2024 Neapolitan oven build

[david s]

With so much insulation blanket the insulation value of your p/v crete doesn’t matter much, so it’s primary function is just to restore an even shape and surface for a final render/stucco. Once it has set and dried a bit (usually around 24 hrs you can carve back any high spots and fill any low spots with the same mix. After covering the whole thing, simply tapping the surface with a trowel produces a very good surface to provide a suitable substrate for the render coat (after drying and drying fires).

[daidensacha]

Oven Insulation Started

I guess to much is better than not enough. When I ordered everything for my oven back before spring, I did some calculations and ordered 3 x 7.4m x 610mm x 25mm rolls of ceramic fibre blanket. Then I saw an offer a month ago and ordered 1 x 14m x 610mm x 13mm roll to use in the vent and chimney. I had about half the roll left when I finished insulating around the vent and flue in the chimney, and I filled the chimney from the arch up to over the vent.

So yesterday I got stuck into covering the dome with ceramic fibre blanket. After covering the dome with 4 layers, I had lots left over. So I added another 2 layers on top of the dome. It now has 100mm around the sides, and 150mm on the top, with about 200mm over dome where it meets the chimney. It wasn‘t planned to have so much, and maybe it’s overkill, but better than having it sitting in my garage and not using it. I covered it with a layer of chook wire, with screws into the calcium silicate under the dome to hold the wire.

In the afternoon I refitted one of the hinges for the doors, and they now fit flush with the door jam under also fitting. I need to patch some of the render around the hinges, and the door jam, but I‘ll do that when adding the final layer of render on the chimney.

Today I started on the first layer of homebrew v-p-crete.

Recipe: (parts

1. 5 parts Vermiculite
2. 5 parts Perlite
3. 1 part cement
4. 1 generous handful of clay powder per 1 liter of cement
5. Water to mix by hand into the right consistency.

I‘m no expert and my experience with this recipe is limited. The clay powder seems to make it easier to work with, besides making the mix a nice brown color. Theres a knack to adding it, and I got that pretty quick. Scoop up enough to fill both hands, then plonk it in place against the dome with one hand holding it against the dome, and the other patting it down flat on top. Adding horizontal layers one at a time until it curves over and on top of the dome. Then I poured a bucket load on top, spread it out, and patted it down to around 35mm thick. Possibly a little more in places. When finished, I scraped back the high points, and filled the low points to get s somewhat even shape over the dome. I put a vent in the top as well. A hydraulic vent (as described in the Forno Bravo forum) screwed into water pipe. It sits on the ceramic fibre blanket, and will give me peace of mind knowing if there is for whatever reason a buildup of pressure in between the dome and outer render then it has a release.

Now the wait for it to dry so I can do the next layer.


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[david s]

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 it’s not exposed to the direct fire, only the. exiting smoke.
[/QUOTE]


Regarding the perlite vs air comparison, as well as the perlite eliminating air flow in the space, it also introduces a barrier for radiant heat. On pondering this I remembered an innovation used in window double glazing of windows where the space could be filled with loose insulation (I think it was polystyrene beads which were pumped into the space when required, or pumped out if views were preferred.
Regarding K values, they are the reciprocal of R values. There are many different units used for insulation value, which makes things confusing. Time is also an important factor, sometimes not included.
Thinking about your problem last night when I was firing one of my ovens, one with a double flue (5”/6”), the outer pipe after one hour of firing, I could hold my hand against the outer pipe for about 2 secs. After 2 hrs only 1 sec.
You should get better insulation with yours because the usual set up with a double flue is a one inch space rather than mine of only 1/2”, which was also just air and not filled with any type of insulation.

[nlinva]

Yes, I have the same fingerprint issue. My computer, which I can normally wake up from sleep with my finger, no longer recognizes it. :-)

[daidensacha]

Jobs for the week

I had my first day off today since I can‘t remember . I covered the oven with tarps last night as the render over the concrete brickwork was drying much slower than that over the old recycled clay bricks. This morning its stiffened up nicely, as it stayed reasonably a warm 12C overnight.

My main task this week is to insulate the dome with ceramic fibre blanket and the first layer of V-P-Crete. I‘ve listed a bunch of other tasks to do when I have spare time.

1. Cover dome with 100mm of ceramic fibre blanket.
2. Fill chimney and sides of arch with Perlite.
3. Mix and cover dome with 35mm of homebrew V-P-Crete.
4. Oak doors: Need to refit one hinge so the doors fit squarely together when closed. Have a little repair work to do on the render around a few hinges.
5. Tile under granite landing.
6. Add final layer of render over the chimney and arch frontage.
7. FIt gutter and downpipe for terrace roof (sheets hopefully delivered end of next week)
8. Fit light on front of oven above arch when render is dry enough.
9. Fit light switch, and connect wires to light and mains. (not connecting into the mains until I have the roof on)

I‘m excited that I begin to see a light at the end of the construction phase. So happy that I cut my last bricks as my hands showing signs of the work they have done. I applied for a new passport a week ago, and had no fingerprints left, haha. I explained I have been building a pizza oven and that the stones are like sandpaper. What to do….

My todo‘s:

1. Measure and order granite for the work benchtop once the oven is rendered and I can be sure of the exact measurements.
2. Source a nice slice of an oak tree truck for the bar at the back of the work benchtop.
3. Order double walled stainless steel flue for extending the flue up through the roof.
4. Add crown on the chimney once the flue extension is done.

[daidensacha]

Looks great, even though it seems like a shame to cover up all your lovely brick work!!

Also, it's amazing how the chimney ends up (temporarily) just dwarfing the actual oven dome. That's going to change with the insulation over the dome, but I am always surprised with how large the chimney structure ends up being, especially given that, strictly speaking, it is basically optional!

I also put my landing on today (slate), so blankets are next for me too.

My plan per my design was to render. The chimney construction ended up consisting of Schamott bricks at the arch, left over concrete paving stones building up the sides, and 200 year old hand made recycled bricks above the arch. I do love exposed brickwork, but with the mix of bricks in didn’t think it was nice aesthetically. So was happy to render it and when painted it should look impressive.

The old bricks are 250x120x65mm on average. I cut the front bricks down down to 80mm thickness, and with a Schamott flue (ID Ø200mm, OD Ø240mm) the chimney ended up 600x700mm at the top. That’s with 35mm ceramic fibre blanket around the flue and approx 30mm space between the inner front and back wall so I could fill with perlite. Building with bricks, I think this is absolutely minimum and next time I would leave more clearance front and back of the flue. I’m keeping my fingers crossed that it will have sufficient insulation the keep the walls to an acceptable heat.

it’s also possible to put just a double walled insulated flue on the vent, and it would not be so imposing. In my case I wanted to to be imposing, a statement. Everyone who sees it is impressed. It is an authentic Neapolitan pizza oven, or meant to be.

I am my harshest critic, for good or bad. If I left the brickwork exposed as it was, I would have been unhappy every time I saw it.

This has been great to build and learn what I like and what I could do differently. Next time I could do exposed bricks, but I would plan for it so the entire frontage is the same type of brick.

[nlinva]

Looks great, even though it seems like a shame to cover up all your lovely brick work!!

Also, it's amazing how the chimney ends up (temporarily) just dwarfing the actual oven dome. That's going to change with the insulation over the dome, but I am always surprised with how large the chimney structure ends up being, especially given that, strictly speaking, it is basically optional!

I also put my landing on today (slate), so blankets are next for me too.

[daidensacha]

Granite landing set, Chimney rendered

This morning I set the granite landing in place in front of the arch. Then completed the first coat of render on the chimney.

It‘s been a big week, and if I‘m honest, i‘ve surprised myself how much I was able to do. Good to have no concepts about limitations and to tackle tasks one at a time as they come. We had our first frost overnight, with a maximum of 14 today and for the next days, then next week it gets warm again, a 19-20C window which is perfect for adding the first layer of v-p-crete. That means tomorrow or Monday I‘ll add the ceramic fibre blanket over the dome. Possible on Monday i‘ll add the second layer of render over and around the chimney if the first layer is dry enough.

Whats nice now is that with the render added, its very clear that the visible space over the dome at the back of the chimney is where the insulation will be added.

As you can see my roof sheeting is still not delivered, but fingers crossed it‘ll be here in a couple of weeks and I‘ll be able to cover the terrace and pizza oven properly. Looking forward to that.

[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.