Brisbane Build

[Hatzi]

The vermicrete mix over the blanket is to restore the hemispherical form, insulate and to provide a firm enough substrate to support the outer tendered shell. The lower the cement content the better it will insulate. Some go as low as 13:1, but I find 10:1 is about as lean as I can go and still have a workable mix. Even a 10:1 mix has more weight in cement content than vermiculite. I also like to use a 50/50 mix of perlite and vermiculite, it makes a more workable mix than either alone.For improved workability I also throw in a handful of powdered clay for every litre of cement added.
So my brew is 5 litres vermiculite, 5 litres perlite, 1 litre cement, a handful of powdered clay and 3 litres of water. Mix dry ingredients then mix in 1/3 of water, knocking down any lumps, add second 1/3 of water, mix and finally add last 1/3 of water. Place from the bottom up making a flat ledge onto which you can build another layer until you get to the top.
You’ll swear it’s not going to work when you place it but in 24 hrs it’s fine.
If needed you can then carve away high spots or fill low spots with the same mix.

thank you david s

Do you use just normal cement?

Now to try and find a supplier of powdered clay.

[david s]

yes normal cement. the temperatures that layer will feel are well within normal cement's capabilities. For powdered clay the cheapest is Cement Australia's Bricklayers Clay. If you can't find that try Potery Supplies and get a 20 kg bag of Ball Clay.You'll need the clay to make the home-brew mortar as well as the small amount in the vermicrete mix.

[Hatzi]

yes normal cement. the temperatures that layer will feel are well within normal cement's capabilities.

thanks again ... now trying to locate where to buy clay in brisbane .... Good luck i say

[Hatzi]

Crisis averted thanks to david s

So yesterday finished my first layer of 25mm thick perlite. Will leave it "dry" for a couple of days before i add my next 25mm layer.

Not sure if people have heard this before, but it was suggested to ne by a refectory "expert" to wet the perlite in the first few days to help it cure.

[david s]

Vermicrete insulating slab copy 2.pdf No need. Because vermicrete or perlcrete requires so much water in the mix there’s plenty of free water hanging around for sufficient damp curing. Only about half of the water added goes into the hydration process so there’s plenty left.Even in dry conditions you need to wait more than a couple of days drying. As the strength of the layer is hardly a consideration, it only needs to be strong enough to act as a firm substrate for the outer render. When the stuff goes from grey to white , you’ll think it’s dry, but it won’t be underneath. You really need to give it a week to dry.
Read my experiment on drying a vermicrete slab to give you an idea how slow it dries.

By the look of your pic your mix is way richer than 10:1. Remember that the more cement you use the less it will insulate. It is especially important to let it dry completely if you are applying it directly over the dome rather than over blanket as any moisture there will rapidly turn to steam creating problems. A layer of blanket takes the sting out of the hot dome, working like an expansion joint and preventing sudden heating of the perlcrete layer.

[david s]

The little attached table gives you the insulation value for various mixes. The 10:1 mix should look like the attached pic with visible voids between the perlite grains. Just make your next layer leaner for more affective insulation.

[Hatzi]

So ended up finding more premix "vermiculite" (photo attached).
1. So since my last post have managed to finish the 2 layers of 25mm "vermiculite"
2. Have also completed a number of slow burns to dry out the "vermiculite".
3. Tile work around oven on the bench

Now waiting on chimney flue and attachments to arrive

[Hatzi]

So, now ready to add a 50mm layer of cement render over the top of my rockwool.

Question though what mix have people used for their render?

At this stage I am thinking of using a sand, cement, lime mixture with the ratio of 6:1:1 Apparently the lime gives the render some elastic properties and reduces cracking ( found this on a rendering website). What do people think?

Also, have been reading about "vent holes" for the water vapor. Am interested to see what people have done and if they are really necessary.

The finish line is getting closer !!!!

[NCMan]

So, now ready to add a 50mm layer of cement render over the top of my rockwool.

Question though what mix have people used for their render?

At this stage I am thinking of using a sand, cement, lime mixture with the ratio of 6:1:1 Apparently the lime gives the render some elastic properties and reduces cracking ( found this on a rendering website). What do people think?

Also, have been reading about "vent holes" for the water vapor. Am interested to see what people have done and if they are really necessary.

The finish line is getting closer !!!!

You are using rockwool as an insulator over the dome and then plan on a render coat over it?

[Hatzi]

You are using rockwool as an insulator over the dome and then plan on a render coat over it?

Yes that is correct.

The rockwool is my second layer of insulation, first layer is perlite mixture.

[NCMan]

So, you compressed it? If so, that's not a very good idea. Is there a reason why you used it there and not ceramic blanket? Just curious.

[Hatzi]

So, you compressed it? If so, that's not a very good idea. Is there a reason why you used it there and not ceramic blanket? Just curious.

I suspect that you will get compression to some degree with either product. I am using a product that WFO manufacturer suggested.

If you look at UtahBeehiver build , there is a debate in his thread about the layering of the dome and when to use the blanket. When reading through the thread and doing some research, it seemed to make sense to me use teh blanket over the top of the vermiculite and then have the render on top of the blanket.

My WFO will not have a separate housing as is seen in many builds in colder climates.

[Hatzi]

So, now ready to add a 50mm layer of cement render over the top of my rockwool.

Question though what mix have people used for their render?

At this stage I am thinking of using a sand, cement, lime mixture with the ratio of 6:1:1 Apparently the lime gives the render some elastic properties and reduces cracking ( found this on a rendering website). What do people think?

Also, have been reading about "vent holes" for the water vapor. Am interested to see what people have done and if they are really necessary.

david s

oasiscdm

UtahBeehiver

what do you guys think?

[oasiscdm]

Hatzi i just used premix bags of light weight render. Costly option. Then i used a spray on granite mix for decorative finish

I drilled 6 holes in bottom of hebel to the calsil board only.

[Hatzi]

Hatzi i just used premix bags of light weight render. Costly option. Then i used a spray on granite mix for decorative finish

I drilled 6 holes in bottom of hebel to the calsil board only.

is this venting for the oven floor?