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**JRPizza** · 02-21-2021, 12:32 PM

Nice progress! It looks like you might have followed the plans that don't specify putting insulation between the oven floor bricks and the slab/hearth. If yes, you may have problems maintaining temperatures due to thermal losses to the large concrete heat sink. Check out the link below to a thread started by a new member with what looks like a similar design.
https://community.fornobravo.com/for...ven-hot-enough

**JimShortz** · 02-21-2021, 04:40 PM

Originally posted by JRPizza View Post

Nice progress! It looks like you might have followed the plans that don't specify putting insulation between the oven floor bricks and the slab/hearth. If yes, you may have problems maintaining temperatures due to thermal losses to the large concrete heat sink. Check out the link below to a thread started by a new member with what looks like a similar design.
https://community.fornobravo.com/for...ven-hot-enough

Actually, if you look closely there is a 12cm slab, followed by 14cm (two 7cm layers) of highly insulating AAC (Aerated Aotoclaved Concrete) blocks to provide a very high level of sub-hearth insulation. The few red clay blocks you see on their side across the front are there just to fill in a gap and aren't beneath the hearth - you can see the AAC blocks down the sides. :-) I've attached a clearer shot here too.

These blocks have been discussed quite a bit on this forum and what convinced me to use them was some data provided by an apparently knowledgeable poster on here, called Carl: "I'm using Thermalite blocks for my hearth insulation (probably very similar stuff - another brand of AAC). Their website gives a value of 0.11 W (m.K) - the value I found on the web for vermiculite concrete was 0.094 W (m.K). AAC bricks actually have a lower conductivity than insulating firebrick, and since the blocks have a very similar conductivity to vermiculite concrete, I've decided to use them."

The quote from Carl comes from post #24 in: https://community.fornobravo.com/for...der-slab/page2

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**JimShortz** · 02-21-2021, 04:44 PM

A few pics of stage two of the build - all three fire brick domes and front and rear walls completed, and a well-earned beer to celebrate, lol.
I only wish I had taken some pictures of the inside of the dome after I went through the contortions required to point it up properly. It looks great, if I do say so myself, and I didn't photograph it! :-(
I have now moved on to begin the insulation - a combination of 5cm of perlite followed by a layer of 7cm of aerated concrete blocks (AAC) - the same blocks as form the two insulating layers beneath the hearth. I will add a further post to share progress on that very soon :-)

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**JRPizza** · 02-21-2021, 07:14 PM

That's great! I could not tell if there was insulation or not. Looks like you are right on top of things.

**JimShortz** · 02-21-2021, 07:32 PM

Originally posted by JRPizza View Post

That's great! I could not tell if there was insulation or not. Looks like you are right on top of things.

Thank you. This forum has been super helpful. I did a lot of reading on here before launching into the project...

Here's some more photos, showing the chimney hole and the chimney in position, as well as my first fires. I'm aware that not everyone thinks it is a good idea to fire before insulating, but I couldn't wait!

The first fire reached a max of 180C on the upper side walls, but I got to 450C on the second - oops, lol. Nothing cracked and all good to go. I think it helped that the main dome had already been built for 2 weeks and had sat under the hot Thai sun (dry season here), so all of the blocks were very dry already anyway.

Insulation layers post to follow... :-)

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**JimShortz** · 02-21-2021, 07:45 PM

This is where I am currently up to. I have begun insulating the dome with a combination of 5cm of perlite (mixed 5:1 with portland cement) and then a layer of shaped and tightly fitted 7cm AAC blocks - giving 12cm (5 inches) of insulation overall. There are actually two layers of AAC blocks on the front wall, but that decision has more to do with how things work out with my base building, the double domes, etc, - to create a sensible and simple shape when finished.

I've used about 50L of perlite so far, with 50L left to finish the main dome. I also plan to insulate the entrance dome, so may need to buy a bit more perlite to finish off. I'll post more pics as it moves forward. :-)

Once the insulation is completed I have a friend who will do the final waterproof rendering for me (with glass fibres mixed into the base layer. I am no builder, and I think rendering a dome may be beyond my skill set. I'm fine sticking blocks together, but rendering that shape might be a step too far, lol.

I know that there aren't many who have used AAC blocks, so I will also post some performance/heat retention figures once th baby is in use...

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**JimShortz** · 02-26-2021, 04:42 PM

Nearing completion - insulation with Perlcrete + full jacket of AAC insulating blocks completed. First firing of insulated oven coming up... :-)

Actually, to say "insulating with perlcrete" is a slight lie. In the Upper part of the dome where the "perlite" layer is deliberately much thicker it is actually chopped up chunks of AAC block offcut sat in a perlcrete matrix. It used up every bit of my left over AAC blocks (chopped up with a machette, would you believe - worked great) and that helped out a lot in filling the spaces - and used up all of the waste; double winner!

At the top of the oven the space for perlcrete reaches about 13cm, whereas only 5 cm on the sides where the oven is less hot. It seemed like the most efficient use of materials to me and I have got away with using only 100L of perlcrete by using the chunks of AAC in the matrix. Perlcrete is fairly expensive here, whilst offcuts are free. unfortunately I didn't photograph this part of the job...

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**JimShortz** · 02-28-2021, 08:47 PM

Help sought with a highly insulating door design...

I have a metal door with a closable gap underneath for use during firing and cooking, BUT, I want a higher insulating door to use when closing the oven up overnight after cooking pizza - ready to bake the next day.

I have seen one with percrete sandwiched between aluminium and with aluminium mesh around the perimiter of the perlcrete. That's a possibility, as is making one out of AAC blocks (or buying an AAC poanel to cut to size). Does anyone have any thoughts on this? Or other ideas?

Thank you :-)

**JRPizza** · 02-28-2021, 09:07 PM

Have you checked out this thread yet?
https://community.fornobravo.com/for...read#post17312

**JimShortz** · 02-28-2021, 11:55 PM

Originally posted by JRPizza View Post

Have you checked out this thread yet?
https://community.fornobravo.com/for...read#post17312

Now that is what I neeeded - thank you! :-)

**MarkJerling** · 03-01-2021, 03:16 AM

I like your use of AACA panel. I used that under the floor of my oven too, but I like that you also used it for your "walls" around the oven. Very tidy!

**UtahBeehiver** · 03-01-2021, 07:54 AM

If you can avoid aluminum as the door material it would be better, it has a very high Thermo Conductivity number (aka K value or the ability to transmit heat). As far as metals, Alum K value is around 220 to 230,
Carbon steel 54 range, Stainless, 14 range. Some builders use wood but it limits temperature range (0.17) you can use for blocking the door. So you can see that AL will transmit heat 16 times more than SS. The insulation will slow the transmission across the middle but not along the perimeter. FYI

**JimShortz** · 03-01-2021, 07:58 PM

Originally posted by UtahBeehiver View Post

If you can avoid aluminum as the door material it would be better, it has a very high Thermo Conductivity number (aka K value or the ability to transmit heat). As far as metals, Alum K value is around 220 to 230,
Carbon steel 54 range, Stainless, 14 range. Some builders use wood but it limits temperature range (0.17) you can use for blocking the door. So you can see that AL will transmit heat 16 times more than SS. The insulation will slow the transmission across the middle but not along the perimeter. FYI

Those figures, and the thought, are super useful in considering the design. Thank you :-)

**JimShortz** · 03-13-2021, 03:15 AM

Door for overnight insulation...
I made this baby today, oven heating as I type! :-)
It's made from 2mm steel and a pair of 7.5cm thick AAC blocks sandwiched between the steel faces and carved to fit the shape. First use later today - I will report back on success, or otherwise...
I have attempted to limit connections between front and rear face, but the 6 bolts are unavaoidable (unless anyone has a better idea!), and along the bottom I made the steel panels only connect along about 30% of the edge (see photos).
Yes, I know I am not a professional welder. I only have an old stick welder, and although not beautiful, it is strong.
We shall see how it works... :-)

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