Thickness of Vermiculite/Concrete Insulation Base

[WalkerW]

Hi,

I am having trouble figuring out how thick to lay my vermicrete slab on top of my concrete support slab. I've seen anywhere from 2'' to 8'' thick of a 5:1 mixture. I would like to keep the thickness and weight down to a minimum, if possible.

I am building a dome, most likely out of firebrick, and I think it is most cost effective for me to go with the vermic/concrete mixture as opposed to Ceramic Fiber board.

Thanks

[UtahBeehiver]

Min 4" 5 to 1 p or vcrete is abt the same as 2" of CaSi or AlSI board.

[david s]

The big advantage of a 5:1 vermicrete slab is in cost. The downside is you have to make it thicker than cal sil. for the same performance. It also contains lots of water that needs drying before building over it.

[Petter]

Sorry for questioning, but how come is the recommended ratio specified to 5:1 (~200 psi) while the FB board is ratad to 75 psi as compressive strength? Has anyone tried a leaner (~8:1) mix which collapsued?

discussion from the past:

https://community.fornobravo.com/forum/pizza-oven-design-and-installation/finding-building-materials/404181-ceramic-fiber-materials

[david s]

Try making an 8:1 vermicrete and I think you’ll agree that it’s not strong enough. Maybe the finer texture of the FB board, which I’m not familiar with, is adequate. Some builders have reported issues with it compressing if it gets wet. Cal sil board can get wet but it’s strength doesn’t alter. Probably the best solution is foamglass which doesn’t absorb water.

[Petter]

Okej, thanks. As I understand, it is quite trickey to value compressive test results of insulating materials as they are so fragile. What the hand feels does not agree with the measured value and according to your description it seems reasonable.

By the way, I got 7 m3 EPS-concrete poured in the basement a year ago. It is 450 kg/m3, 0.08 W/mK & 2 MPa (300 PSI) and I agree its pretty brittle to the touch.

When I calculate the pressure from the dome (32" ID, 300 kg) , I get a silly low pressure of about 0.012 MPa (1.7 PSI). By just reading the numbers, the safety margin is high, hence the question.

[Amirlb]

Hello all, I am a stonemason from Lebanon who trying to build wood fired oven, and I have question if 8 cm slab of vermiculite 5:1 mixture enough for isolation under pizza oven

[SableSprings]

Welcome to the forum. The recommended minimum for the 5:1 concrete insulation base is 10-11 cm (4 inches). Be aware that standard bricks may not work very well for the oven as they may spall with the normal, high temperature changes that occur during the firing cycles of these ovens.

[Amirlb]

Thank you for your help, I am using fire brick offcourse, should i add more vermiculate or can I use ceramic wool blanket because in my region I didn't find ceramic fiber board, or I continue with the 8 cm vermiculite because this project for personal use not commercial?

[SableSprings]

Glad to hear you are using fire brick...just wanted to make sure because of the red bricks in your picture. I strongly advise adding another layer of vermicrete (2 to 4 cm, 5:1) on top of your existing 8 cm of base insulation. If you don't, you will lose significant heat to the concrete hearth below and end up using more wood to fire the oven or maintain cooking/baking temperature. The ceramic wool blanket is highly recommended to cover the dome, but if you put it underneath the oven it will compress and not provide much (if any) insulation. You should start your own build thread instead of using this one (started by WalkerW in 2020). It is always good for other builders in your region to learn from your experience on how to cope with finding substitutes for difficult to find materials/equipment.

[Amirlb]

Thank you, yes sure I will start my build thread

[UtahBeehiver]

Mike gave you the correct advice. Also the dome walls also need to sit on the vcrete and not the concrete hearth so there is not a heat sink.

[SableSprings]

Lots of variables determine the drying time for the 4-6" perlcrete/vermicrete base insulation of a WFO. You want to have 5-6 drain (weep) holes in the supporting concrete top slab, mosaic sheet tiles (upside down), and then your insulation base. If you use pieces of porcelain instead of the sheets, you'll want to put a layer of garden cloth on top of the tiles. This ensures you've got little channels for moisture from the wet insulation to escape down through the weep holes. Best practice is to set up this lower drainage system, pour your insulation mix over it and let it air dry for a couple weeks (obviously during a dry, sunny period).

Remember, you want this insulation under the entire oven structure not just the cooking floor. You can use a cheap, plant moisture meter if you want to check the drying process. Although it will appear dry on top, there's a lot of water still contained in the deeper sections. After a couple weeks of air drying, you can lay down your cooking floor bricks and start your dome.

Hope that helps...looking forward to your build.

[The foxdale phantom]

Is there any benefit having vermiculite base and cal-sil on top of that? Then the brick floor?

[SableSprings]

The primary goal for floor insulation is simply to isolate the oven structure from heat loss. Having vermicrete (not just vermiculite) below a sheet of Calsil is more of a cost saving method than anything else. Note, using just vermiculite or perlite does not provide adequate compressive strength...that's why using 5:1 (*/cement + a dash of clay) is recommended for base insulation. Also, you need x2 as much of the 5:1 mix to provide roughly equivalent insulation value as offered by Calsil (2.5 cm Calsil offers about the same insulation value as 5 cm insulating concrete mix). Recommended minimum insulation for the base of a WFO is 5 cm (2") of Calsil or 10-12 cm (4") vermicrete. Although you can save some money using a layer of vermicrete, it takes significantly longer to cure and completely dry (which is essential to creating an effective insulation).

Hope that helps, comes down to creating your balance of time & cost in creating the all important adequate insulation base for your WFO.