Insulation necessary around top of the oven?

[jezzaa]

Hi,
I'm a newbie and have just started my research on building an oven so here goes with my first stupid question
If I build a dome oven out of firebricks is it completely necessary to have insulation around the dome? I ask because we are quite space-constrained - and the insulation adds width, and both of us prefer the look of the brick dome without the render covering it.
Thanks!

[Gulf]

Jezzaa,

Welcome to the forum. Imo, insulation is necessary over the dome for the oven to perform properly. A water shedding render of some kind is also. Depending on what you intend to cook, you may be able to get by with a minimum of insulation. I also wanted "the look" of a brick dome. Applying brick spilts over the stucco is how I achieved that look.

[SableSprings]

Welcome Jezzaa! As Joe (Gulf) noted, the insulation and enclosure (to keep the rain off) are pretty important. I did a similar thing as Joe (to achieve the brick look I wanted) but applied splits over my insulation instead of first doing the stucco. I also built a dome but preferred the look of a vault oven, so my brick facade allowed me to change the visual effect of the build. I don't know what size oven you are thinking about for your space, but a really useful oven doesn't have to be very big. David S, in Australia is able to do a lot of fabulous food in an oven that's 23"-24" in diameter (if memory serves me correctly ). So consider a smaller oven...not one without insulation and/or a water "resistant" enclosure.

Looking forward to hearing about your WFO quest.

[JRPizza]

Jezza, I cured my oven without any insulation, and really had trouble getting the dome hot enough. The outside of the bricks got up over 200F, and I was unable to get the inside hot enough to burn off the soot, which is the usual starting point when firing the oven for baking. As soon as I insulated I was able to achieve baking temps. Like Mike says above, you might want to start with how big your outside could be and see what that leaves you for a dome inside diameter.

[david s]

If you have space issues then build a small oven. Not only will it cost you less,easier and faster to build, but it will use way less fuel, so you'll use it more frequently.
i cooked and prepped 60 two min pizzas on my own to feed eighty people in my 540 mm diam (21") mobile oven two weeks ago. It's footprint is 910x910 mm.

[jezzaa]

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

[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!

[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]

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

[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

[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]

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.

[jezzaa]

Some photos of the progress so far

[jezzaa]

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

[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!