Dome Insulation - Vermicrete Ratio and foil???

[kypreos]

If the weather holds out today, we'll be having a go at the ceramic blanket insulation...

Following that, the Vermicrete insulation layer - for which I have a couple of question, pls:

What's the recommended ratio for Vermiculite to Portland for the dome insulation?

Also...

What's with the foil over the ceramic blanket that I've seen some people using...? surely hat's just asking for trouble with allowing the oven to breathe and the escape of steam...

Cheers!
Kyp

[turkey]

no idea on the foil, but I would not worry about the steam it will punch its own crack in the cement render so a thin bit of foil will not hinder it in any meaningful way I doubt, I am also not sure on its purpose but I guess its to keep the blanket from absorbing water from the vermicrete?

for the mix ratio you want it as lean as possible as cement does not insulate I think I went the easy route and did something like 2 buckets to 2 flowerpots of cement (wanted an easy life and worrying over the insulation factor was no fun, I can say the outside is less than luke warm during pizza cooking so working fine) in a big builders bucket and mixed it all up there as did not have anything larger, Terry swears by the 1ton builders bags which mean you can make a lot up at once and really mix it well, you can probably go even leaner on the mix then

[spinal]

From what I've read, dome insulation is 10-20 parts of vermiculite for 1 part of portland (by volume). The difference is on how structural you need it to be. If you can get way with 20 - better...

The foil is an old method of creating break so that things underneath could move/expand without impacting the vermicrete dome...

Some swear by it, some say it's useless...

I'm not putting it in as in British weather I'm more worried about the humidity than a bit of movement...
M.

[faz]

I did a 9 or 10 : 1 mix with a dash of lime for good measure It turns in to a loose sloppy mixture which does not resemble mortar or concrete at all - so don't be concerned that it looks different.

I also did the builders bag mixing method and found it to be a good way of getting the materials mixed without breaking up the vermiculite up too much. I found it set quite hard after a few days, so you could probably get away with a slightly leaner mix, as long as you aren't looking for any significant structural strength.

Take a look at a few threads and see how people have applied the vermicrete to get some tips on what the best way of getting the sloppy mix from the bag to the oven is

[tonyb]

I wasn't convinced about the foil so didn't use it. The Scott designed barrel ovens use the foil , which is placed between the brick and concrete layers to create an expansion layer, but we generally don't have a concrete layer and have the ceramic blanket next to the bricks so not needed as far as I can see.

I've never seen anything reported about 20:1 mixes, I doubt they would be mechanically stable. I have seen people have used up to 10:1 but more often 5:1, which is what I used. In general the higher proportion of vermiculite the better the thermal properties but the weaker the mechanical properties and also the ease of use.

[kypreos]

Thanks guys

Did the dome vermicrete at the weekend... I was aiming for a 12:1, but a mix-up with quantities meant that we're not quite sure what we have... Probably somewhere between 10 and 12 though...

We tried to keep it as loose as possible with temporary forms and din't worry about teh aesthetics too much... End result, is that the shape still needs some work and it's a bit crumbly in places, structurally. So I'm planning to mix up another slightly stronger batch - and go over it again, if possible to add some strength and shape.

Here's a couple of pictures in the meantime:

[kypreos]

Before:

Attachments:

[kypreos]

After:

Attachments:

[cannyfradock]

kypreos

I've used vermiculite initially after reading the Guru's on the FB forum who say a ratio of 6 to 1 (presumably by volume) of vermiculite to cement with water added to bind. The first time I used it I found it difficult to bind so on more recent builds I have added lime which seems to hold it together a bit more.

The mixing in an empty ton builders bag came from someone on our forum and I have always copied this as an easy way of mixing this stuff. I have also picked up many tips from comments from Tony B and Turkey....they always have valid points.

The last time I mixed up vermerete was very close to Faz's version....and I agree with Spinal on the foil layer......I have always left it out.

Terry

[cannyfradock]

Addition for "Baker Girl"

Vermecrete is a mixture of vermiculite, cement and water. Some people add lime which helps in binding this "devil's porridge" as one of our members has called it. If lime is added, make sure to wear marrigold or similar gloves as the lime is not friendly to the skin.

The finer vermecrete is best for our purposes although any grade will work. It is available from refractory suppliers like Kilnlinings, or most builders merchants stock it as a "stock item". Also many garden centres stock it. It comes in 100ltr bags (it's very light) and normally 3 bags will give a 3" covering on a 90cm internal diameter oven.

The ratio is between 6:1 and 10:1 vermiculite and cement with water to bind. If using mixing by volume, then it's more like 10 buckets of vermiculite to 1 bucket of dry cement. The whole idea is to coat each granule of vermiculite with a little cement so as when it dries...it resembles a rice crispie cake.

Mixing.....

A cement mixer method will work but you need to keep the back of the mixer wet for the stuff to mix correctly.....it can't be to wet or you won't be able to form it. It can't be too dry or it will fall apart. The "devil's porridge" description is good....just imagine a stodgy version of it.

An old plastic dust bin is also good for mixing and.....I use an empty 1 ton builders builders bag. I empty the vermiculite into the bag the night before, and throw a couple of buckets of water on it. The next day I add the cement and pull the bag from side to side to mix the stuff. If it's too wet dry it up with "hydrated" lime....don't forget to use gloves when working with lime.

All the best with your build.

Terry

n.b...The forum has it's own discount account with Jewsons (being revamped at the moments, but discounts on vermiculite should still apply. see here...
ukwoodfiredovenforum.proboards.com/index.cgi?board=membersdiscounts&action=display&thread=82

[wishy14]

Hi all. I'm very knew to the forum and the world of wood fired ovens to be honest. I've bitten the bullet and made the dome itself from vermicrete over a gym ball. The mix I used was 5:1 but have found it difficult to find advice regarding the water ratio to this mix. I read somewhere that just enough water is best to form a loose ball in the hand. I did this and I have now formed the dome and I'm waiting for it to cure whilst keeping the surface dampened with wet blankets.

A couple of questions are;

• I intend now wrapping the formed dome in situ on the ceramic board and firebrick base. Should I wrap in blanket once set then put another layer of vermicrete over and then screed, I have a certain manufacturers, V----s,  screed mix?
• Also, is it possible to screed the inner of the dome with this same product to smooth off and maybe stabilise the delicate Vermicrete inner dome itself?

I suppose the last question of all you experts is will this dome even work or will it fail miserably and end up resembling an earthquake proof ant hill?

Thanks,
W

[albacore]

So are you saying that your dome is entirely made of vermicrete? Normally the dome is made of a) firebrick b) modular precast dense refractory sections or c) cast in situ dense refractory.

All these materials have a high thermal mass to store the heat from the flames and release it to the food being cooked. They are insulated at their external surfaces with ceramic blanket/vermicrete to retain the stored heat.

I am not sure that vermicrete alone will make a suitable dome, though it may work in some fashion. It may also be structurally weak.

Sorry not to be the bearer of better news!

Lance

[wishy14]

Thanks for that, the dome has certainly set and seems structurally sound. Having said that I've yet to deflate the support

Many have made ovens from vermicrete, just YouTube Vermicrete pizza oven.

I just haven't seen anywhere where people have screened the inside with heatproof screed. However after reading the directions the screed is designed for both internal and external applications up to 1200 C

[albacore]

OK, good luck with it! Let us know how you get on - and show us some pictures!

Lance

[wishy14]

Will do Lance, might just need some luck ;-)