42" Pizza Build.

[lowspeed]

I was thinking of doing a perlite cement as insulation, would a tiered approach work well?
So first layer of 5" using 5:1 mix and then a 2" layer using 10:1 mix and then just perlite and lay the bricks directly on perlite.

Can you point out any negatives?

[SableSprings]

Welcome to the forum community! Take some time to read some of the well documented builds in the Newbie section thread "Hidden treasures..." To answer your question, 10:1 perlcrete or vermicrete is not strong enough for supporting the oven's weight. The 5:1 mix is used underneath the cooking floor and the 10:1 is used to insulate the topside where you don't need much, if any, compression strength. Be aware that although less expensive than ceramic insulation board, it takes twice as much to match the insulation value and it takes much, much longer to remove the moisture.

Our best practice advice for any hearth is to drill or form 3-5 weep holes through it to allow any moisture an exit route out/under the oven footprint. We also suggest laying some broken tiles on top of the hearth to provide a small air gap between the hearth and your base insulation. The threads in the forum contain a wealth of good advice, great pictures to illustrate techniques, as well as successful and not so successful build results... take your time to develop a good plan before you buy or build anything!

[lowspeed]

Welcome to the forum community! Take some time to read some of the well documented builds in the Newbie section thread "Hidden treasures..." To answer your question, 10:1 perlcrete or vermicrete is not strong enough for supporting the oven's weight. The 5:1 mix is used underneath the cooking floor and the 10:1 is used to insulate the topside where you don't need much, if any, compression strength. Be aware that although less expensive than ceramic insulation board, it takes twice as much to match the insulation value and it takes much, much longer to remove the moisture.

Our best practice advice for any hearth is to drill or form 3-5 weep holes through it to allow any moisture an exit route out/under the oven footprint. We also suggest laying some broken tiles on top of the hearth to provide a small air gap between the hearth and your base insulation. The threads in the forum contain a wealth of good advice, great pictures to illustrate techniques, as well as successful and not so successful build results... take your time to develop a good plan before you buy or build anything!

Thanks for the answer, I meant the 10:1 mix just under where the floor is, not under the dome.

Would that not work?

[SableSprings]

The 10:1 has virtually no compressive strength. So even if you only were putting under the cooking floor bricks, you'd very likely see settling and an uneven floor after a short period of oven use. It is just more practical and structurally sound to have a consistent insulation base (5:1 or ceramic board) for the entire oven (dome outer footprint & cooking floor).

[lowspeed]

The 10:1 has virtually no compressive strength. So even if you only were putting under the cooking floor bricks, you'd very likely see settling and an uneven floor after a short period of oven use. It is just more practical and structurally sound to have a consistent insulation base (5:1 or ceramic board) for the entire oven (dome outer footprint & cooking floor).

How do you explain this oven using loose perlite under the floor?
https://youtu.be/CZRX61uz9Y4?t=350

[david s]

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.

[fox]

How do you explain this oven using loose perlite under the floor?
https://youtu.be/CZRX61uz9Y4?t=350

Hi lowspeed, that oven you have linked is nothing like the mass majority of ovens featured on this forum!
Apart from the massive size it does not appear to have any insulation over the dome just three layers of brick! I can only imagine the oven is kept permanently hot and in permanent use?
If that is in fact just plain perlite under the slabs, then that is the first time I have ever seen it done that way and it is most certainly not a method I would use, however that is an amazing build just not a good method for a garden home, pizza oven!

I also feel that might not be just ordinary perlite as the stuff I have used seems even more aerated and light but in any case I can not think of any logical reason to use dry perlite under fire bricks in a more Conventional wood fired oven!

[Pizzarotic]

So first layer of 5" using 5:1 mix and then a 2" layer using 10:1 mix and then just perlite and lay the bricks directly on perlite.

Can you point out any negatives?

You've heard the negatives, a couple of ideas that may help.
Mix different grades of perlite to minimise the voids.
Use larger floor bricks if possible to spread the load.
Try to vibrate or compact the perlite prior to laying the floor bricks.
Consider using a fiber cement sheet on top of the perlite to minimise movement or maybe even put the 5:1 on the top instead of the bottom with the fibre cement sheet between layers.

I really think you may be able to make it work. Please document it if you go ahead and the results, so we can all learn

[david s]

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

[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.

[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]

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.

[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.

[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]

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.