Dome Insulation Question

[llamabucket]

Hey everyone! Firstly, thank you for all the amazing information on this board. I've been lurking and reading for quite a while and I've already learned so much!

I'm in the process of building a barrel vault WFO in which we plan to use to cook pizzas primarily. Our foundation is done, the main concrete slab that the oven will sit on is done and we just finished pouring our 4" perlcrete on top of that! The floor and the whole interior will be firebrick so we're good to go.

I have a question regarding insulating the dome and walls of the oven. Before reading up on how to do it properly, we initially were only going to do two brick layers: fire brick (using fire mortar) and on top of that regular red bricks (using regular mortar). But since getting educated, we decided it would be best to at least insulate the floor properly, which we did. Now, I know it would be best to insulate the whole oven on top with perlcrete as well but as I've never used a WFO before, how crucial is it? Will our pizzas simply not cook?

If that's the case, then would this order make sense?: fire brick on the inside, perlcrete, then red brick on the outside of the oven

My first concern is how the perlcrete would stick to the firebrick? Secondly, what purpose would the red brick serve other than making it look nice? Would the red brick actually play a role in keeping the oven working correctly?

Or do we just simplify it and just do firebrick then red brick and it'll work fine?

Thank you all for your help

[SableSprings]

Welcome to the process! There is an important consideration here and that is the oven's heating mass. If you don't insulate the fire brick layer from the red brick layer, then you will be attempting to bring both layers up to temp...way too much mass to heat for a "normal" oven (i.e. lots of wood) and definitely overkill for pizza. By putting a layer of insulating material around the oven core, you are isolating the heat sink for the oven. Think of the oven core as your single firebrick thick layer that surrounds your food. Since you have isolated the floor, it will accept and store heat much more quickly than an uninsulated hearth that will be constantly bleeding heat to the "outside". One brick thickness on the oven's walls is plenty adequate for heat retention, so insulate that barrel vault and you have effectively isolated the oven core and gotten yourself much improved oven efficiency. Once insulated from the core, the outside (red brick) is simply a facade and doesn't affect the operation or efficiency of the oven. One question you need to answer yourself is "Are we going to do anything other than pizzas?" If you want to retain heat for baking bread, turkeys, bean, cookies, etc. after the pizza, you will need to insulate that single brick core to maximize stored heat from your initial firing.

You certainly can cook pizzas without insulating your barrel vault...however, it will take a more wood to bring the vault dome up to temp and having the outside of your oven at 600F is not recommended. So in answer to your questions

1) Yes, you want fire brick on the inside, then an insulation layer (either ceramic batting, perlcrete, or vermicrete), and then the outside facade...in your case, red brick.

2) Perlcrete would "stick" to the firebrick, but you do want it to slide on the firebrick vault shape which will expand and contract with normal firing. If you use ceramic batting, it's flexible and there is no need for slip. If you are using a material that becomes hard (perlcrete or vermicrete), normally a layer is added to allow slippage. I used use perlcrete for my dome insulation and used a layer of diatomaceous earth between the firebrick and the perlcrete. I've also seen a single layer of aluminum foil used to provide a slip layer between the firebrick and the perlcrete/vermicrete. You may have also noted several builds here that simply built an enclosure around their dome/vault and filled the void with loose perlite or vermiculite...same idea, the oven core may expand and contract freely and the insulation keeps the heat from leaving the core.

3) The red, outside brick is simply decorative when insulated from the oven core. Don't just put the red brick directly over the firebrick oven core...you simply are increasing the amount of mass you are heating...basically a waste of wood and your time.

Sorry to be so wordy...I hope this helps answer your questions

[llamabucket]

Wow, what amazing help, thanky ou SO much Mike!!!! You just convinced me that we absolutely have to insulate the top in one way or another.
I'm also debating simply covering it with a ceramic blanket of some kind, I just need to source one here in Canada (specifically Montreal). Do you think this would work just as well? If I go this route, what thickness should I be looking at at a minimum? I see blankets being sold in 1" thicknesses.

And don't worry, the wordier the better! That way you were able to explain everything in great detail to a newbie like me, who may otherwise misinterpret something

[llamabucket]

Oh! One other question before I go too far. With the space and the bricks we have, our design will allow for the following dimensions: 23" wide x 32" long x 14" dome height. These are interior oven dimensions that don't include the mouth of the oven. We'll be making the door opening smaller than the inside as everyone suggests. Just off the bat, do you see anything that could pose an issue for us? I know it's a small oven and that's okay! It'll heat up faster and we don't plan on making more than one pizza at a time

[SableSprings]

As to your first question (post #3), the ceramic blanket is the most effective & efficient way to add insulation to a wood fired oven. Although it tends to be on the expensive side, its advantages are quite impressive. The consensus on the forum is to use 2" of the batting (2 layers of 1" material) for oven insulation. Do be aware that the main caveat is that wearing gloves and a GOOD breathing mask/eye protection are a must when working with this stuff. It's really not that bad to work with as long as you are aware that the brittle fibers are not something you want to get up close and personal with (skin, eyes, nose, & throat wise). Also you need to provide a protective, waterproof, cover to keep the fibers from "wandering off" and to keep the blanket from absorbing water in the years to come. The outer red brick facade you plan would work pretty well keeping moisture out especially if you use one of the masonry coatings to reduce/eliminate water absorption. Of course the best option to keep your oven dry is to build a cover for it...

If you are only going to cook pizza ( and I would bet that won't happen...bread and Montreal bagels come to mind ), is there a reason that you are doing a barrel instead of a dome? A consideration here is the barrel vault does need some sort of buttressing for the sides. The curved top pushes to the outside and so either the sides are built quite thick or have outside support. Look up Barrel Vault for more explanation (Wikipedia does a good job of explaining the engineering aspect). Basically, the weight of the top bricks push out and potentially will force the side walls to collapse outward if not braced or extremely strong...not good. The dome would give you a great pizza experience and doesn't need the buttressing. There is a builder in Australia (David S or CobblerDave on the forum who has a 23" inside diameter oven and cooks just about everything in it...). Size doesn't really matter here...it just determines how you work it for "the pizza party".

For your second question, if you are doing the barrel vault, the 14" inside height means about an 8.5" height on your oven opening. Working the back of the oven at 32" deep might be a bit of a reach...another point for the dome style. If that's what you've got to work with, then it will work. How much actual space will you have (outside dimensions)? I think at this point you need to be thinking about the actual final outside dimensions that you've got. Remember that you need to double (both sides) when calculating the completed size. Your 23" wide oven would be 23+(2x2)+(2.5x2)=32" if you added 2" of insulation and then laid red bricks on the outside as a facade. AND you still need some room in which to work-I made the mistake of carefully calculating my build's completed width, but then forgot I would need to actually work behind it...hence, I couldn't get a power screwdriver into the space behind my stand !

Hope this helped...glad the other info made sense.

[llamabucket]

I initially wanted to simply cover it with perlcrete, but with perlite being $9/bag, it's not cheap to cover the whole oven with a 4" layer! So I found a local supplier of ceramic blankets and they have 2" thick rolls, 2 feet wide for $9/square foot! Since it's cleaner and lighter to work with, I think we'll simply go this route. And thank you, yes, I know how harmful it is and we'll be taking precautions when handling it!

So the current plan is to lay the firebrick, let it set, then apply the 2" ceramic blanket, hold it down with some kind of wire mesh, then lay the second dome of red bricks with regular mortar. We just laid the hearth and used some commercial grade fire cement and I'm quite surprised by how creamy it is?! I always pictured it grey and rough, not off white and creamy. Anyway, we'll see how that sets before we use it for the fire brick arches.

As for barrel vault, it was just a lot of different factors that went into it. We originally wanted to make a budget oven (as I'm sure many people initially do haha!) so we're repurposing a lot of materials we already have. From the huge stones that make up the foundation, to the limited firebricks and red bricks we acquired from a neighbor to the space we have. If I had a big budget I would absolutely do a proper dome oven, but we also don't have the time to dedicate to it. So, we're trying to make the best barrel vault oven we can! And I know it can be done!

Yes, I too am concerned about the forces bearing down and out on the arches. Which is why we're currently exploring what the best arch radius should be based on the area we have to work with. I do think it will be possible to make the arch solid enough to not break down on us. I mean, has anyone here suffered from a collapsed vault oven due to this? I would think that it would take years before something like this collapsed? With each arch interlocking with the next one, then the tightly wound ceramic blanket and finally the red brick layer, I would hope that it would be sufficient.

I will get back to you about the outside area but we have enough space for everything so far! Unless we run into another unforeseen issue

Oh yes, I still have to figure out where the best place for a chimney would be. I'm still researching but would you know if putting a chimney in the main vault near the front of the opening would be okay?

Thank you again!!!!!

[llamabucket]

Here's the refractory mortar I bought at the masonry store, it came ready to apply in a bucket. The guy recommended this for what we're doing...but I found the product sheet and I'm concerned about the limitations
When laying the arches, I will surely have thicker application between the bricks than 1/8". From builds I've seen, fire mortar isn't thinly applied, especially on the outside of the ovens where the gap between bricks is wider.

Like I said before, the consistency is that of very very smooth cream. Like soft serve icecream. So it worked perfectly fine on the hearth as it even matched the firebrick color! But I'm not sure how it would work for the dome.

Should I be concerned?

http://www.cpd.ca/wp-content/uploads...ve-Furnace.pdf

[SableSprings]

As for the vault arch radius, the half circle is the best compromise (IMHO). The flatter the arch, the more pressure is exerted to the outside.

Chimney placement: Convention is for the chimney to be in front and just outside the cooking chamber. Most folks want to be able to close off the cooking chamber and be able to retain maximum heat for baking (without an actual fire or live coals present). That does mean that you will need to make a landing and a reveal (for a firing/cooking door) beyond the cooking chamber/hearth. I know you are talking about just limited firings for primarily pizza...but I hate to see you limit options when the oven could do so much more.

I used the same type of premixed refractory mortar for my oven dome and had the same concerns. Once the bricks are actually set in place in your vault arch, they really don't need a lot of mortar to keep them in place. That's the beauty of arch's design strength. The main issue I had using the pre-mix stuff is that with the thicker applications it does crack and open up as it dries...hence the call for thin layers. There is a homebrew high heat mortar described in the forum that is much cheaper to make and doesn't have the same cracking issues. I initially bought a couple of 5 gallon pails of the pre-mix and ended up needing to buy a lot more. I simply kept filling the big gaps and then the following cracks with more mortar. Eventually I got them all filled, but it was more expensive than making my own mortar would have been. The homebrew heat mortar doesn't have anywhere near the same issues. One note here, if you are cutting the vault bricks with proscribed angles for the arch radius, they fit more tightly together and you obviously use much less mortar. Cutting the brick is more work, but worth it in many ways. Look up the homebrew formula on the forum--it's fastest to google "home brew mix forno bravo" or "homebrew forno bravo" to get a better idea of its advantages...note: make sure to add the forno bravo or you get a lot of beer references......not bad, just not on point. Also you might look at the other barrel vault builds documented in the forum for chimney placement.

As to collapsing barrel vaults...don't remember any and since your oven doesn't have a huge dome vault as is on the smaller side it probably will not have any major problems with the outer oven facade as support.

Pictures of your build and oven location (and even a scanned sketch of what you're thinking about building) really help us to help you.

Here's just one link to a build on site that shows some of what you'll have to consider during your build...his has a little flatter arch than semi-circle so he has thicker side walls that do the needed buttressing.

https://community.fornobravo.com/for...el-vault-build

[llamabucket]

So yeah, I agree that a semi circle arch would be best. Based on how many bricks we have available, here is the ideal layout. I'd love to lay the bricks on their 4" side, but I simply don't have enough. I am hoping that this will hold properly! Especially since I plan on interlocking each arch with the next one by offsetting the bricks nicely. I'd love to cut the bricks lengthwise but I think it would be unbelievably hard and time consuming so I'm trying to do it the way they are. This dome height below is 14.5"

As for the mortar, yeah...it's very expensive in this premixed form And since I don't have super thin gaps, I'm concerned that not only will I use a TON of the stuff but the watery consistency won't be good enough to hold it together. It was fine for the floor, but I think I'll either make a thicker home made one or buy some dry refractory mortar and create the thick consistency myself. That would be best.

Thank you for the help! I really appreciate it If you can point out any issues from the above photos please let me know!

[SableSprings]

I can't see your pictures in the above post. Did you use the little camera icon, click upload, select your pictures, and then upload them?

I wouldn't make lengthwise cuts on each brick...it doesn't leave you much contact area for the adjacent bricks. You might just have to find some more used firebricks, source some new ones, or go less than 32" deep.

[llamabucket]

Let's try this again, here are the photos:

[SableSprings]

Pictures came through just fine that time, thanks!

That looks pretty workable to me. Remember that making a form for the inside arch will make putting this together a lot easier. Also if you make the form so you can shim it up into place, do your row of bricks, pull the shims to get enough space to pull the support form forward for the next row it will go pretty quickly. If you can grind along each side of the brick just a little, you'll get more contact with the next (upper brick) and you'll need less mortar. You can look at your layout photo and hopefully "see" what I mean to increase the brick to brick contact and it will ultimately lower the whole structure a little as well.

Don't mortar the back wall to the arch/barrel as things are going to need room to move during the firing. If you do mortar it, do so lightly as it will break free down the road no matter what you do. (Also don't mortar the barrel bricks to the hearth...they need to expand/contract as well. Just sitting on the hearth works just fine.)

[llamabucket]

Thanks so much for the prompt reply! Oh thank goodness that's workable! I thought it was, but I guess I needed a little confirmation from someone who's gone through this process already just to be sure
I'm surprised about not attaching the dome to the hearth?! I know the dome will be super heavy and surrounded by other things, but that seems like something could go wrong?

Another thing that's worrying me a little bit is the fact that there will be this "thin" firebrick dome free standing inside, then insulation and then another freestanding red brick dome on the outside. Do you think that the structure will be solid enough?

So yeah, I think I'll be avoiding using this creamy fire mortar and just make my own. I'll need a lot of it to fill the "V" shapes on the outside of the firebricks so I might as well save some money and make it thicker all together. I hear soaking the firebricks is necessary so they stick to the fire mortar better?

Oh and as for grinding the bricks, I understand exactly what you're suggesting! I'll test it out on a brick to see how doable this is with my diamond blade, but I suspect it may be a little hard to get the angles perfect with my handheld angle grinder :P

[llamabucket]

I forgot to add, thank you for the form suggestion! So the way we came up with doing it is the following way. I bought 2" thick styrofoam which we cut into the inner dome shape. We'll cut a whole bunch of these that will run densely through the whole barrel vault. I did a test and it holds the bricks up perfectly! Having many of these side by side will allow us to keep them in place while the mortar cures. I'm also planning on interlocking each arch with the one next to it by offsetting the bricks. It'll be a little trickier but I think it'll help with the stability of the whole thing?

[SableSprings]

RE: not attaching the dome to the hearth. There is enough movement (expansion) during heating that the vault will move/expand enough to break any mortar bond you apply at 90 degree joints...the weight will keep it in place just fine and it will settle back into position as it cools back to air temps. That's the beauty of the arch design...the final piece (keystone) in a dome or arch settles the structure and makes it essentially a single strong unit of support. Basically, your styrofoam is important until you add the last piece in each vault row...at that point the arch needs no more support at that row, the bricks are now self supporting.

The mortar and design of the vault arch will make for a pretty solid structure. As I noted before, many people add some buttressing to offset the outside push but a smaller structure as you are creating is pretty stable. I'd just be putting in a wider base or some side support in the middle side to keep movement to a minimum 'cause I over-engineer and I'm not an engineer Personally, I'd be adding a ring of perlcrete 3"-4" high around the base of the vault to provide some of this support. It also would be a good way to establish the distance to the outer facade bricks.

I think your interlocking of the barrel vault bricks will indeed increase the structure's stability. Also, when you build the outside facade you can and should mortar the back bricks to the back arch to help contain the structure. There won't be the expansion issues on the other side of 2" of insulation.

When you build your front entry, it should be buttressed and/or wide at the base. Connected with your barrel vault, this front entry will help contain the vault structure, especially if you carry the interlock out into the entry and chimney area.

Again, I hope this is helping answer your questions...