vermiculite

[morrishuckeba]

I have a question on insulating the dome. I am using the ratio of 1 to 10 of vermiculite and with that mixture I?m having trouble getting it to stay on the dome. It seems to be very crumbly and hard to work with. Do you have to let it set up overnight and then add more? If you have any suggestions please let me know.

[Neil2]

Re: vermiculite

Go up in 6 inch "lifts". Allow to set for an hour or so. The next lift then rests on the lift below.

[david s]

Re: vermiculite

Make sure that you mix the correct amount of water with it ie for every 10L vermiculite add 1L cement powder and 3L water. Mix it really well and work from the bottom up making sure you leave a ledge to hold the next lift. Also pressing and holding the mix in place for a couple of seconds helps. It seems like the mix won't hold but it does set up firm enough to take the outside stucco.

[turkey]

Re: vermiculite

dont people alot use mesh to give it something to grip into for the first layer?

[dmun]

Re: vermiculite

dont people alot use mesh to give it something to grip into for the first layer?

It won't stick to mesh. It's not like mortar. It's like stacking up oatmeal. As Neil says, you have to do it in courses, giving a while for the previous layer to dry, or the whole mess will collapse in a heap.

[morrishuckeba]

Re: vermiculite

It is like pushing a car up a hill with a rope.

[Dolf]

Re: vermiculite

Will it be much worse insulating when mixed 1:5 for instance?

[david s]

Re: vermiculite

A1:5 mix will set up better, but you are adding heaps more weight to the resulting mixture, increasing its density and hence reducing its insulation value. I usually cover the whole dome in one go,with a layer about 35 mm thick, leave it for a few days so nature will do most of the job of removing the water, then do a second layer. Tapping with the flat of a trowel will give you a perfectly formed hemisphere.

[Dolf]

Re: vermiculite

Thanks, I will definitely go weaker then. A tad worried now as I did my floor 1:5. While that is lower & thus less prone to heat leak, I am also worried for another reason. I use 2nd hand bricks which are very!! hard (8 cuts per diamond blade :-() so I used a build-soldiers-on-floor approach so as to get away with chisel break ends only. Problem comes in that I have casted my vermiculite in a circle within a concrete border and the excess length and corners of the floor bricks which are "sticking out" the soldier outer circumference is now resting on the concrete floor! Thus small ends not insulated at all. In my enthusiasm at the time & focussing on starting round & first encounter with fire clay etc (which hid the outer perimeter & thus allowed me to forget), so no excuse other than neglect at the time, but now 5odd rows up, its too late to redo.
On the positive side, I see many entrances not insulated to a big degree, so perhaps the loss to the lower & outer fringes may be little enough to get by.
Unfair to ask for anybody with such experience as you guys all do it right, but I am somewhat concerned.

[david s]

Re: vermiculite

Dolf,
Don't worry you'll be ok. You won't lose too much heat from those bricks in contact with the concrete. You are also correct with the floor mix. It needs to be about 5:1 for the strength required as it supports the floor and dome. I was referring to the mix (10:1) over the dome.
CheersDave

[david s]

Re: vermiculite

Dolf,
your observation about entrances is also correct
In fact most ovens have no insulation between the inner parts and the outer arch which by conduction must also be a good path for heat loss and resulting temp differential and potential for crack development. sorry this is one of my little theories/innovations that I believe can improve the efficiency of our ovens. A small break to prevent conduction to the outer arch with vermicrete also provides some cushioning to prevent outward expansion pressure on the outer arch.

[Dolf]

Re: vermiculite

Relief!! :-)

Thanks Dave

[Dolf]

Re: vermiculite

Dave
Not sure if you saw my other posts. In

http://www.fornobravo.com/forum/f8/raised-insulated-chimney-13335.html

Here, I checked my logic with you guys and then the pics on my execution/progress on the plan is at

http://www.fornobravo.com/forum/21/dolfs-bushveld-oven-12810-2.html

In essence I gathered from (amongst others your own :-)) postings that the insulation is an issue and am now inserting insulating bricks to do the trick, while pillars and lintels then carry the chimney.
Perhaps the mix between what I gain here will further compensate for what I lose at the bottom :-)

[david s]

Re: vermiculite

Excellent plan. You will be amazed at how much heat is captured by this thing.

[Dolf]

Re: vermiculite

Glad you think so - the plan may be mine, but it was through folks like yourself that I became aware of the need and could still do something about it. I guess (will check :-)) that most folks here builds only for themselves & thus needs to do it right first/only time round :-)