42" Pizza Build.

[Pizzarotic]

Maximum 1.7°

[david s]

The food inside the oven won't care if it's not perfectly level, but if the supporting slab isn't you might end up with water pooling at the base of the dome which can result in water entry to the under floor insulation, so keep an eye out for any cracks at the base.

[Sixto]

Its your call, if you are talking about the counter slab level, I think a few degrees won't matter, you can make it up with sand below the floor bricks. If you are talking about the dome, that's round so a course or two off level by 1/2" or so should be fine too, and can also be corrected with mortar.on subsequent courses if you want.

If your chimney is 10 degress off vertical, you might have a visual issue, but you could always call it the tower of Pizza! .

Bottom line is that level construction is what we all strive for, but often fall short, and that's not all that critical to the operation of the oven.

[lowspeed]

Is 1 degree ok? 2,3?

[UtahBeehiver]

This table is from Strong Vermiculite out of the USA (est 1957) so it highly likely that the K values are in Btu•in/(hr•ft2•°F) which make more sense when comparing K values of CaSi

[david s]

If it is Watts per Metre Kelvin then loose perlite is a 10 times better insulator than 5:1 mix. Now my head is spinning. If we can't find a method to work for under floor, I'm thinking 2" loose perlite then 2" of 10:1 over the dome. Could be worth the effort.

As I said it is difficult to make comparisons because not only are different units quoted (you can do conversions here)

https://www.engineeringtoolbox.com/t...tor-d_857.html

but the values change for different temperatures. A comparison of thermal conductivity for any given material at ambient may be different at 200C or 400C. eg one material may be an excellent insulator at ambient, but not so good at 400C. Another complication is the grade of perlite selected and which grain size was used to do the testing. Post what you can dig up.

[Pizzarotic]

If it is Watts per Metre Kelvin then loose perlite is a 10 times better insulator than 5:1 mix. Now my head is spinning. If we can't find a method to work for under floor, I'm thinking 2" loose perlite then 2" of 10:1 over the dome. Could be worth the effort.

[david s]

Thanks David,
I'm not sure what the "K factor" is in your chart.
Is this W/mK?

No, I’m not sure what units they’re using, there are so many. But it does give a relative comparison of different mixes. You can have strength or insulation value, but not both, so it’s always a compromise.

[Pizzarotic]

Thanks David,
I'm not sure what the "K factor" is in your chart.
Is this W/mK?

[david s]

My solution employs a 5:1 vermicrete as the bottom layer of the two piece floor which sits on top of the loose perlite contained in the supporting slab basin.

Thermal conductivity comparisons are difficult because they vary at different temperatures and there are lots of different units of measurement.
Perlite at 400C is 0.08 W/mK while isowool blanket is 0.09 according to Engineering toolbox, which places it as a slightly better insulator than blanket.

https://www.engineeringtoolbox.com/p...es-d_1173.html

Once you start adding cement the value changes rapidly (eg a 10:1 verm. or perlcrete contains more mass from the cement than from the vermiculite or perlite.

[Pizzarotic]

It's certainly an interesting concept David.
Does anyone know the difference in insulation values between loose perlite and 5:1 mix. At a guess I would think 2:1 which could make it a desirable option if it can be made to work. I'm leaning towards a loose perlite on the bottom half with a 5:1 mix on the top as a possible solution if the R values warrant it.

[david s]

Here's my own solution which takes advantage of the higher insulation value of loose perlite in a recess in the supporting slab, but incorporating a two piece, triple sandwich, tongue and groove floor. But probably not suited to a single hand built project.

[lowspeed]

Here's more information: https://hessperlite.com/use-insulation.html

In simple applications like rocket stoves or pizza ovens—whether poured loose into a cavity space or used to make a lightweight cement—perlite provides resistance to thermal transfer. For example, the fire rating on a typical concrete block wall is doubled—from 2 hours to 4 hours—when the block cores are filled with perlite. (See UL Designs U901, U904, U905, and U907.)

In foundry applications, expanded perlite is used as an insulative cover over molten metals, mitigating heat loss. Perlite is mixed with exothermic powders and used to prevent shrinkage cavities in ingots and castings. Perlite is added to foundry core and molding sand mixtures to cushion and compensate for expansion during phase change, minimizing casting defects.

Other applications include fire-rated door cores, insulation boards, pipe and duct insulation, ceiling tiles, and concrete roof decks.

[Pizzarotic]

A fibre cement sheet will certainly not cope with the heat if placed directly under the floor bricks.

No it won't, you are correct, it will deteriorate but it will maintain most of its compressive strength which is what's important.

[lowspeed]

The internet abounds with videos that give poor advice. Most are by folk who have only made one oven. As SS has already advised, floor bricks sitting on loose perlite are likely to compress the perlite over time, resulting in an uneven floor that can make working the oven more difficult.

The oven in the video that i linked is probably built by an expert oven builder. That's not his first build or 2nd or 3rd... Sure it's not a pizza oven and not traditionally insulated. But notice they made a clay slurry almost like a cob like that probably insulates and then they added 2 more wall layers. So there's a good amount of top mass/insulation to keep that oven going for a long time.

Point is, this is a master builder putting loose perlite under the stone and it seems to be holding just fine.

I should do a test with perlite a few bricks and run it over with a cars tire see if it crushes.