28" homebrew cast oven in walled enclosure Belgium

[Kris S]

Hi all, I plan to post my progress in making my first cast oven, after having plowed through the masses of excellent advice and opinions.

Just some of the threads that I found particularly useful:

https://community.fornobravo.com/for...-castable-dome
https://community.fornobravo.com/for...-uk#post426012
https://community.fornobravo.com/for...e-design/page2
https://community.fornobravo.com/for...st-midlands-uk

I plan to go for a small size oven of 70cm – 28” internal diameter, because it will be mainly used for cooking like 6 or 7 pizza’s for the family.

I really, really like the look of an igloo, and really don’t like the look of an enclosed oven, but living in Belgium means I really have to protect it permanently from the elements. Our weather is comparable to the UK: rather wet than dry . I looked hard for a perfect weather proof final render hoping to avoid enclosing, but the general consensus is it doesn’t work because sealing it shut means vapor inside can’t escape which in turn means slower heat up times needing multiple drying out fires… so a walled enclosure it is…

I’m going for a homebrew cast oven with 5cm / 2” thick walls

This is my plan, dome internal diameter: 70cm – 28”, dome outer diameter: +/- 104cm / 41”.

Foundation:
1,35 x 1,65m concrete slab 12cm / 5” thick reinforced with rebar

Stand:
1.25m x 1.5m dry stacking hollow concrete blocks, filling every other hole with rebar and concrete.

Width: I’m thinking 10cm / 4” extra space around the outer dome edge to accommodate enclosure walls so 104 + 10 + 10 = 125cm / 49”.
length: I’m guessing I’ll need an extra 20cm / 8” for the vent landing / flue gallery (like to keep it as short as possible) and the extra 10cm for decorative arch and walls so 104 + 20 + 10 + 10 = around 145 – 150 cm?
I’ll have to draw it out on cardboard first to get an exact figure for the length.

Table surface:
Dimensions are the same as the stand:1,25 x 1,5m.
concrete slab 9cm / 3.54” thick reinforced with rebar

Hearth:
Was first looking at the CaSi boards, but thery’re expensive and bulky (2.4 x 1.2m) so I guess I’ll go for a 10cm / 4” 50/50 vermiculite/perlite and cement layer in ratio 7:1 (should be strong enough to support oven while also insulating enough)

Dome:
Homebrew (sand, fireclay, lime, cement) cast with PP fiber, but without SS needles. Walls 5cm / 2” thick.
Probably cast the flue gallery separately
5cm / 2” fire blanket on top
5cm / 2” vermiculite/perlite and cement layer in ratio of about 8:1
Final 2.5cm / 1” render of some cement/sand mixture.

Door opening:
Using the 63% internal dome height rule the door opening height will be 22cm / 8.7” and the width will be 48cm / 19”.

Vent/chimney:
The vent is part of the cast dome/entrance structure, 15cm / 6” diameter single walled stainless steel pipe 1m long +/- 3 feet.
I’ll try and cast the gallery separately with a flat area on top on which the chimney would simply rest unattached. Fireblanket up against the pipe, also PVcrete up against it because it has enough give when the pipe expands. Making sure I render the PVcrete high enough upwards against the pipe so the pipe has something to sit into and not fall over. Final cement based render not touching (using a cardboard spacer around pipe) and later filling the gap with heat resistant silicone.

[Kris S]

I still have plenty of questions:

1)
Can I put the concrete slab directly on the dug out soil (with plastic foil underneath), without first putting a layer of compacted gravel under it?
We do not live in an earthquake area and my soil is pretty hard and sandy – loamy. The oven being a cast instead of heavy bricks, with a lightweight enclosure made of hardiebacker panels shouldn’t be too heavy? We usually don’t get a lot of frost around here so I’m not too worried about frost pushing the concrete slab upwards.

2)
Since the oven will be enclosed, is it necessary to put a 5cm / 2” vermi/percrete insulation layer on top of the 5cm / 2” fireblanket insulation to act as 1) insulation and 2) a firm surface on which to put the final cement based render?
is it possible to skip the vermic/percrete layer and final render layer and instead just add 5cm / 2” fireblanket extra, so: cast + 10cm / 5” fireblanket. And then just enclose it?

3)
I like the idea of a removable chimney, that way I could get away with keeping the walled enclosure lower, and making it less bulky. This is one design I kinda like, and it looks like a low profile roof with quite a short chimney. So I was thinking of a removable cover stone on top to access the ‘free standing’ pipe and insert / pull it out when in use / not in use. Is this a good idea? Anyone done anything like that?

[Kris S]

Also:

As a possible alternative to dry stacking the hollow 40 x 20 x 14 cm (15.7 x 7.8 x 5.5 inch) concrete blocks, I like the thinner look of the smaller full concrete blocks as used in these 2 builds:

https://community.fornobravo.com/for...ble-dome/page2
https://community.fornobravo.com/for...-cast-scotland

Is it pure coincidence that both builders used an 'E' pattern with a middle wall? Is this necessary from a strength point of view with these thinner blocks, or is it just for aesthetics?

thank you for any feedback,

Kris

[david s]

I still have plenty of questions:

1)
Can I put the concrete slab directly on the dug out soil (with plastic foil underneath), without first putting a layer of compacted gravel under it?
We do not live in an earthquake area and my soil is pretty hard and sandy – loamy. The oven being a cast instead of heavy bricks, with a lightweight enclosure made of hardiebacker panels shouldn’t be too heavy? We usually don’t get a lot of frost around here so I’m not too worried about frost pushing the concrete slab upwards.

2)
Since the oven will be enclosed, is it necessary to put a 5cm / 2” vermi/percrete insulation layer on top of the 5cm / 2” fireblanket insulation to act as 1) insulation and 2) a firm surface on which to put the final cement based render?
is it possible to skip the vermic/percrete layer and final render layer and instead just add 5cm / 2” fireblanket extra, so: cast + 10cm / 5” fireblanket. And then just enclose it?

3)
I like the idea of a removable chimney, that way I could get away with keeping the walled enclosure lower, and making it less bulky. This is one design I kinda like, and it looks like a low profile roof with quite a short chimney. So I was thinking of a removable cover stone on top to access the ‘free standing’ pipe and insert / pull it out when in use / not in use. Is this a good idea? Anyone done anything like that?

Vermicrete insulating slab copy.doc.zip

It depends somewhat on the soil you are laying your concrete foundation slab on to, but yes generally you need to compact it with some aggregate first.
I don't like those thin 100mm solid blocks, give them a decent push and you'd be able to knock them down. However for the size of your oven I think the 150mm wide hollow blocks are sufficient, no need to go to the wider 200m blocks. By filling the cores with concrete you tie the blocks together and produce enormous strength especially if some steel bar is used as well.

The big advantage of the enclosure is that you don't have the issues of drying verm/perlcrete insulation. The enclosure can be filled with dry loose perlite or vermiculite over the blanket.

It is generally recommended that a 5:1 vermicrete be used for the underfloor insulation. Anything leaner is not considered strong enough to take the weight of the over sitting on top of it. The mix over the dome however is better at a 10:1 mix as anything richer is a poorer insulator (if doing an enclosure you don't need a a wet mix over the blanket)

[Kris S]

Thanks for your reply David!

Allright, I'll stick with the 14cm wide hollow bricks.
Also better safe than sorry, so I will look into putting some compacted gravel under the slab.
I'll use a 5:1 ratio for the underfloor vermicrete (instead of 7:1)

So, when enclosed, I can skip the vermicrete insulation, that's helpfull.


Still undecided about the chimney pipe, any thoughts about a removable chimney pipe anyone? I would like to avoid a chimney pipe that's too high, but understand it might be necessary to create a good draft for the smoke. The chimney height in the above picture looks rather short imo.

What I'm most unsure about is how to flash the area where the rectangular chimney walls and the other parts of the roof come together.

[Kris S]

So here's what I think it would look like: any gross errors?

5cm / 2" wall
10cm / 4" fireblanket (no vermicrete or final render layer because oven will be enclosed)
oven opening 48 cm / 19"
vent gallery would be like 2.5cm or 1" wider dan the oven opening to have a rebate for the door
likewise the decorative arch would also be a bit wider so I can put in a 'storm door' during winter.
over the vent gallery I would reduce the insulation to 5cm / 2".

my main question is about the depth of the vent gallery (correct terminology?): the single walled chimney will be 15cm / 6" in diameter so at the moment I'm thinking 5cm / 2" either side the chimney makes for a 25cm / 10" vent gallery depth.
Does that sound about right?

thanks in advance for feedback!

[Boogie-D]

Aloha Chris... I am somewhat stuck in this area too.. and trying to figure out the galley length... waiting for feed back too... at least you know your chimney size... I get so confused with my 33.5 oven and the conflicting advice in chimney but I am leaning towards a 6 inch chimney diameter too..

then in an effort to keep the galley short and easier to work with I am going to add enough room for casting as your thinking... in my case 2 1/2 thick but to the front and back of the pipe so totaling 5 inches for me..

my thought process too is the diameter of chimney plus casting room then can figure out galley floor... I am thinking if I go with 6 inch chimney... add 5 inches for my casting... then 11-12 inches should do my galley floor.. 12 inches galley floor would accommodate a resting place/warming place for my 12 inch pots and pans... but I am still waiting for the expert advise here

[Boogie-D]

I like your idea of a reveal on your decorative arch for a storm door... might be useful for smoking food too.. we have pretty similar plans and at similar stages... I’ll be following along... cheers

[Kris S]

Dug the foundation, it's a little bigger (1.35 x 1.5 m) than the hearth dimension (1.2 x 1.3 m).

Pouring the slab is for next weekend, maybe.

[blink]

If the dome is going in an enclosure wouldn't it be easier to skip the 100mm concrete insulation layer and just loose fill the enclosure with loads of cheap insulation?
I'm planning something similar - a 30" dome in an enclosure with aerated clay loose fill insulation of about 150mm all round, and probably 200mm in top.

[UtahBeehiver]

Yep, several builders have just poured in dry perlite or vermiculite in the enclosure. Blocking out the corners with spacers, IE plastic soda bottles or scrap sheet rock. Are you talking leca balls as aerated clay?

[Kris S]

concrete slab poured...


f3.jpg

[Mullster]

Hi Kris - great to see your build underway and really glad my thread gave you some info. Let me know if any questions on what I did on mine - there are plenty of way more experienced builders for any general questions though!

Just addressing the bricks I used for the stand - to be quite honest I was limited by the fact it was early in COVID and building supplies were impacted. I’m confident the structure has turned out strong though with the E formation.

good luck with the build

[Kris S]

Hi Mullster, nice to see you popping by.

Concerning the bricks: I'll go for the slightly larger 14 x 20 x 40 hollow conrete bricks.

One question I have right now is what grain size of sand I should use for the cast. Shoudl I go for sand with grain size 0-2mm or 0-4mm, does it even matter much?

[david s]

Hi Mullster, nice to see you popping by.

Concerning the bricks: I'll go for the slightly larger 14 x 20 x 40 hollow conrete bricks.

One question I have right now is what grain size of sand I should use for the cast. Shoudl I go for sand with grain size 0-2mm or 0-4mm, does it even matter much?

The larger the aggregate, the less workable will be the mix. A range of grain size is also preferable. The proprietary aggregate size for the castable refractory I use is max 3 mm. I don't think it will matter much because the high clay content in the home-brew assists greatly in making the mix pretty sticky, but I'd be going for the finer 0-2mm. If you are using any of that as a mortar then sieve out the courser sand with a sieve.