Dome finishing - Vemicrete and render advice for an amateur pls

[This lass builds UK]

Hi all

Writing from the UK - I've got to the stage of having my dome built of fire bricks, the ceramic fibre blanket is on covering the dome and chicken wire on top of that (got to this stage last year! but haven't progressed it as I'm now stuck).

What do i do next? i think i have to do a vermicrete insultation layer but some guidance would be appreciated.

I have bought some Micafil vermiculite which i was going to mix with some cement and potentially fireclay but i don't really know what I'm doing.

Is this the right materials to use? What would the mix be? is it 10:1 vermiculite:cement? Do i need to add lime or fireclay into the mix? How much water - would this not just make the ceramic blanket wet?

ande then - what can i render the dome with and what mix do I use?

I'm happy to go and source something else to render over the chicken wire and fire blanket if something else is better.

Any help with how to render the dome - or essentially what i need to do after the chicken wire stage would be so helpful and much appreciated - amateur girl builder but have loved putting something together with help from this forum.

have tried to add photos below - not sure if they will work but will give it another go if it hasn't.

[david s]

Some folk render directly onto the blanket over the chickenwire, which provides some reinforcing for the render layer. Unfortunately the blanket does not provide a firm substrate for the render to adhere to and the surface rather lumpy and out of shape. For these reasons I prefer to add a layer of vermicrete over the blanket which restores the form back to a perfect hemisphere. The surface can be accurately smoothed by tapping with the flat of a trowel while eyeballing the profile. If a 10:1 mix is used it results in around the equivalent insulation value as the blanket, so also saves on additional blanket cost.. Whether a vermicrete layer is added or not, it is better to do the drying fires after insulation, but before rendering over it with a cement render, becausethe moisture can more easily escape this way. Once the drying fires have been completed the cement render can be applied.
Some folk use chicken wire as reinforcing for the render layer, but the difficulty applying it to a compound curve is very time consuming, as you’ve no doubt already found. My approach is to use randomly mixed AR glass fibres added to the cement render when mixing, which do an outstanding job of reinforcement making chicken wire reinforcing redundant.

[This lass builds UK]

david thanks for your reply, much appreciated, I'll try the 1:10 vermicrete mix, I keep reading that it's really difficult to work with and people have gone down to a 6:1 mix. I'll give it a go though start at the back.

Do you think about an inch depth of vermicrete would do it?

Many thanks

[david s]

By the look of your rather uneven blanket and chicken wire a 2" layer would be better. Start at the bottom and press a handful against the side of the oven and at the same time use the other hand to make a flat top. This will allow you to put on another row so it has a ledge to sit on. If you've never made this mix before it is probably best to make up a small batch and do one row then leave it overnight to harden, because you'll swear it doesn't look like it'll work, but it does. This gives you a firmer base to hold the next row. Once you get the hang of the mix and you've got high enough, the dome is leaning in and the application gets way easier.
For a 10:1 mix start with 10 litres of vermiculite and dry mix one litre of general purpose cement to it. In order to improve workability throw in one handful of powdered clay. Remember that the more cement you add the poorer the mix will be to insulate. Add water slowly to the dried mixed materials until water just begins to pool in the bottom of your barrow. Once you get the hang of this mix you can make up double or triple batches. Do not use a mixer as it degrades the grains of vermiculite and sticks to the sides of the mixer and its blades. Best to fold the mix gently with a spade so you can see and feel its texture.