Insulation necessary around top of the oven?

[jezzaa]

Couple of pictures of progress.... Things seem to be going well!

I have started driving out the moisture and am on my third fire this evening. I can feel some hot spots when moving my hands around the top of the dome. Is this likely because there is a gap in the insulation?

Also, I have bought a flue but, as I did not have it when I cast the hole for the flue, unfortunately my flue does not quite fit in the hole. Is it safe to use a grinder bit on a drill to finesse the hole so the flue sits nicely?

Thanks

[jezzaa]

Thanks so much David. Will keep you posted

[david s]

And when wrapping my mould with wet newspaper, presumably I should also wrap the edges of the tiles, so they don't stick to the cast dome?

If building the dome around the floor tiles, most builders like to leave a small gap between the floor tiles and dome to allow for floor tile expansion, so you can use one thickness of corrugated cardboard box to do this.

[david s]

If building a hemispherical oven the internal height should be half the diameter from the top of the floor tiles to the internal crown of the dome. The oven door opening should likewise be 63% of the internal height measured from the top of the floor tiles. If you get a stick cut to the internal height and place it in the centre of the floor you can then easily build your sand castle around it until you reach the top of it.

[jezzaa]

And when wrapping my mould with wet newspaper, presumably I should also wrap the edges of the tiles, so they don't stick to the cast dome?

[jezzaa]

Before I build my mould for the cast, should I be calculating oven height and entrance height based on measurements from the top of the tiles? Or the top of the calsil? I have an internal diameter of 750mm and was going to make the height half of that, but not sure if I should build it 375mm above the top of the tiles, or 375mm above the calsil.

Likewise should the top of the entrance be measured from the top of the tiles, and be 63% of the height of the oven from the top of the tiles?

The tiles are only 40mm in height, so maybe it is not too important, but funny the questions that hit you in the middle of building something.

Thanks in anticipation!

[jezzaa]

Hopefully rain will hold off tomorrow and I can get the cast done

[jezzaa]

Some photos of the progress so far

[jezzaa]

Excellent, thanks David for this and all your advice.
I have the materials for dome building and plan to get started on the dome tomorrow! I will be using a commercial mix - Densecrete 135 it is called.

[david s]

That all depends on what kind of cement you used in the casting. If OPC then you need to damp cure it for a min of one week. If Cal. aluminate cement then you can begin driving out the water 24 hrs after casting it. It is not a good idea to drive the water out before insulating it IMHO because there will be a huge difference between the inside and outside temperatures which can lead to cracking as occurs in heating an uninsulated chimney. The insulation will help even these temperatures out, although will also make the process take longer. It is best to drive out the water after insulating, but before applying the outer render, then after rendering the outer shell, wrap it up (I use cling wrap) to hold moisture in that outer layer for a week,

[jezzaa]

Folks, can you advise how long I need to allow a cast dome to cure before I put insulation blanket on it and render it?
Would I be safe if I cast the dome on Saturday and then came back 24 hours later and added the blanket and the render? Or should I wait until next weekend?
Thanks

[jezzaa]

OK great thanks David. So it sounds like I will need about 130mm for render and insulation plus 100 for the bricks - cheers

[david s]

My walls are only 50 mm thick (cast oven), if you are building in brick then they need to be 100 mm thick because of the need to have enough strength in the mortar joints, thin brick walls tend to fall to bits from the heat cycling. Insulation and outer shell in my oven take it to 900mm in outside diameter.

[jezzaa]

Folks, was wondering what the your advice for dome wall thickness is?
David, it sounds like your walls are (910-540)/2 = 185mm thick - what are they made of? I was assuming that my walls would have to be 115mm(brick)+50mm(insulation blanket)+50mm(render) = 215mm at least.
Thanks!

[jezzaa]

Fantastic, really appreciate the advice everyone... The (re)search continues!