Advice about dome shell.

[angelboy]

The dome shell I've build is made up of 5:1:1:1 of perlite, portland cement, sand and hydraulic lime (over 50mm of fibre insulation). I don't know the actual thickness but I used 200l of perlite in total - I'm guessing anything from 25mm to 50mm.

I want to know if I can now add a regular exterior render?

Should I cure the perlite shell first then add the render later or should I do the render first then cure the dome?

The render I was going to use is waterproof but breathable if that makes a difference?

[david s]

The mix you have used does not result in a dense casting. Consequently it is not particularly strong which results in it being very subject to damage if bumped as well as abrasion around the entry. Additionally it results in a low thermal mass which results in an inability to store a lot of heat which is a requirement for retained heat baking and roasting. Because the mix also results in low thermal conductivity it will be slow to both heat the material and slow to cool down. Experience from oven design prefers a dense, conductive inner casting covered with a highly insulative layer to preserve the stored heat, over which is a weatherproof outer enclosure or igloo shell.
A 5:1 mix that you have used is somewhere in the middle between strong / dense and weak / highly insulative. If it were mine I’d be adding a 25mm layer of blanket which, in addition to being a good insulator, will also act as an expansion joint reducing expansion pressure on the outer shell, (skip the chickenwire) over which another layer of highly insulative 10:1 vermicrete, allow at least a week to dry, before doing the drying fires, then do your cement rendered outer shell fortified with AR random mixed glass fibres. Most commercial renders/stucco are partially waterproof, but once it’s perfectly cured and dried a further fully waterproof coat can be applied.
Others may disagree with this approach as there are many ways to achieve your ends.

[angelboy]

Sorry, I didn’t explain myself properly. I’ve build a brick dome (100mm thick), then 50mm of blanket then the perlite shell.

[david s]

Ah, ok, then disregard my advice. As you've used a 5:1 mix for your over the blanket layer, it will take quite a long time to dry because it is far denser than a much leaner mix. See attached experiment drying a 5;1 vermicrete/perlcrete slab. Builders have found for adequate strength to hold the floor and the weight of the dome a 5:1 mix is required. However over the dome you only need a weak mix, enough to act as a firm substrate for the render/stucco, to maximise insulation value (see table) I've found a 10:1 mix is about as lean as I can go whilst still being workable and holding together enough to stay in place, but results in insulation value about the same as blanket for a given thickness.
The drying fires should be done before the render/stucco is applied, otherwise excessive moisture can get locked in and cause problems.

Vermicrete insulating slab PDF copy.pdf

[angelboy]

That's great David, thanks.

I'm going away for a few days and back Monday, so was rushing to get the dome complete so I could start the curing fires ready for the weekend. At least now I can leave the render until after the curing.

I'd already cured the dome a few weeks back so I could use the oven for a birthday party (worked great for cooking pizzas) but I understand I should repeat the process now I've added the blanket and perlite shell.

[david s]

If you used around 200 litres of perlite then you would have added around 70 litres of water. That's more than a rubbish bin full of water! As less than half of that would be used up in the hydration process you are left with a huge amount of free water to remove. If you can leave it in the sun and wind for a week it will do much of the drying from the outside, but you still need to push the moisture out from the inside with some prolonged slow fires.

[angelboy]

Thanks David. Makes sense when you put it like that.

[Pizzarotic]

Angelboy, given that you have insulated the dome with ceramic blanket, you will need considerable prolonged heat for it to transfer through to the vermiculite layer to aid drying. After 1 to 2 weeks of natural drying I would just use the oven to cook a few pizza then seal the door and let it cool slowly. You should then be ready to apply your final render, let that dry then waterproofing and/or final finish.