So my oven cracked. Advice needed.

[Kris S]

Did you add 2 layers or more?

I put 3 layers, and I've still got some left so I'm gonna put like 4 layers on the top of the dome (hottest part), and fill up my enclosure with leftover pieces of rockwool as well.
The 25mm blankets are really easy to manipulate, and they seem to completely keep the heat inside.
Not cheap but seems to work great.

[ebbbe]

I put 3 layers, and I've still got some left so I'm gonna put like 4 layers on the top of the dome (hottest part), and fill up my enclosure with leftover pieces of rockwool as well.
The 25mm blankets are really easy to manipulate, and they seem to completely keep the heat inside.
Not cheap but seems to work great.

Alright thanks. I'll give it a go.

[ebbbe]

Some folk have rendered directly over the blanket, but as it’s lumpy youend up with using way more render to get a smooth hemisphere. It is also not a firm substrate to render against. I’ve only used the 25mm thick stuff so not sure how you’d go with 50. The perlcrete layer allows you to restore the perfect form again as well as providing additional insulation, saving on an additional blanket layer.

Hi again David,

I just thought of another round of questions related to the video linked below:

1. The perlcrete render Melbourne Firebrick Company uses seems way thicker than the 1:10 stuff I used to insulate my old oven with. What ratio would you go with? For info, I have bought insulating blanket for two layers of 25mm.
2. Do you have any recommendations with regards to sealing between the bottom insulation slab and the table top itself?

https://www.youtube.com/watch?v=OSEJIysnn1k

[david s]

1. The perlcrete mix Ben uses is way denser, thicker and stickier than what I use. The 10:1 mix is far crumblier and harder to apply, but is far more insulative (see table)
I Then use a much stronger render to apply over the perlcrete in a thinner layer. Notice I don't use any chicken wire over the blanket as it's way too time consuming.

2. Often a crack can develop where the outer dome shell meets the supporting slab. My solution is to coat the whole dome and the supporting slab with a flexible waterproof acrylic layer around at least 1mm thick. I use flexible pointing diluted 20% with water so it can be painted on.

[ebbbe]

1. The perlcrete mix Ben uses is way denser, thicker and stickier than what I use. The 10:1 mix is far crumblier and harder to apply, but is far more insulative (see table)
I Then use a much stronger render to apply over the perlcrete in a thinner layer. Notice I don't use any chicken wire over the blanket as it's way too time consuming.

Alright. I think 10:1 is too lean for me to work with. I can see from the instructions to the MFC oven, that he uses a 5:1:1:1 of litefill perlite, cement, hydrated lime and sand. If I want it to be more workable than the 10:1 I used before, would there be any benefit to adding sand and/or lime? I know you recommend fire clay as well to make it stickier.

2. Often a crack can develop where the outer dome shell meets the supporting slab. My solution is to coat the whole dome and the supporting slab with a flexible waterproof acrylic layer around at least 1mm thick. I use flexible pointing diluted 20% with water so it can be painted on.

Could I maybe ask for a link to the acrylic product you use? just so I have an idea of what to look for.

[david s]

Adding sand or cement to the mix simply makes it denser and therefore a poorer insulator (see table). Skip the sand and lime, keep the cement proportion as low as possible and add powdered clay to make it more workable. Even a 10:1 mix has more mass from the cement as from the perlite. A 50/50 mix of perlite and vermiculite seems to work better than either of the two alone.

Flexible Pointing was developed in the 80’s to coat the mortar on tiled ridge caps on houses. As the root is continually expanding and contracting the mortar would often crack resulting in roof leaks the roofers often had to repoint. Since the invention of flexible pointing, an extremely flexible acryli, containing fine sand for high build, the roofers never have to do that job any more. As it is more flexible and cheaper than an acrylic render and it contains plenty of UV resistance it is ideal for our use. It is designed to be troweled on but I find it easier to apply by diluting it 20% with water so it can be painted. 3 heavy coats result in around a 1-2 mm thick layer.
https://www.bunnings.com.au/selleys-...E&gclsrc=aw.ds

[ebbbe]

Adding sand or cement to the mix simply makes it denser and therefore a poorer insulator (see table). Skip the sand and lime, keep the cement proportion as low as possible and add powdered clay to make it more workable. Even a 10:1 mix has more mass from the cement as from the perlite. A 50/50 mix of perlite and vermiculite seems to work better than either of the two alone.

Alright. I'll go for 50 mm og insulating blanket and around 25 mm of perlcrete. Probably 8:1 (+ clay) at the bottom, vertical part of the oven and 10:1 at the top, where gravity will keep it in place while it's wet.

Flexible Pointing was developed in the 80’s to coat the mortar on tiled ridge caps on houses. As the root is continually expanding and contracting the mortar would often crack resulting in roof leaks the roofers often had to repoint. Since the invention of flexible pointing, an extremely flexible acryli, containing fine sand for high build, the roofers never have to do that job any more. As it is more flexible and cheaper than an acrylic render and it contains plenty of UV resistance it is ideal for our use. It is designed to be troweled on but I find it easier to apply by diluting it 20% with water so it can be painted. 3 heavy coats result in around a 1-2 mm thick layer.
https://www.bunnings.com.au/selleys-...E&gclsrc=aw.ds

Okay. I can't seem to find this in Denmark at all. I can find something called "silicone render" which is based on silicone resin (directly translated). It should be breathable and weather proof. The problem is I can only get 25kg tubs, which are expensive for this use.

Alternatively I could maybe use a render that is normally used for the rendering the above-ground part of the foundation. It has "water proof" in the name. I would pair this with a water/weather proof paint made for rendered walls. Either resin based or acryllic.

In any case I plan to pair it with a fitted tarp cover, if I can figure out how to sew tarp together and make it durable.

Another question:

The sand I use for the homebrew is sharp sand made for paving. It's the finest quality of sharp sand I can find at a 0-4mm grade. There are not may particles that are 4mm but there are some. Is that alright?

[david s]

"The sand I use for the homebrew is sharp sand made for paving. It's the finest quality of sharp sand I can find at a 0-4mm grade. There are not may particles that are 4mm but there are some. Is that alright?"

If you are making up your own render mix a 4:1:1 sand, cement, hydrated lime is pretty good. There are plenty of commercial renders but they're more expensive. Sand with a range of size is actually better, but 4mm might give you problems applying it. I'd be sieving out the coarse stuff. Holding the moisture in the layer will enhance its strength so cover it for a week.

[ebbbe]

"The sand I use for the homebrew is sharp sand made for paving. It's the finest quality of sharp sand I can find at a 0-4mm grade. There are not may particles that are 4mm but there are some. Is that alright?"

If you are making up your own render mix a 4:1:1 sand, cement, hydrated lime is pretty good. There are plenty of commercial renders but they're more expensive. Sand with a range of size is actually better, but 4mm might give you problems applying it. I'd be sieving out the coarse stuff. Holding the moisture in the layer will enhance its strength so cover it for a week.

i was planning on buying render.

With regards to sand I meant the sand for the homebrew. I can see that wasn't clear from my phrasing.

[david s]

Not sure how you are using the homebrew, is it to fill a crack, use as a castable or use as a brick mortar? If using to fill a crack or as a mortar it will perform better if you sieve out the coarse sand. If using it as a castable a variety of grain size is preferable.

[ebbbe]

Not sure how you are using the homebrew, is it to fill a crack, use as a castable or use as a brick mortar? If using to fill a crack or as a mortar it will perform better if you sieve out the coarse sand. If using it as a castable a variety of grain size is preferable.

Hi David,

I'm using it as castable. As per your advice I'm casting a new dome.

[ebbbe]

I have now made some progress on my oven. I have cast the inner dome, damp cured it for a week and then further cured for another two. I have then added 75mm of CF blanket to the dome and 50mm perlcrete around the opening arch to hold the flue in place.

My plan is now to add chicken wire and then render on top.

My question is now what type of render I should be buying for this? Just any render I would use on a house facade?