I think the objective is .....insulate as best you can with a combination of blanket and vermicrete. If you have 4 inches of blanket, I would use it and cut down on the verm layer, to accommodate. On the other hand if you only have 2 inches of blanket then increase the verm to take up the available space.
The vermicrete on an open to the elements style pompeii serves 2 purposes, firstly as insulation and secondly as a solid structure on which to put the outer, weather-proof coating. I wouldn't have vermicrete as the outer layer, it insulates because its full of air gaps but that also means its not waterproof.
I'd be interested to hear what others have to say on this.
Jonaero I had same size restriction on mine too. For the blanket + vermiculite (or not) issue I guess it depends on the quality of the blanket and what you want to use the oven for. For my oven I just used blanket and no vermiculite. The blanket was high quality though. The outside of the oven gets up to maybe 25c (on a cold day) hours after I have cooked pizza at 450c. So there is no issue in getting the oven up to temp for pizza and no danger of burning people if they touch the outside. But does it reduce my cooking time if I want to do long slow cooking? yes, probably but it is still 100c in the morning after a pizza burn the previous night so it's still good for many hours of cooking. Does it risk cracking the outside with temp fluctuations? yes, probably but none yet on mine.
both offer insulation, blanket is more efficient so less is required (by thickness), I think the various options presented in the pdf are to achieve about the same level of heat retention so is very much option 1 or option 2 etc, it sounds like you are considering having all options, which space and budget not being an issue is great, but overkill.
I thought the blanket was more expensive the vermicrete but I suppose it depends on suppliers and what not. If it is cheaper for you that's great as it takes less space and that is what you have an issue with here
you can also get some sort of resin that sets over these blankets to give them a hard outer crust , I am not sure how that cost compares to the vermicrete layer, but at least the vermicrete adds even more insulation, and is a nice rough surface to render against and I guess the render will leave a smoothish slippy surface so possibly not as great.
I probably have about 3" blanket and 1" vermicrete and can say the outside does not really warm up all that much under full firing so can only assume the insulation is working well, if you want to bake with retained heat remember to ensure you have a decent door.
I'd add that only 1" of Vermicrete will be a bugger to apply and probably not going to provide enough of an intermediate layer for a render. Maybe Perlite would be better, but i've no experience of using it.
You may have to concede that your insulation layer is going to be compromised and heat retention will be less, but probably still adequate or you'll have to concede cooking space and reduce the oven dimensions.
The final option is to cantilever, or extend, somehow the base layer of your build.
I do not think you can skim (with a float etc) with vermicrete unless you are a real pro, I stuck on some rubber gloves and then moulded the vermicrete onto my blankets and chickenwire by hand, this then make it slightly easier.
I don't think the 1" will make a massive insulation difference but its better than nothing and I think possibly easier to apply because of the larger aggregate in it (the vermiculite), I found the lower part of the dome where the wall is basically vertical really hard as gravity pulls everything off and the render does not grab well to the blanket so I ended up more making a vermicrete ring wall round this part and letting it dry a bit, this then could support the rest of the first layer, then once this was set my second layer went on much easier. You would not need the second layer if you did not want but I think it would be easier if slower to do that way in steps, I think others have managed a full coat in one go but for me it just kept falling down / off.
Has anyone here rendered direct to the blanket? how hard was it?
we completed our build in January and used the body soluble blanket from kiln linings as insulation. We used 2 layers of the 50mm thick blanket to give a total of 100mm insulation. It was very easy to do, put one layer in place cutting with scissors as required then covered with chicken wire to hold then second layer again cover with chicken wire. We then rendered using the castable from Kiln linings. We have fired the oven about 8 times since then and the exterior doesn't get hot at all even with the fire burning all evening for a pizza party so that is definitely sufficient insulation and thinner than vermicrete.
We chose to use blanket rather than vermicrete firstly to reduce space and secondly because by that point we really had mixed enough concrete for a lifetime.
You can see some photos of us fixing the insulation if you look at my Flickr album by following the link in my signature.
If you are only planning to cook Pizzas insulation is not such a big deal, However don't underestimate the need for an insulated hearth, My brick oven has no insulation and still retains heat for a very long time