Help with Insulation for vault oven

[Rusty Nuts]

Hi All, I am building my first oven (its been on the to do list a long time) and have some questions around the design. I have an old open fireplace in the garage (house was built in the 50's and original owner built plywood racing boats so threw offcuts in the fire, heated the house, ran the wetback, and generally kept him warm. I don't build boats and have no need to keep my car warm so am converting it to a pizza oven which can use the same chimney.

The attached pic is what I have so far, the base layer of refractory bricks is on a thin layer of sand, below that a layer of tin foil and below that 250mm of concrete (then the garage floor which is probably 150mm thick). I asked my question about insulating the walls/roof on another forum and the only reply I got was that I should insulate the floor more. I hindsight that would be great but that ship has sailed so hopefully I can get some more information to help complete the build here.

Once I have laid the refractory bricks for the walls/roof I will grout with fire mortar. My Question is what should go on top of this insulation wise? i.e. do you use a layer of render, then some insulation like vermiculite mixed with concrete, then tin foil? and then more render?

Or can I just use Vermiculite/concrete mix straight on the bricks? What would be the ideal thickness

Many thanks for your help. I have posted some pics of the layout I am going for. Because of the opening in the wall and size of the garage I was a bit limited with design options but any constructive feedback would be appreciated.

Cheers

Russell

[UtahBeehiver]

Russell,

Going to address a couple issues, first by the pics it looks like the floor bricks are dry fit which is good, because it is really important to have insulation under the floor or the un-insulated floor and garage floor will act as a heat sink and the oven will never perform well. With the floor bricks being dry fit you can remove and add a minimum of 2" ceramic fiber or 4" of perl/verm- crete mix then place the floor bricks back. The sides of the oven must also be isolated from any bricks that do not have insulation under them for the same reason. There have been several builds that have had similar issues and moved forward as is only to find out and realize "we told you so". As far as the outside of the oven, perl/verm-crete mix can be applied directly to the outside of the brick no need for tin foil min 4-5" with a ration of 5-8 to one, imho or you can go with a min of 2" although 3-4 inches would be better of ceramic fiber blanket. Rule of thumb, CF blanket twice as effective as perl/verm crete, ie 2" CF = 4" Perl/verm. Good luck, but rethink seriously about not insulating under the floor bricks.

[Rusty Nuts]

Thanks Utah, yeah I am considering lifting them but the space constraints mean I would need to dig down to get insulation in or perhaps increase the height of the wall opening. In terms of performance what is going to be the impact of the solid floor? Will it be something like only gettin it up to 500 degrees instead of 700? Or is it more the retained heat for longer cooking. Ideally I want to be able to use this for pizza, baking breads, slow dishes like glazing a ham, pork shoulder and lamb, and even pan dishes like lasagne and moussaka. Is there a particular area where this flaw will make doing any of these impossible? I'm not too fussy if it is just increasing the pizza cooking time by a minute.

Thanks for the info on insulation. I am presuming it is the coarse stuff that is used as a hydroponic medium

Cheers

Russell

[UtahBeehiver]

If you are just cooking a pizza here and there then not having insulation under the floor "may" not be a critical. But as you mentioned, you want to be able to bake breads and other dishes. Insulation under the floor and around the dome or barrel in you case allows you several days worth of cookng at various temps. As mentioned, the concrete under the floor will act as a heat sink and suck the heat right out of the bricks and the oven's ability to be used over several days will be compromised. Even a couple inches of ceramic fiber board under the floor will make a huge difference. But it is your choice, just hate to see you do all that work and effort and not have an oven that works for what you listed you want to do.

PS I used perlite that was about the size of large BBs

[Rusty Nuts]

Thanks, yeah I hear what you are saying and ideally I would have asked these questions prior to the base going in. I'll have a think about it as I design the rest of it and source the perlite/vermiculite.

Cheers for the input

Russell

[SableSprings]

Welcome to the forum Russell. UtahBeehiver & the other forum folks are right on target with the need for complete isolation of the cooking chamber bricks & hearth with insulation. Especially when you'd like to experience the full potential of the WFO baking breads, roasts, cookies, pies, etc. My concern is the height of your hearth...did I miss something or are you thinking you will be cooking with the hearth just three bricks off the floor level? The insulation under the hearth will be easy to install when you build a higher base to support your oven. Don't want you to blow out your back working the oven below your knees...

Again, I hope I just missed something here...but like UtahBeehiver, I hate to see you invest time in an oven that won't live up to your needs or one you won't be comfortable using.

[JRPizza]

Rusty, what is the target oven size you are shooting for (length X width) and how big is your flue opening in the wall? Is raising the height of your vent an option? It looks like you are going to end up with at least one 90 degree bend, possibly 2 (depending on whether your vent is straight out the wall or has a 90 going up), and if you are trying to push too much smoke up you could end up with a bunch of smoke in the garage.

[Rusty Nuts]

Thanks all for the input, so much good knowledge here and real works of art (I just looked at your album UtahBeehiver, blown away by that one) I guess I am going for something a little different as a project to utilise an existing space. SableSprings, yes you are right that it is pretty low to the ground. This was to do with the existing hearth and old fireplace opening. Though not ideal I am OK with sitting on a bench to cook, or even giving the kids the job. If I had built outdoors I would have made it a standard height but this position will mean I can use it year round regardless of weather.

JRPizza the main chamber will be 780mm x 590mm (30in x 23in) with a slightly smaller width for the opening of 390mm (15in) The flue ID is about 180mm (7in) and my intention is to run short stubs from the wall and front arch then a longer 45 degree pipe. It goes into an existing chimney that is probably 300mm ID (12in) and 2 storeys high. Something like what is in the attached pic.

I am revisiting the base insulation and will mull this over while on holiday as to the best way to get some ceramic fibre board under there. In terms of using ceramic fibre blanket on top of the oven do you then create a wire frame and chicken wire/mesh to then apply some hard render to?

Also a question on supporting the walls. Will the arches push these out? do I need to use some sort of bracing?

Many thanks for all the helpfull pointers and identification of pitfalls. It must be frustrating for the pros to watch but I'm quite happy bashing my way through this.

Cheers

[Rusty Nuts]

As an aside this was the video I came across that got me on to thinking about doing this. https://www.youtube.com/watch?v=D3Ivz-utYak

[UtahBeehiver]

Russell,

Thanks for the compliment, after 4 years I am still putting around with the finish work. BTW, although I am very detailed oriented and a DIYer, I never laid a brick in my life until this project.

There have been several builds with vents like your latest post #8, sometimes called a squirrel tail. The 45s will help reduce any flow restriction.
I used used 2" CF then covered with chicken mesh (some people use stucco lath, others use a more rigid 1/2 X 1/2" wire mesh) then I put on 3" of Perlcrete then 1" stucco and finally copper. but you should be able to go CF/Wire/Stucco.

The shape of a barrel and arch has a tendency to have lateral forces pushing on the vertical portions of the side walls of the barrel and arch. Some form of reinforcement is probably necessary. There are several ways, Look at Tscars, build or search for other barrels. This also applies to the front arches too. I think Tscar had a technical link on his thread that showed the lateral and horizontal loads of various type of arches.

We are here to help, why reinvent the wheel, Everyone on the forum wants to see new and old build great ovens.

[Gulf]

Welcome to the forum Russell,

You are getting some great advice. In your drawing, you show the walls of your oven and your your entry laid as shiners. (you can use this same link for the other highlighted definitions for brick courses) Aside from being weak consruction, shiners will not give you the heat retention that you are looking for. I read where you are interested in breads and slow dishes. The video that you linked states that it is a pizza oven. The walls of that oven are laid as a soldier course. That gives about twice as much thickness for heat retention. Though it will take a little longer to heat up, those type walls will hold the heat much longer for those slow cooks. In the video, the roof of the oven was laid as shiners. That is very thin, but probably still ok for a "pizza oven". Pizza ovens rely on a live fire during the cook to replace the heat loss. Breads and slow cooked roasts do not always rely on a live fire. The slow released heat from the walls and the dome/roof are what makes a WFO/Bread oven successful (along with sufficient insulation). In your pic, youn only show the top two courses as stretchers.

If I were you, I would lay the sides and roof of the oven completely as stretchers. That would give your build more strength and much more heat retention for breads and slow cooks. I also noticed in the video that the the builder inserted the thimble much higher than you have shown. I would not hesitate to plug the one that you have and insert one a little higher. Getting that oven up to a more ergonomic height is critical when you get to my age. And the kids will grow up and leave before you know it. Even if I had to loose the room of the existing fireplace, I would build the oven a little further out in the room if that is what it took to get it up to specs.

My main reason for this post is to back up what Russell, Mike, and John are saying, think it through. Don't rush your decisions, You will be cooking in it much longer than it takes to build it right .


.

[Rusty Nuts]

Great info guys, keep it coming. So today I mad the call to grind out the small amount of mortar around the edge of the base and I will lift this to get some ceramic fibre board insulation under it. Probably 50 - 75mm or there abouts (2-3in) Unfortunately I cant lift the whole oven right up as the chimney structure splits just above the hole i drilled as there is a log burner upstairs with an internal chimney that uses the same cavity and also I need to still park my car in the garage so cant come out further. I am OK with that compromise.

Gulf, thanks for the laying info. I was trying to be economic as fire bricks in NZ cost $7 each! but I'll go back to the design and see what I can make work. My question on the vertical walls of the oven is do I need to use mortar between these? or can I stack them butted up against each other? (is that called dry fit?) I will use some form of buttress/support that would either be steel (if i use a ceramif fibre blanket option) or cement/vermiculite around the whole structure. The end result is that I will build up a brick box the same length and width as the red bricks in the pic and with a flat top. This will serve as a prep surface and allow me to add ample insulation around the walls and roof of the oven.

Going to take it a bit slower and see if I get get this as right as possible.

Cheers

Russell

[UtahBeehiver]

Good decision, it is a tough pill to swallow but you will be glad you did so. Do what you can to make sure the barrel walls get insulation under them too. I am not sure if you have a Craiglist or equivalent to look for surplus fire brick. I would be cautious on used (since you don't know where or what the bricks were used for) but old or surplus unused should be fine. I feel for Southern Hemi builders having such high material cost.

[Gulf]

Rusty, (I have to keep you two separate in my mind) .

I forgot to look closely at your location. I read Plymouth, but did not associate that with New Zealand . My appologies and condolences for the price of fire brick in your country. If you can't source the extra brick as RussellI advises, I'm going to suggest something that is a little above my paygrade. I think that you can lay the entire oven as shiners. But you will need to add a 2" thick cladding layer for thermal mass and for some structural strength. You can form the side walls to achieve that. The roof can be just applied to depth. If you can source some fireclay, you can make a home brew with portland cement, hydrated lime, fireclay (brickies clay), sand, and the cuttings from your saw to clad the vault. For the sides, you will probably have to sandwich the vault with a ridgid insulation between it and a stretcher laid face brick for buttressing. That insulation could be vermicrete, perlcrete, calcium silicate board, or ceramic fiber board insulation.

I hope this helps.

[Rusty Nuts]

Ok, so a new year and the build is back on. i have got 2 inches of ceramic fibre board to go under the base. My question is can i use sand on top of the board to level up the bricks? Also under the board I have used more sand to provide a level base. Is this OK or would i be better pouring a vermiculite mix here?