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Generally not recommended if you look around this forum. Foil keeps water out, sure, but it also keeps water *in*, which you don't want. Best to assume that some water will get in some time, not to mention all the water you will introduce through the mortar, soaking bricks etc. during the build. You want to make sure that any moisture has a way to get out without damaging anything. Weepholes and an air gap (e.g. from mosaic tiles) between the slab and the insulation on the bottom, and a vent of some kind through the stucco on the top, seem to be the way to go.
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Thanks for the input! Very helpful. Isn't it funny there are so many plans and experts out there saying completely different things. haha!Comment
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Check out this thread.Originally posted by totalnewbie View Post
https://community.fornobravo.com/for...use#post391542
Frankly, the extra trouble you have to go to in supporting a form with structural weakness is not worth the effort and extra expense IMHO. The popularity of the hemisphere, with its superior structural form, as an all round performer, has been tested for centuries. There must have been millions made and many efforts to improve on it, yet builders (including me) return time and again to the hemisphere. The Ancient Romans kept building circular arches for centuries because its simple to build and works well. A Catenary arch is self supporting, stronger than a half circle, but more complex to form and unsuited for an oven because of the more severe angle at the base.Kindled with zeal and fired with passion.Comment
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That's an incredible thread. Thanks for pointing it out.
David, do you have an opinion on using aluminum foil around either the floor insulation or the dome insulation to keep water out?Comment
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Yes and my opinion is based on personal experience because I used it on the oven I have at home which is now 16 years old.Originally posted by totalnewbie View Post
My opinion is not to use it, for exactly the reasons explained by rsandler in post #31
I placedfoil over the vermicrete layer in an effort to prevent the dry vermicrete from drawing moisture out of the cement rendered layer to be applied over it. Knowing that it was likely to trap moisture I perforated the foil in two places around 25cm2 3/4 of the way up frm the dome base and at the very top. I thought it could be instructive to see what happened there. It certainly was instructive because if moisture gets into the insulation space, and it will just by accumulated humidity, the fire driving any moisture outward and then accumulating in the insulation space. At the two points where the foil was perforated the outside of the dome is hot to touch while the rest of the dome is just cosy warm. This indicates to me that steam is building up in the insulating layers and pushing through where the perforations are. My outer rendered shell is quite thin (approx 12mm thick so heat can be felt through it more easily.
I now do not use foil in builds. It may work ok if the foil is perforated all over by either buying perforated foil or slapping the foil silly with a flat stick that has several nails protruding from it.
To overcome the problem of the dry vermicrete drawing moisture from the outer cement rendered layer, I simply wrap the whole oven in cling wrap to retain moisture in that layer for a week. It does a fantastic job with visible beads of moisture held under the plastic.
Regarding a foil layer as additional insulation, forget it because as aluminium is highly conductive, heat will easily transfer by conduction if its in contact with another conductive material like a dense cement render. Foil requires an air space either side of it to act as an insulator.Kindled with zeal and fired with passion.Comment
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A thoughtful and experienced answer as always. Thanks!Comment
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Hello Utah or other moderator. Can you please move my thread to the Other Ovens Types category. I've decided to use 3:1:1:1 over a mold. There are ton of good threads on this, and I think the ease of installation and low cost are the deciding factors. I don't really want to spend weeks cutting bricks (even though I have a pile of free bricks). Plus, I finally found fireclay locally. And I can build a relatively low dome by designing my mold.
The current plan is 3.5" of 5:1 vermiculite insulation layer (formed with 2x4 to be lazy), a standard 9x4.5x2.5 firebrick floor. Some people have recommended Whitacre low duty because of lower conductivity. a 2" dome, and ceramic blanket on top. I also downscaled to 36" interior. I've found vermiculite and the blanket on Amazon with free shipping, and just have not been able to find a good source in DMV.
Built in 36" grill and concrete block "cabinets" and work surface on the side.
Comments appreciated.
And a couple of questions.
Is "best practice" 1% poly burnout fibers and 1% basalt fibers?
And where do I buy them in the US? I'm guessing I will buy them online.
I am planning on needing about 600 lbs of 3:1:1:1 castable for a 36" dome and vent. Is that a good calculation? Hoping to not under or over buy materials.
Has anyone calculated (or built a calculator) for how much vermiculite (cu ft) I need for a 36" oven, roughly 6" vent and a 3.5" floor insulation layer? It comes in 4 cu ft bags (about $50 each including shipping).
The back terrace is about done and there is hole in the ground for the oven.
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Sorry. Last question. How many firebricks do I need for the cooking floor? 9x4.5x2.5. 36" oven, 6" landing. I'm sure this has been answered a hundred times, but it's hard to find things on the forum.Comment
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Probably plan on 40-50 bricks for the floor, depending on whether you cut them to fit inside your mold or not. Basic math says your floor area is 18^2*pi =1017 square inches, which at 40.5 square inches per brick comes out to about 25. But you're trying to fit rectangles into a circle, so there will be some waste, plus you need a few for the landing. I had 35 numbered bricks in the floor of my 36" oven which was cut to fit inside the dome. But several of those were fragments around the edges. Add ~7-8 bricks for your landing. You may need a few additional bricks if you are casting on top of the floor. So, 40-50.Comment
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Excellent math. Thanks! :-) They're about $2,00-$2.20 here.Comment
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Regarding vermiculite quantities, work out your volumes using volume of a sphere formula 4/3 x pi x r3 but remember to halve it for a hemisphere. Calculate using outside radius minus inside radius. Also there is about a 20% reduction in volume on mixing with water and cement.Originally posted by totalnewbie View Post
on fibre quantity 1% by volume is about right for the fine polypropylene burnout fibres, but they don’t disperse easily so require around double the mixing that you’d normally use.
I use both basalt and AR glass fibres, which have similar performance for our service temperatures in strength, but (for the ones I have) I way prefer the AR fibres as they disperse well while the basalt fibres tend to clump.
If you are buying the blanket on line then it’s probably cheaper and the old generation which is not the “exonerated as a carcinogen” type. This can be a consideration if there’s any exposed blanket A class2 carcinogen means that tumours have been recorded in lab rats given very high doses of material to inhale, but none have been recorded in humans. I always use the slightly more expensive/safer blanket.
Kindled with zeal and fired with passion.Comment
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Hi David,
What are AR glass fibers?
And to be sure. You are recommending three fibers? (1) poly for burnout, (2) basalt structural and (3) AR glass for [I'm not sure]. Is that right?
And last, for you US builders, does anybody have a lead on where I buy these three fibers online? My local brick company laughed when I asked them.
Did my spreadsheet come through?
Thanks all.Comment
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1. Polypropylene fibres: the really fine ones, finer than human hair used primarily for standard concrete to control early shrinkage (not for strength enhancement) but melt over 160C and therefore leave network of micro pipes through which moisture can escape) check Sika product range
2. Basalt fibres are for strength enhancement and resistant to our temperature range. Mixed length and thickness.
3. AR alkaline resistant fibreglass fibres are of uniform length and thickness 19 x 0.5 mm, also for strength enhancement and resistant to our temperature range. Used widely by the concrete countertop manufacturers.
4. melt extract fibres (stainless steel needles) are the industry standard for refractory reinforcement. They are resistant to temperatures way above the range of WFO’s service temperatures .Expensive and not particularly user friendly, particularly when placing a mix by hand and for the first time. 19 X 0.5 mm.Last edited by david s; 02-10-2024, 01:53 PM.Kindled with zeal and fired with passion.Comment
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Hi again David,
Ok. I have a choice of 4 fibers. haha. I'm doing a box standard 36", 2" thick dome that I would like to last a very long time. We're planning on staying in this house for a long time. Which fibers would you add if this was your oven?
All builders -- does anyone have an educated (or experienced) guess for how much a 36" dome, 6" cast vent, 2" thick is going to weigh? I need to calculate my raw material purchase. Is the US, Portland cement comes in 96 lb bags, fireclay and hydrated lime come in 50 lb bags, and I'm not sure about fine mesh masonry sand.
Thanks again!Comment
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