42 build in queensland

[SirClutzz]

Couple of pics of the insulation blanket, a wire mesh. 3 layers of 25mm blanket for 75mm of insulation.

[SirClutzz]

First layer of perlcrete render. I've never worked with perlcrete before. Mixing was interesting. I used a mixer to mostly mix it, then finished in the wheelbarrow. Found adding the perlite to the mixer first with some water, then adding the other dry ingredients gave me the best result.

Some cracking the in the render so far, but it's pretty much only due to movement in the mesh, it's nice and hard this morning, so the mesh won't move when I do the second coat, therefore I'm expecting to be able to close those cracks up nicely (crack occured before it was dry, so I really don't think it was shrinkage).

Covered the oven with wet sheets and a tarp 30 min after I finished rendering, to slow the cure. This morning it's looking all nice and solid. Will do the second and final coat today.

[Gulf]

I'm not sure what you mean by "movement in the mesh". I haven't experienced that. However, I have not applied vermicrete or perlcrete in thin layers. I apply it in full thick courses much like laying brick from the ground up. As soon as one course (3" to 4" high) around the dome is finished, I go back to where I started. That allows the mix to firm up a few minutes to a couple of hours in order to support another course. Patting the mix downward and outward toward a curved blade with a marginal trowel helps to shape and firm the mix as you advance around the dome. I use a fixed curved blade. Russell UtahBeehiver did this with a hand held version. Either method will limit any compression of the insulation and most probably "movement of the mesh" as you describe.

I would hold off on the drying/heating schedule until the perlcrete has cured and allowed to dry for a while. There is a lot of moisture in this stuff and it does not give it up very easy. I would include a vent in the apex of the dome to help eliminate moisture. You phrased this layer as a "render". I call this layer another insulation and/or a shaping layer. I have been under the assumption that "render" was a UK/Australian term referred to as "stucco" here in the US.

When you get back to the drying/heating schedule, the old heat bead method works great for gently bring the dome up to the early temps without direct flame impingement on the dome. A search of "hat beads" on the forum will give you lots of info and it works great. Karangi Dude was the first that I saw using this method and that is what I did on my oven. It worked great for me. I will try and get a better presentation of what what I did with the stainless steel starter chimney. (I will probably call it "Heat Beads 2.0" lol). Take your time in fabricating the ss starter chimney. I have have found one to be another great cooking aid in the oven..

I like the rounded back of your dome. I recently completed one very similar. It allows the water to shed without hitting a shelf which can be a route for water intrusion.

I'm a little internet bandwidth limited at the moment. However, I will try to add some pictures to explain the above as an edit or in a later post.

[david s]

First layer of perlcrete render. I've never worked with perlcrete before. Mixing was interesting. I used a mixer to mostly mix it, then finished in the wheelbarrow. Found adding the perlite to the mixer first with some water, then adding the other dry ingredients gave me the best result.

Some cracking the in the render so far, but it's pretty much only due to movement in the mesh, it's nice and hard this morning, so the mesh won't move when I do the second coat, therefore I'm expecting to be able to close those cracks up nicely (crack occured before it was dry, so I really don't think it was shrinkage).

Covered the oven with wet sheets and a tarp 30 min after I finished rendering, to slow the cure. This morning it's looking all nice and solid. Will do the second and final coat today.

Not sure what mix ratio you used, but as a guide the stronger you make it by adding more cement the poorer the insulating value will be. It can be strong or insulating but not both. Roughly doubling strength halves insulation value. The attached table explains this best.
Using a mixer to make up vermicrete is not recommended as the lightweight weak grains are degraded from the mixing process. The stuff also sticks like crazy to the sides and blades of the mixer. I find mixing by hand in a barrow is much better because you can feel the texture of the mix, inspect it as you go, folding in gently without degrading the grains and get the water quantity just right. If you end up with water pooling in the bottom of the barrow you've added a tiny bit too much. That can result in washing the cement off the grains, producing an inconsistent mix.

[SirClutzz]

The mesh was able to compress a little bit in places, generally where the was a thicker layer of it (like on the curve of the dome you cut triangles out of the insulation to follow the curve, with the mesh I just folded it over in those places, so it was a couple of layers of mesh, they did always sit down as tightly on the blanket, place the blanket also can compress, these 2 things gave potential movement. Only really happened in one place. For the second layer, once the first was firm so nothing would move anymore, I opened that crack a bit wider, so the second later could get into it better, thus eliminating me from have a really thin fill.

Yeah, stucco is called render here.

I had read somewhere I need to let the perlcrete cure for 2 days at least before starting the curing process. Is that too short? I also considered a vent early on, that then didn't get around to sorting it out, so will have to see how I go without it.

Thanks for the info on heat beads, I def going to do that, cause I want to use them up. I don't really use them anymore, and I've had them for years.


Render mix is 5:1:1:1, percrete, sand, cement, lime. I'm mostly relying on the ceramic blanket to do the insulating, just figured this would be a bit extra. My last oven (was a precast kit), only had 50mm of blanket, and now render/stucco over that, it only ended up with 2 hairline cracks after 2+ years of use, so I'm happy if that's the worst it gets. As the oven is undercover, I'm not really concerned about rain getting in cracks like that, and I will probably finished the oven after curing with an acrylic render paint to seal it all up.

Anyway, did the last of the perlcrete today, got the render out flush with the slab at the back, which was my main goal.

Next to move on to a door, planning to make 2, one basic, single sheet steel one for convenience when firing and making pizzas, and an insulated one for slow cooking or heat retention. Just make the simple one first. The second door gives me another thing to over engineer.. :P

Thanks all for the help and guidance

[david s]

That’s a pretty rich mix for an insulating layer, but a fairly weak one for a finishing render. If you want to enhance the layer’s strength, then hold the moisture in it for a week. Because it’s under cover you would be better not to seal the outer surface so the oven can breathe. Sealing the outside, as well as preventing water entry also prevents its escape.

[david s]

Here is the firing schedule for my 21" ovens, so you will probably need to adjust it for your large oven, also bearing in mind that you've partially dried the inner dome.A sheet of plastic thrown over the dome during firing will tel you if water is condensing under it.
If too much heat gets to the vermicrete, it can swell and crack, although that's highly unlikely with all your blanket layers.
firing schedule before render.docx

[SirClutzz]

Thanks David, it's not a finishing render really, was more a bulking render, as I just wanted to fill out the slab lips around the back more then anything, and if that gave me a bit more insulation, then great. I initially wasn't intending on doing perlcrete at all, rather relying on the 75mm of blanket to do the insulation work.

I've read about sealers that some have used in the us, that are vapour permeable. I think the point being that water can't pass through unless in gas form (steam). Hence rain would not pass in, but as you fire the moisture could see pass through. Have you seen anything like that down here in Australia?

I do still plan to do a finishing render over the top later, after the benches are poured and tiled.

Thanks for your firing schedule, I'll take a look.

[david s]

Thanks David, it's not a finishing render really, was more a bulking render, as I just wanted to fill out the slab lips around the back more then anything, and if that gave me a bit more insulation, then great. I initially wasn't intending on doing perlcrete at all, rather relying on the 75mm of blanket to do the insulation work.

I've read about sealers that some have used in the us, that are vapour permeable. I think the point being that water can't pass through unless in gas form (steam). Hence rain would not pass in, but as you fire the moisture could see pass through. Have you seen anything like that down here in Australia?

I do still plan to do a finishing render over the top later, after the benches are poured and tiled.

Thanks for your firing schedule, I'll take a look.

No, I haven't seen anything like that. I don't think it works that way. If it's a permanent or partial barrier to moisture it will work equally in both directions. Maybe if it's some kind of silicone coating that repels water but allows vapour perhaps. I think Thompsons sealer works like that but you need to keep reapplying it at least every 12 months. Most commercial renders contain additives that make them partially waterproof.This is usually enough to prevent water soaking into a wall while at the same time allowing some of it to escape. You can tell when you try to mix water with commercial renders, they rest it mixing in, unlike a render you make up yourself where the water penetrates immediately.
Because your oven is under a roof allowing it to breathe by having a porous outer shell would be your best option I'd say. Our oven is out in the weather and although the exterior is fully sealed, during the wet season (Townsville) even if it hasn't rained, the really high humidity gets into the oven and sometimes requires a slow fire the day before a decent cook up. My oven being small does not consume much wood so that's not a problem. My insulation layers are also vented to the atmosphere for better drying and steam pressure relief.

[Giovanni Rossi]

I have in my notes that Mongo used Laticrete - Hydroban Liquid, a product that is waterproof but vapor permeable. Not sure if it's avail in your hemisphere.