Finally decided on 32in castable dome

[Stan74]

Excellent, thanks for posting that Nick. Will check them out straight away, I was going to proceed without needles but with a sense of unease.

I second that, thanks for the heads up. It says on the site there are 3 sizes, 12, 25 & 35mm, does anyone know the recommended size to use ?
Good luck with the builds Mullster & Mesoiam, I'm loving the updates, tips and links etc

[WoodywWun]

Just found this thread! I'm on a lockdown build as well and going through the same decisions. So far I've got a house-brick stand constructed, with a nice little arch, which I'm pleased with! Now the serious stull...

I'm gearing up to pour the base slab this weekend. It's 75mm thick, reinforced with a bog-standard cement mix about 1m square.

Wish I'd joined the "Thermalite" debate earlier as I can see possibilities here but they are very thick and using them would probably throw out my height calculations. Did anyone get to the bottom of how good an insulator they are? They are ridiculously cheap but I've only found 100 mm thick ones. The "FB" ceramic-based boards for insulation look the bee's knees but what a price! I may go the in the Vermiculite cement direction for underfloor insulation but it's 100 mm minimum thickness that is required so it's going to throw out my height calculations.

I'm going to go the extra expense of fire bricks for the cooking surface and the Vitcas folks seem to know what they are doing.

As for the dome, well I'm humming and harring on this as well! My original intent was to use half house bricks (as seen on many YouTube videos) held together with a good refractory mix of mortar. Then I started to get some negative views on the long-term durability on these as they heat and cool. Any views? The cost of proper fire bricks is out of the question for me but is obviously the best. So what about the cast refractory dome idea? Any info on how durable they are or how much heat they hold compared with any type of brick?

Looking at the dome insulation, the ceramic-based blanket (again from Victas) looks like a good solution as they have good insulating properties and would be fairly easy to put on. ...or are there home-brew alternatives such as another layer of fire clay/lime/cement sand mixes.

I'll render the final covering with a standard cement mix.

Sorry to pitch in so late but any learning experiences, info or ideas would be great!

[WoodywWun]

...just found this about the thermal conductivity of Thermalite blocks. Does it mean anything to anybody!? "Designed thermal conductivity (λ) 0.11W/mK"

[WoodywWun]

I've dug a little further and Here is some more information. Apologies if this isn't news to you!

The thermal conductivity ("U value") of a 100mm Thermalite Turbo block is 0.11 W/mK and the U value of the Victas Ceramic Fibre Board is 0.08 at 400C (Lower is better insulation). From another forum, I have found that in the "CIBSE" guide that vermiculite concrete has a U value of 0.17 (not sure for what thickness). I understand that you need a minimum of 100 mm thickness for Vermiculite concrete?

So the order is FB > Thermalite > Vermiculite.

A Thermatlite Turbo measuring 100 x 215 x 440 will cost around £1.70. I'm not sure how stable a Thermalite would be under a cooking floor. Any ideas?

[UtahBeehiver]

Woody,

It is time for you to start your own build thread rather than hijacking this thread.

[aaronpizza]

I've dug a little further and Here is some more information. Apologies if this isn't news to you!

The thermal conductivity ("U value") of a 100mm Thermalite Turbo block is 0.11 W/mK and the U value of the Victas Ceramic Fibre Board is 0.08 at 400C (Lower is better insulation). From another forum, I have found that in the "CIBSE" guide that vermiculite concrete has a U value of 0.17 (not sure for what thickness). I understand that you need a minimum of 100 mm thickness for Vermiculite concrete?

So the order is FB > Thermalite > Vermiculite.

A Thermatlite Turbo measuring 100 x 215 x 440 will cost around £1.70. I'm not sure how stable a Thermalite would be under a cooking floor. Any ideas?

Will be interested where this goes I'll look out for your thread!

[WoodywWun]

No highjack intended UtahBeehiver! New thread about underfloor insulation started.

[Mullster]

Finishing the Vermicrete under floor insulation

So managed to finish off the vermicrete today having bought a 2nd 100l bag and some more cement.
Struggled to get it level at first and decided to do what I think is called a slurry? The last few millimeters are a mix of about 1:1 vermiculite and cement and quite wet - allowed me to get a flatter finish. I’m assuming it’s ok because I still have about 95mm of normal 5:1 mix there too.

Really looking forward now to starting the fire bricks layer. I’ve decided to go for the proper bricks instead of the storage heater second hand ones. I’ve spent so much by this point it doesn’t make sense to compromise now.

Hoping I can share a brick oven floor next weekend....

[david s]

Don’t be in too much of a rush. Refer to post #50

[Mullster]

DavidS you must have to say that so often to us first-timers
I can’t view that doc on my phone but I will take a look later. I did drill myself some of the escape holes last weekend though - made sure they went through the concrete slab and up into the vermicrete.
We have some surprisingly warm weather right now - I kept the vermicrete covered with plastic last week - do I need to do that or just leave it uncovered this week? Apologies if the answer is in the doc!

[david s]

Because the grains of vermiculite are so porous the mix requires way more water than required for the hydration process. That means lots of free water so it’s not really necessary to hold it in for damp curing, but it is a nuisance to get rid of a large amount of it by fire once it’s been covered over with floor bricks. So leave it uncovered to let it dry more before covering it with brick.

[Mullster]

More on the Vermicrete insulation layer

So just taken off the frame - it’s been on for more than a week for everything other than the top inch and with the warm weather it’s drying out quickly.

Just a technical question though on next steps. I’ve seen it’s not uncommon to have built this layer straight edges but what is the best tool/method for cutting away those back corners (indicated in yellow on the photo). And should I do that before or after I’ve shaped the bricks that are going to sit on top - I’m thinking afterwards - I.e. cut the brick floor to shape and then mark on where the insulation layer can be trimmed away?

[UtahBeehiver]

There is more water in the vcrete that you visually see. Once you cover it with fire brick the it is just that much harder for the vcrete to dry. Cutting, I don't know, maybe a angle grinder with a cut off wheel.

[Mullster]

In the meantime

So I’ve let my materials management slip and I’ve been sat waiting for bricks to arrive so I can start the next step of laying the floor. Never mind - it’s probably ensured that the vermicrete gets time to dry.

I did really enjoy my birthday recently though - check out the gift of peels and floor brush etc!

Also got a great looking book called From The Wood-Fired Oven which I think is going to turn out to be excellent - seems to cover multiple aspects of oven build, management and then lots and lots of different ways of using the oven - broadening my horizons beyond pizza and definitely emphasis good using the full heat cycle so that firing and heat doesn’t go to waste.
Author is Richard Miscovich

[aaronpizza]

I did the exact same with my materials!
As you mentioned it is good to make you let the vcrete dry more.

Great gifts - fathers day soon so might put in a book suggestion to my wife haha!