
2 x 3000x50x1.5 V2A stainless sheet strips.

This will last my lifetime.

All I have to do is drill the holes, and join the bands with solid stainless rivets. But I’ll leave that until when I have the dome and arch up, and know the exact measurement I need to fix them. Having a plan with the details worked out puts me at ease.