Efficient dome insulation in a tight space

[crowx]

I am building my first oven in our small garden, hemisphere Pompeii style.

I've built the oven base with 4inch vermiculite concrete as an under-hearth.

The oven base is as large as I could get away with in our small space - 1.1mx1.3m.

I am about to start on the hearth and firebrick dome, but really not sure how much space to leave around the outside edge of the dome for insulation. It must stay within the 1.1x1.3 base.

I'm using half-firebrick as dome depth, so roughly 11cm.

Advice seems to vary from multiple laters of Ceramic Fibre blanket, chicken wire, and vermicrete, to just a few inches of the latter.

What is the single best readily available insulation type for a given thickness?

Would the oven really suffer if I set my interior diameter at say 70cm, so 92cm external diam, allowing then for a thin layer of fire cement over the oven dome fire bricks to keep its integrity, and then just under 4 inches of vermiculate concrete to clad the whole lot?

Or is that just not enough and better to compromise on cooking area?

Or is there a better way to use that 4 inches than just one layer of vermicrete?

Advice is not hard to find, but there is so much different advice it is hard to read it for your own specific situation as a novice!

Any more tips on this hugely appreciated!

[5leafclover]

That's a great sized base for an oven in my opinion. Larger ovens will just consume more building materials and later on firewood.

I'm working with a similar size constraint at 103cm base width and the same 11cm brick width. I've chosen 5cm thick ceramic blanket which I plan to top with a 2-3cm render layer, which Im hoping will be strong enough.

This gives me a 64cm square shaped hearth, which is probably similar in feel to a 70cm circular hearth. In my opinion its a sufficient size, but probably as small as you'd want to go.

Of the high temp insulations, ceramic blanket will perform better per thickness than vermicrete (if kept dry). CalSil board is good too, but not appropriate for a dome.

An 8cm rebate around the outside of the brickwork is probably a sensible minimum for either the blanket plus render or pure vermicrete options.

If you really still need more hearth, i'd look to trim the bricks, have a small overhang at the sides, or opt for an elongated dome, rather than drop to a 1 inch ceramic blanket.

Hope that helps. Hope you get some other replies too for a balanced view.

[cobblerdave]

G'day
Ceramic isupation is the most effective per its thickness. 2 inchs of ceramic is equal to 4 inchs pear lite cement in insulation value. That level of insulation seems to be what most homebuilders use and most commercially made ovens only include enough for 1 inch
I here's a link to a test of insulation on the FB site
Regards dave
www.fornobravo.com/community/forum/pizza-oven-design-and-installation/getting-started/281-insulation-efficiency

[crowx]

Thanks folks. That gives me quite a bit more confidence that I haven't built it too small and will be able to fit the insulation I need to make it worthwhile.

Sounds like a couple of inches of blanket is enough so can probably go slightly larger than 70cm internal diam.

I have also realised I can start to put more insulation on the top once the surface of the dome veers away from the vertical. I imagine there is no problem with having more insulation over the top than the bottom edges.

[oblertone]

The single best insulation material (imo) is loose dry vermiculite, however it has to be contained when used loose which normally means building a dog-kennel type structure over the oven and back filling. To keep it in place where no outer structure is used it gets mixed with cement at 10/1 and is known as vermicrete, which can be a pain to work with.
The usual compromise is to use ceramic blanket over the dome, held down by a chicken wire net that is then either rendered direct (as I did - see below) or covered in vermicrete, then rendered.
There are other variations using foam-glass, rockwool or clay/straw mix and doubtless other things too, but the above covers the most common techniques.