2024 Neapolitan oven build

[daidensacha]

Aren't you worried about freezing temperatures at night? Here it's going to be ok for the coming week, but as we are going to be on holiday I would have no way of providing heat to prevent any uncured cement or vermicrete from freezing and destroying itself.

I’m not too worried yet. „Eisheiligen“, first possibility of frost is 10 October here, and although we can have frost, it’s usually not much under 0C. We can still have days mid teens to 20C or so. In my experience covering the oven will go a long way towards protecting it from the odd cold night. It won’t get consistently cold untill usually Dec or Jan. If it’s looking like it’s going to get freezing I’ll put some drop blankets over it with a tarp. I’m almost certain no frost will get under that.

Tomorrow I’m putting my granite in front of the arch, and doing first coat of render on the chimney. Weathers looking good Tuesday-Thursday so planning on getting stuck into the first layer of v-p-crete then. Have everything ready to go.

[Toiletman]

Toiletman Thanks for your spreadsheet examples. Given you showed one for Perlite and one for Ceramic Fibre Blanket, it would be interesting to see how Vermiculite compares. I have all, and can fill the remaining space with any of them. Bits trickier to get the ceramic fibre in now though, so dry Perlite or Vermiculite would be my choice at this stage. Building the oven has introduced me to things I never considered before. Heat movement for example, as explained very clearly by david s early on when explaining the function of the heat break. Heat loss through convection and conduction are to be considered.

A double walled stainless steel flue has 30mm of insulation in it, so my reasoning without going into it scientifically like you, was that 35mm (1.5“) would be sufficient around the flue. The reason I want to fill the remaining void is to prevent/limit the walls heating. Hence I wanted to fill the void to prevent the transfer of heat via convection though the air. I thought that would be more than conduction through dry Perlite filling the empty space.

A question arose reading your chart, where your top end temperature range is 300C. Is that a realistic possibility, that the flue gets that hot? I thought it would be less, given its not exposed to the direct fire, only the. exiting smoke.

Several answers:

• Vermiculite vs perlite: when I did the initial estimations for my oven I came across values for vermiculite of the order of 0.064 W/m.K --> so slightly worse than the blanket. So going for perlite seems the best and most practical option. Indeed fillling it will prevent any convection to occur.
• Going for 300C smoke temperature seemed indeed to be a quite conservative estimate. The flames inside the oven will be much hotter, but they will mix with ambient air going up the chimney + heat loss through the chimney wall. I don't know if anyone ever pointed their IR guns into the chimneys, but it would be interesting to find out what kind of temperature you get there. Anyway: for the calculation you like to err on the side of caution: ie. overestimate the initial temperature, overestimate the convection coefficient and if it's then well safe to the touch you know you don't have to worry too much. And for a relative comparison (ie. blanket vs perlite vs air, it's even less of an issue, as you're more interested in the relative performance than the absolute numbers).

Note: I did take the convection heat transfer coefficient into account for heat transfer between two vertical plates at the stated temperatures and doubled it to be conservative. However on top of that I calculated the heat transfer via conduction through the air, as that's typically what you do for small airgaps between parallel plates (ie. double glazed windows, or oven windows are the typical example).
As an exploration I doubled the resulting heat flow from 140W to 280W and calculated how fast the air would need to flow to transfer that heat across the gap. This amounts to roughly 6cm/s which seems like a very realistic order of magnitude (ie. consider smoke rising from a cigarette). So indeed, David is right and the contribution of convection might be significant --> doing the calculations this would put air closer to the performance of the blanket or slightly worse.
This confirms that putting perlite in the gap is the best solution.
Btw I also completely disregarded radiation heat transfer, which at these temperature might also make a significant contribution.

I don't know if it would work in your situation, but it would be an interesting experiment to do a before and after perlite with test fires to verify the performance.

Putting a light was in my plans, and I had only considered running the cable up the flue and then through the wall. Heat being the major problem, and not knowing exactly how hot it would get. As I planned on rendering the outside of the chimney I decided to put the cable conduit in the wall from outside. Yes its ugly, 100%. But it wont be seen. It’s been a good process, learning lots, and I would do some things slightly differently with wisdom of experience. Like for example, my lintel. I would make my one that follows the shape of the dome next time, to an extent, to allow for good clearance. I would also set the rear all another brick width back, to allow for more space inside the chimney at the front and back of the flue. What I have is functional, but it can be improved.

I see your in Holland. Similar weather to here a bit I guess. I became used to working through rain now, with a tarp over my oven.

Same. Finished the arch with the rain pattering on the tent covering the oven. Aren't you worried about freezing temperatures at night? Here it's going to be ok for the coming week, but as we are going to be on holiday I would have no way of providing heat to prevent any uncured cement or vermicrete from freezing and destroying itself. If you have a heat source (old construction light) for instance you could prevent this from happening.

Fully appreciate the learning experience of building an oven. It has been a great education as well, and like everyone we end up with things we would do differently on a next oven.

[daidensacha]

Cut and set my last bricks, onto the dome insulation.

This morning I picked up more cement and set the last 2.2 bricks on side of the chimney.

Then I drilled 4 dome weep holes on the sides and back of the dome. 2 out of 4 I hit rebar. Luckily I had a good drill and it went through the rebar, although one of them took ages. From the position where I drilled the hole, I remember putting a extra thick on there.

Once the holes were drilled, I custom cut 16 concrete wall stones to taper the tops so I could lay them around the back of the dome insulation. These are inspired by david s who explained per his experience how water condenses and flows down the inside of the dome.

I‘ll give it a day or so to set and take a well earned breather, before jumping into putting the ceramic fibre blanket over the dome, followed by 4 x 35mm layers of v-p-crete.

Some pics from today.

[daidensacha]

Toiletman Thanks for your spreadsheet examples. Given you showed one for Perlite and one for Ceramic Fibre Blanket, it would be interesting to see how Vermiculite compares. I have all, and can fill the remaining space with any of them. Bits trickier to get the ceramic fibre in now though, so dry Perlite or Vermiculite would be my choice at this stage. Building the oven has introduced me to things I never considered before. Heat movement for example, as explained very clearly by david s early on when explaining the function of the heat break. Heat loss through convection and conduction are to be considered.

A double walled stainless steel flue has 30mm of insulation in it, so my reasoning without going into it scientifically like you, was that 35mm (1.5“) would be sufficient around the flue. The reason I want to fill the remaining void is to prevent/limit the walls heating. Hence I wanted to fill the void to prevent the transfer of heat via convection though the air. I thought that would be more than conduction through dry Perlite filling the empty space.

A question arose reading your chart, where your top end temperature range is 300C. Is that a realistic possibility, that the flue gets that hot? I thought it would be less, given its not exposed to the direct fire, only the. exiting smoke.

Putting a light was in my plans, and I had only considered running the cable up the flue and then through the wall. Heat being the major problem, and not knowing exactly how hot it would get. As I planned on rendering the outside of the chimney I decided to put the cable conduit in the wall from outside. Yes its ugly, 100%. But it wont be seen. It’s been a good process, learning lots, and I would do some things slightly differently with wisdom of experience. Like for example, my lintel. I would make my one that follows the shape of the dome next time, to an extent, to allow for good clearance. I would also set the rear all another brick width back, to allow for more space inside the chimney at the front and back of the flue. What I have is functional, but it can be improved.

I see your in Holland. Similar weather to here a bit I guess. I became used to working through rain now, with a tarp over my oven.

[david s]

Find a quick exploratory calculation attached. The difference between air and perlite is not very much --> around 5W of heat loss saved on a 1m2 area, assuming thicknesses as in my exploratory calculation. On the outside wall that amounts to a 1 degree difference in temperature (hotter for air). For comparison: a 4 m/s wind will decrease the outside wall temperature by 4 degrees, so it's all kind of in the margin.
If my thickness estimations are completely off it might change a little bit, but as you can see the R-values of perlite or air don't differ that much, and they're the main contributor to the total resistance. Anyway, if you have the perlite laying around, might as well dunk it in the chimney .

Edit: just noticed I compared perlite against air instead of blanket --> added another picture to add that case, and also assumed that the insulation thickness is much smaller (ie. 50mm) instead of the 150mm I was considering previously. It's great that you have cheap blanket, but it's probably not that much to cover the whole chimney with 150mm thickness.

It changes the conclusion as well: definitely go with perlite or just air instead of stuffing blanket into the gap.

Thanks, that's useful info. Your comparison with insulation vs air does not take into account the movement of air in the space which allows heat transfer by convection. With the space totally sealed off, even expanding air as it's heated will create some movement which allows convection heat loss. Filling the empty space with a very low density/conductive material actually produces a superior insulative result compared to air. Ideally a vacuum would be the best, but I doubt that would be possible for a WFO build.
Here is a very useful table (often shared on this site) which shows the relationship between strength, insulation value and material density.

[Toiletman]

So racing against time to do what I while the weather allows.

Fully recognize the feeling

This last week the project at hand is working on the chimney. Monday I insulated the vent and flue as it wouldn‘t be possible when I brick it up. Got a deal on some ceramic fibre blanket which I got specifically to fill the chimney around the vent and the flue. Yesterday I finished the last course at the top of the chimney.

I used hand made recycled bricks for the main chimney. Lots of cuts to form the curves I wanted, and a little complicated given the variance in sizes. The wet cutting saw was really handy to straighten some bricks when needed. Still was really difficult to keep my walls straight with some bricks bulging, twisting or curving, thicker one end than the other, or one side than the other. This morning I hit the brickwork with a concrete grinder disk to take out the bulges, and whilst super dusty, it came up really nice.

I‘m adding a light to the front of the oven, so I cut a slit in the brickwork, installing cable conduit and a switch box in the brickwork. Backfilled with cement to fix the conduit inside the wall. Then clean up with the messy work done. So much dust.

Lastly today, started bricking up the sides to finish the general curves in the side walls that I want. Ran out of cement with 2.2 bricks to fix in place to finish, so that will be first up tomorrow. Then I‘ll fill the empty space in the chimney with perlite, and move on to the back of the oven. One row of tapered bricks to add around the oven, along with some dome seep holes before starting on insulating the dome.

Wonderful brickwork again man. Really jealous of the curves and angles, great job!

The electricity conduit is kind of an eyesore, but I assume you will plaster/stucco across everything anyway? Too bad that you're hiding that beautiful masonry then, but it will match the bottom of course.

[Toiletman]

Hey, thanks. Am I right then that dry Perlite at 0.03 is has a higher resistance than ceramic fibre blanket (0.06)? If i’m correct then I would use it to fill the empty space around the flu in the chimney.

Find a quick exploratory calculation attached. The difference between air and perlite is not very much --> around 5W of heat loss saved on a 1m2 area, assuming thicknesses as in my exploratory calculation. On the outside wall that amounts to a 1 degree difference in temperature (hotter for air). For comparison: a 4 m/s wind will decrease the outside wall temperature by 4 degrees, so it's all kind of in the margin.
If my thickness estimations are completely off it might change a little bit, but as you can see the R-values of perlite or air don't differ that much, and they're the main contributor to the total resistance. Anyway, if you have the perlite laying around, might as well dunk it in the chimney .

Edit: just noticed I compared perlite against air instead of blanket --> added another picture to add that case, and also assumed that the insulation thickness is much smaller (ie. 50mm) instead of the 150mm I was considering previously. It's great that you have cheap blanket, but it's probably not that much to cover the whole chimney with 150mm thickness.

It changes the conclusion as well: definitely go with perlite or just air instead of stuffing blanket into the gap.

[daidensacha]

Chimney insulated and brickwork almost finished.

So racing against time to do what I while the weather allows.

This last week the project at hand is working on the chimney. Monday I insulated the vent and flue as it wouldn‘t be possible when I brick it up. Got a deal on some ceramic fibre blanket which I got specifically to fill the chimney around the vent and the flue. Yesterday I finished the last course at the top of the chimney.

I used hand made recycled bricks for the main chimney. Lots of cuts to form the curves I wanted, and a little complicated given the variance in sizes. The wet cutting saw was really handy to straighten some bricks when needed. Still was really difficult to keep my walls straight with some bricks bulging, twisting or curving, thicker one end than the other, or one side than the other. This morning I hit the brickwork with a concrete grinder disk to take out the bulges, and whilst super dusty, it came up really nice.

I‘m adding a light to the front of the oven, so I cut a slit in the brickwork, installing cable conduit and a switch box in the brickwork. Backfilled with cement to fix the conduit inside the wall. Then clean up with the messy work done. So much dust.

Lastly today, started bricking up the sides to finish the general curves in the side walls that I want. Ran out of cement with 2.2 bricks to fix in place to finish, so that will be first up tomorrow. Then I‘ll fill the empty space in the chimney with perlite, and move on to the back of the oven. One row of tapered bricks to add around the oven, along with some dome seep holes before starting on insulating the dome.

Interesting, tonight I stuck my head in the front of the arch and said a few words. There is an echo now, which I can only put down to the chimney with insulated vent. I liked it

[daidensacha]

If you google on 'Thermal conductance and resistance' you'll get a wikipedia article that gives examples of how to do a thermal resistance calculation of a composite wall. I've used that method to estimate the insulation thickness and cooldown time for my oven. Once I'm done building my oven I'll turn it into a proper google sheet that can be used by everyone. I could send it to you already 'as is' if you like (at least you're also using metric ).

Basically a lower k-value gives a higher 'thermal resistance'. Typically these thermal resistances are in series (ie. brick/cast - blanket - vermicrete - plaster) and in such a configuration the heat flow is mainly determined by the higher resistances in this series.

Hey, thanks. Am I right then that dry Perlite at 0.03 is has a higher resistance than ceramic fibre blanket (0.06)? If i’m correct then I would use it to fill the empty space around the flu in the chimney.

I finished bricking up my chimney today. Yesterday I insulated the vent and flue with ceramic fibre blanket. The entire top of the arch in side the chimney is covered with ceramic fibre blanket, up to the top of the vent. Then 35mm of ceramic fibre blanket wrapped around the flu. Between the flue and the chimney wall from the vent up it is empty space, Which I guess is also good, but filling it with perlite or vermiculite would stop the walls of the chimney heating up. Your spreadsheet sounds intersting. I think i’ll understand it more when I finished this process of building my first oven.

[Toiletman]

Can you explain in brief what the “K“ values mean? Perlite (0.03), Vermiculite (0.065), Ceramic Blanket (0.06). Excuse my ignorance, what is high and what is low. I saw in one build (in a cold climate) on Youtube where the builder used dry vermiculite totally covering the dome to insulate. He used a lot of it. It’s not for me now where I’m at, but I would like to understand.

If you google on 'Thermal conductance and resistance' you'll get a wikipedia article that gives examples of how to do a thermal resistance calculation of a composite wall. I've used that method to estimate the insulation thickness and cooldown time for my oven. Once I'm done building my oven I'll turn it into a proper google sheet that can be used by everyone. I could send it to you already 'as is' if you like (at least you're also using metric ).

Basically a lower k-value gives a higher 'thermal resistance'. Typically these thermal resistances are in series (ie. brick/cast - blanket - vermicrete - plaster) and in such a configuration the heat flow is mainly determined by the higher resistances in this series.

[daidensacha]

To relieve pressure build up the position of the vent can be placed anywher. However, this pressure build up can also be utilized to help assist the removal of moisture.
I'm convinced that efficient venting of dome insulation moisture (and under floor insulation moisture for that matter) is more efficient with the vents placed at the bottom of the oven for the following reasons.

1. Heat rises by convection, so the fire in the chamber heats the crown of the inner dome faster than any other part of the inner chamber.
2. Moisture travels away from any heat source.
3. Gravity makes water fall.

Pressure build up from steam or just expanding hot air forces moisture present to the outside of the insulation, away from the heat source, as well as down. Some of that moisture will condense against the inside of the cooler outer shell where it will run down or make its way to the base,
Having observed this process firing a new empty kiln, as recommended by the ,amufacturere, I was surprised to see water dripping from the front corner when the kiln's internal temperature reached around 400C ( 750F), which is way above the temperature water turns to steam.

I think the drawing explains the process. While a vent at the apex may seem the most logical, it is the build up of moisture lower down that is more problematic in creating a huge temperature differential and thereby thermal expansion issues. This problem is also evidenced by owners discovering the effidiency of venting the underfloor insulation.

That’s interesting and useful info David. I do have mosaic tile sheets turned upside down under the 10cm thick calcium silicate insulation which is under the oven dome and floor. That extends 10cm further than the diameter of the dome, with a min gap of then 30mm followed by a row of bricks. Any water that flows per your sketch should then flow back out under the floor insulation and through the hearth drainage holes.

Addition: After contemplating your drawing, and theory about the steam condensation and flow of water inside outer render, I decided to add an additional course of bricks around the outside of the calcium silicate insulation. The additional course will have tapered tops so and condensation will flow down the bricks to to the floor, where I could add dome insulation weep holes per your sketch, or allow the water to flow under the insulation before draining through the existing floor insulation weep holes.

A sketchup illustration integrating your knowledge.

[daidensacha]

Are you going to fully enclose your oven with brick? If so you could consider dry filling the space between the dome and the outer brick wall with dry perlite or vermiculite (if you have not already bought your ceramic blanket). Both perlite (0.03) and vermiculite (0.065) K values are pretty good when comparing to ceramic blanket (0.06) and probably less expensive.

I’m only bricking up the front 600mm to enclose and insulate the vent and schamott flue. Here is a plan I drew up and am working to replicate. I do already have the ceramic fibre blanket, plus vermiculite, perlite, cement and dry clay powder. I will fill the empty space in the front bricked up part with dry vermiculite or perlite, or mixture of both.

Can you explain in brief what the “K“ values mean? Perlite (0.03), Vermiculite (0.065), Ceramic Blanket (0.06). Excuse my ignorance, what is high and what is low. I saw in one build (in a cold climate) on Youtube where the builder used dry vermiculite totally covering the dome to insulate. He used a lot of it. It’s not for me now where I’m at, but I would like to understand.

[david s]

To relieve pressure build up the position of the vent can be placed anywher. However, this pressure build up can also be utilized to help assist the removal of moisture.
I'm convinced that efficient venting of dome insulation moisture (and under floor insulation moisture for that matter) is more efficient with the vents placed at the bottom of the oven for the following reasons.

1. Heat rises by convection, so the fire in the chamber heats the crown of the inner dome faster than any other part of the inner chamber.
2. Moisture travels away from any heat source.
3. Gravity makes water fall.

Pressure build up from steam or just expanding hot air forces moisture present to the outside of the insulation, away from the heat source, as well as down. Some of that moisture will condense against the inside of the cooler outer shell where it will run down or make its way to the base,
Having observed this process firing a new empty kiln, as recommended by the ,amufacturere, I was surprised to see water dripping from the front corner when the kiln's internal temperature reached around 400C ( 750F), which is way above the temperature water turns to steam.

I think the drawing explains the process. While a vent at the apex may seem the most logical, it is the build up of moisture lower down that is more problematic in creating a huge temperature differential and thereby thermal expansion issues. This problem is also evidenced by owners discovering the effidiency of venting the underfloor insulation.

[UtahBeehiver]

Are you going to fully enclose your oven with brick? If so you could consider dry filling the space between the dome and the outer brick wall with dry perlite or vermiculite (if you have not already bought your ceramic blanket). Both perlite (0.03) and vermiculite (0.065) K values are pretty good when comparing to ceramic blanket (0.06) and probably less expensive.

[daidensacha]

As long as you feel comfortable that there is an egress point for water vapor to escape you should be good. Liquid water when it sublimates to gas increases volume by about 1500 times hence a need to have some type of mechanism to reduce pressure building up under the finish coat of the oven.

Thanks Russel. Being my first oven, its all new to me, and I‘m working through things like this as best I can.

Attached are some images I took after finishing my work on the oven today. I did my first course around the chimney, and set the lintel in place to support the bricks at the rear of the chimney wall. You can see under the lintel is where I will place the ceramic fibre blanket insulation down to the back of the vent. The vent and flue will also be covered with ceramic fibre blanket. I was going to fill the empty space around the flue in the chimney with vermiculite. The V-P-crete will cover the ceramic fibre on the dome up to the back of the chimney wall. It will create a seal, but as it is open under the lintel to the space between the chimney wall and flue, I guess I could add a copper pipe in there that would go from the ceramic fibre to the top of the chimney.