Construction advise please

[Catman]

Hi, this is Catman. I’m new to the forum and appreciate being able to reach out for information from the more experienced. I am planning to build an outside Igloo style pizza oven. I have done significant research but I am needing advise and clarification on a few items that I haven’t found clear or consistent information on, so would appreciate any advise I can get.
Q1. My design will have 4” concrete/vermiculite base on top of a 4” concrete slab. I have found a reasonable supplier for vermiculite. It comes in 4 grades from coarse to fine. I’m leaning towards the more coarse side which I believe will increase the insulation value vs fine. Thoughts?
Q2. I’m assuming the mortar covering on the exterior side of the fire blanket does not need to be refractory mortar, but I’d like to confirm this. I’m assuming an exterior rated type S mortar is sufficient.
Q3. I would like to use mosaic tile on the exterior. Is there any advise on how to ensure durability in Canadian weather? I was thinking of a layer of parging, then tile, then a several coats of sealer. Thoughts?
Q4. Last question, I have seen conflicting info on when to perform the curing stage. Should this be done at the very end or post fire blanket or even earlier?

Again, thanks in advance for your advise.

Catman

[deejayoh]

Hey Catman - welcome to the forum

Not sure about your Q1, as I did not use vermiculite. On your other questions
Q2 - no, it's normal stucco/mortar. You should not have any heat issues outside the fire blanket
Q3 - if you're in canada - I'd recommend against a tile covered igloo. Tile + grout is porous, you really can't seal it 100%. You will likely have water issues, especially with frequent freeze/thaw. Unless your oven is under some other cover - leave that style for those living in Mediterranean climes. (FWIW, I'm in Seattle - so I built a doghouse style covering)
Q4 - you will need to start curing before you cover the oven. You'll want to drive out as much moisture as early as you can. Even after sealing, best practice is to put a vent on the oven for igloo style.

[UtahBeehiver]

Q1 - medium grade at 5 to 1 ratio for floor
Q2 - same answer as DJ
Q3 - ditto
Q4 - clarifying what definition of covering the oven. cure with insulation on dome but NOT stucco. This allows for a better temperature differential on the dome, reducing cracking potential. Once dome is cured then you can stucco over the insulation blanket. Also, install a breather vent in the stucco. Do a search on the Forum in you unclear on this.

[david s]

Q1 There is hardly any difference in insulation value between the fine, medium or coarse grades. However there is a difference in the quantity of water required in the mix. The fine grade requires more, the coarse less. The finer grade does make a more workable mix which is useful if making a lean mix (10:1) Those ratios are by volume not weight by the way. As the water removal is a problem I go for the medium grade, but find a 50/50 mix of vermiculite/perlite makes a better mix than either of them alone. Also a little powdered clay added to the mix makes workability much better. The correct water addition for the medium grade is 3 litres for every 10 litres of dry insulation. If water pools in the bottom of your barrow you've added a little too much and you risk washing the cement off the grains. (See attached experiment on slab drying.) Vermicrete insulating slab copy.pdf

Q2 Applying a stucco over the blanket is difficult as it is a bit springy and lumpy. The usual method is to cover the blanket with a 10:1 mix about an inch and a half thick. This covers the lumps and bumps of the blanket, allowing you to restore the hemispherical form and sets hard enough to provide a firm substrate for the stucco as well as being an additional insulate layer. It is important to allow this layer to dry thoroughly, then do your drying fires before applying the stucco over it. Otherwise water trapped in the insulation layer can build up enough pressure to crack the stucco layer.

Q3 ? i live in the tropics

Q4 see answer Q2

[Catman]

Thanks to both for the info. Very helpful. I will search out the stucco vent. Haven’t completely ruled out the mosaic tile as yet. I’m not worried about the absorption of moisture into the tile as I’m using glass tiles. As for the grout, there are swimming pool grade grouts available. Not sure how much that might help, but I have some experience with using sealers and they can be very effective. Still exploring options.

One additional question to the forum, has anyone found reasonably priced Fibre board in Alberta? I have not.
Agains thanks for the comments.

Catman

[UtahBeehiver]

Fibre board? Q1 said you were using 4" of pcrete. Are you talking CaSi for under the floor? If so it is the best and high insulation value and also the most expensive (not sure what you consider reasonable). Pcrete is a good budget insulation but takes roughly twice the thickness and more labor. Mosaic tile will not last and will pop off in a freeze/thaw cycle on an exposed oven.

[Catman]

Just looking at Fibre Board as the simpler option to the vermiculite base, but since I couldn’t find a cost reasonable option I was leaning to the vermiculite, but I thought I would ask the forum to see if there were any better price options from what I had found. And thanks to the respondents on the best grade of vermiculite and mix and water info was informative. I had read the mix ratio many times and the recommended 5-1 is pretty consistent.

thanks

Catman

[deejayoh]

Thanks to both for the info. Very helpful. I will search out the stucco vent. Haven’t completely ruled out the mosaic tile as yet. I’m not worried about the absorption of moisture into the tile as I’m using glass tiles. As for the grout, there are swimming pool grade grouts available. Not sure how much that might help, but I have some experience with using sealers and they can be very effective. Still exploring options.

One additional question to the forum, has anyone found reasonably priced Fibre board in Alberta? I have not.
Agains thanks for the comments.

Catman

I have experience with sealers on my concrete countertops, and none of them are up to the elements. They all wear out in a less than a season. So if you can get it to work, it's a maintenance nightmare. you also have the issue of the joint at the bottom of the oven where it meets the hearth. What you really need is a tile membrane that goes under the tile and grout - like what you would use in a shower. Heat shouldn't be a problem, but the curved surface means that stuff designed for the shower like Schluter won't work. There do seem to be some brush-on membrane products. If I was going to try an igloo I'd use one of those

[Catman]

Thanks for the input Deejayoh. I’m doing more research to see what products and options are available. I have considered the dome hearth joint and believe I can manage this without concern.

Russel, I looked through your photo album. Excellent craftsmanship. Very well done. That’s certainly a benchmark to strive for.

Catman

[UtahBeehiver]

As far as a mosaic tile dome, if you can put a open structure with a roof over the dome so most of the rain/snow does not sit on the dome then a mosaic should be okay with good waterproofing. See Gulf's build, there was also some other timber open frame coverings done lately on the blog, think in the UK.

[Catman]

A few additional questions. I noted the drying experiment used a mix of vermiculite and perlite. Is this recommended or is 100% vermiculite acceptable. If not what is tha benefit with the mix?

I have seen conflicting information about wetting the fire bricks. Some masons recommend against it in general, so trying to understand if it should or should not be done when building the dome.

lastly, I have read about having several holes in the hearth base at the blanket level to allow moisture and pressure to escape. Is this viewed as a reasonable approach vs the breather vent?

thanks again

Catman

[david s]

I usually use half perlite half vermiculite as I find it makes a more workable mix. This is really only a concern when making a more insulative mix that contains far less cement. The 5:1 mix has enough cement in it to hold the grains together sufficiently, but a far leaner 10:1 mix does not. Some folk end up adding more cement to make it stickier but this reduces insulation value. A little powdered clay, say a handful for every litre of cement added to the lean mix as well as the half perlite half vermiculite seems to improve workability enormously.

Regarding holes in the outer shel, they are the equivalent of the hole in a saucepan lid to relieve pressure. Obviously more is better but unless designed well can provide an entry for water too. There is a case for putting some lower in the dome because as the moisture is driven out by the heat it travels to towards the outer shell, condenses when it hits the cooler surface then runs down the inside of the dome to accumulate nearer the base. I don’t really think it matters too much about their location and holes in the outer shell are an invitation for wasps to make homes.
The bricks should be damp and dust free but not wet, to get a good bond, so there’s sufficient water left for the hydration process (damp curing) If using refractory cement which relies on calcium aluminate cement rather than calcium silicate cement (Portland), the stuff is cured within 24 hrs so it probably won’t matter.

[Kvanbael]

For what it’s worth. My uncovered mosaic iglo design just survived a very wet and freezing cold belgian winter and performed dry as ever on my first firing last weekend. I used a combination of materials and techniques to get it waterproof.
Don’t know if it will last over the years of course. I realize it is a risk.

[mongota]

Welcome to the group Catman

Prior to veneering my dome with stone, I did apply a stucco coating of sorts. After I applied four 1" layers of blanket insulation over the dome, I then wire tied expanded diamond mesh over that. It was pretty easy, conformed to the shape of the dome nicely, and gave a great base for applying the cement coating.

After the stucco coats were applied, because the dome was going to sit for a while, exposed to the elements, I used a product called Thoroseal for waterproofing. It's a cement-based coating and is not affected by temperature. It can be tiled upon. Powder form, comes in a bag. Mix it with water and brush it on. I applied several coats. It's waterproof, but vapor permeable. The vapor permeable aspect allows water vapor to pass through it if you have a fire and there is moisture within your dome. I would never recommend using a water AND vapor proof coating because of the vpor permeable coatings that are available. If you did use a vapor-impermeable coating, then it would be wise to include a vent tube.

Because you are applying glass mosaic over the stucco, I'd agree with the others who recommended a vent tube that passes through the stucco and tile at the apex of the dome. Even though the dome is essentially an open system, the vent tube would allow moisture vapor in the blanket insulation to easily escape from the shell, versus potentially trapping vapor and having the steam super heat and cause negative issues with the stucco and tile. I installed a vent in my dome just for insurance. I vacillated, but did install one. The downside is I might have over-worried and wasted a few dollars on a nipple and cap and 30 minutes of my time installing it. The upside? When I fire up the oven after a long period of non-use (after winter, for example) I have had no issues with moisture vapor being trapped in my dome.

[Catman]

Mongo, , thanks for the information. I have found a number of waterproofing options, but great to get your knowledge and experience and advise.

As a small side note. I love the look of you oven with the stone. Looks great.

Catman