Oven cladding?

[mleuck]

When building the Pompeii oven, has anyone used cladding over the entire oven? Or, if not, have you used only the heat blanket?

Thank you

Michael

[UtahBeehiver]

What do you define as cladding? Mortar, stucco, brick, tile, perlite or vermuclite/concrete mix? In any case, good insulation on the dome and under the floor is critical for a good performing oven. By heat blanket, I am assuming CaSi fiber blanket, which does not have to be covered "If" your oven is in an enclosed structure and fastened securely to the dome.

[mleuck]

I define cladding as a concrete/vermiculite mix. I guess my question should have been.... Is cladding necessary if the CaSi fiber blanket is used?

[SableSprings]

Michael, the cladding (perlcrete or vermicrete) over the CaSi blanket is normally applied to 1) Improve/define the dome shape prior to applying finishing stucco/render coats, 2) Protect and encapsulate the blanket...fibers can be very irritating to the skin and lungs...and squirrels & other rodentia love to use or steal it for nesting , 3) Provide some protection from the weather...primarily wind & rain, and 4) Provide a minor insulation supplement.

So, in answer to your question--No, the cladding is not required over the insulation blanket, but without knowing your final design plan for finishing the oven, it's impossible to give you any specific answer or recommendation for your situation.

If you are going to enclose your oven with a structure, many builders have just made sure the blankets are secured in place with chicken wire or other loose mesh metal screening first. Then either just left "air space" or filled the open space between the structure walls/ceiling with perlite or vermiculite. Remember that both the Ceramic fiber board and batting love to absorb moisture, so protecting them from the elements is a major element in your final design of a WFO.

Hope this helps

[david s]

For an igloo design the vermicrete layer over the blanket, as well as evening out the lumps and bumps of the blanket surface, also provides a firm substrate to render against. Some folk have successfully rendered up against the blanket surface, but as it's rather flexible it is much harder to apply and you have to adjust the thickness of render to account for the uneven surface if you want a good hemispherical form. For these reasons, for an enclosure as Mike has pointed out, there is no need for the vermicrete layer.

[mleuck]

Thank you very much. This clarifies my issue. By the way, what is the best source for the blanket?

[UtahBeehiver]

Any refractory supplier will have ceramic fiber blanket as well as mail order. You just need to call around.and price out.

[mleuck]

Since I do not understand how to calculate the amount of fiber blanket I need, I must ask.... Is a roll of 25' fiber blanket (24"x1"x25') enough for a 42" Pompeii?

[mongota]

Long answer: the formula for the surface area of a sphere is 4 x 3.14 x r x r. Don't forget the radius used is the outside radius of your dome, and it's a half dome, so divide that answer by two. The vent arch fudges things up a bit, but in general, it's not a worry.

Short answer: yes, a 50sqft blanket should provide more than enough to get a single layer of insulation over a typical 42" inside diameter dome.

Making that short answer longer: A 50sqft blanket should be more than enough when the dome is just 4-1/2" thick firebrick. If that is the case, your outside radius will be 21" + 4-1/2" = 25-1/2". If you already have added a shell of of vermicrete or something else over the exterior of you firebrick dome, run the numbers with your actual exterior radius. If you use inches for your radius, your answer will be in square inches. Divide that square inch answer by 144 to get square feet.

Unnecessary answer: My firebrick-only dome is a 42" dome sitting atop 4" of board insulation. I used about 3-1/3 rolls of 1"x 24" x 25' blanket insulation to get four layers (4") over my dome and two layers (2") over my vent arch.

[mleuck]

Thank you very much for the math lesson (must have been sleeping in class when that was taught) and for your answer. It seems it would be better to add more than 1" of blanket insulation. Would you recommend up to 4"?

[mongota]

Thank you very much for the math lesson (must have been sleeping in class when that was taught) and for your answer. It seems it would be better to add more than 1" of blanket insulation. <B>Would you recommend up to 4"?</B>

Personally, yes. I used 4" under and over my dome. I'm using my oven for cooking even though it's still not done with regards to construction. I still need an insulated door, a veneer of stone over the dome, and a roof over the entire thing.

I ran full temp fires when I just had a naked brick dome with no insulation. And I've run full temp fires since I insulated it a few weeks ago. Even though I have no insulated door on it at this time, the difference in heat retention and the temperature retention curves post-fire is significant. Since insulating, I've cooked on the third day after firing with residual heat. Even did a breakfast frittata on the morning of the fourth day.

Insulation can be pricey. For my build, where the finish surface is intended to be an igloo-style veneer of stone, I had one shot to insulate. If you're surrounding your dome with a four-walls-and-a-roof, a house-type enclosure, you can certainly add more loose fill perlite or vermiculite into the enclosure at a later date. Easiest would be through an access panel high on the back gable end wall. If surrounding with an enclosure, you still want SOME insulation, as you don't want the interior of your enclosure to be subjected to high temps radiating from a marginally insulated dome.

In the end, it comes down to how you intend to use it. Insulation will allow better heat retention and longer cooking sessions off of a single fire. Or you can slow feed a long continuous fire with less fuel. I built my oven for extended cooking off of a single fire, thus my desire for 4" of insulation.

[mleuck]

I really appreciate your detailed explanation. After reading, it makes sense to add extra layers. I plan to enclose my oven with the "four walls and roof" concept, so I will also add the vermiculite in the open spaces. This leads to another question. I have seen builds using metal framing, but wondered if that is necessary. I have previously built two Allen Scott ovens using wooden framing without issue. What are the exterior temps of your oven? Do you have an opinion regarding the metal vs wooden framing?

Thank you again for your knowledge and helpful information.

[david s]

For clarification the area of a sphere is 4 x 3.142 x r x r (halve it for a hemisphere)
The volume of a sphere is 4/3 x 3.142.x r x r x r. ( halve it for a hemisphere)
If you have more than one layer of one inch blanket plus vermiculite I can’t see any reason why a timber frame could not be used as an alternative to a steel one, but keep timber away from the oven mouth.
To save on vermiculite you can fill the corners with something else like empty PET bottles with their lids screwed back on.

[UtahBeehiver]

Here is an excerpt from Forno Bravo regarding framing of oven enclosure. Also, your local building inspector may have a say as well.

[mongota]

I plan to enclose my oven with the "four walls and roof" concept, so I will also add the vermiculite in the open spaces. This leads to another question. I have seen builds using metal framing, but wondered if that is necessary. What are the exterior temps of your oven? Do you have an opinion regarding the metal vs wooden framing?

I shot the exterior of the shell yesterday while we were cooking, it ranged from 94-117F. For grins, I shot my stone pool patio, it was 98F, late afternoon, a sunny day.

If you were building a forno bravo kit and not insulating, I'd recommend you follow the manufacturer's instructions and use cement board sheathing over steel.

If you are fully insulating, I see no problems with a wood stud frame enclosure covered with cement board sheathing. As Russell noted, there are code requirements when combining wood framing and a fireplace-type structure.

If using wood, for wood/concrete contact points, use pressure treated lumber where needed, example, for your wall sole plates (depending on your structure).

I do recommend you take a look at steel studs. For an outdoor enclosure, it'll eliminate moisture, rotting, insect concerns, etc. Steel stud framing is actually quite easy. For starters, they are all straight! They are lightweight, easy to transport. Often times they are less expensive than wood studs. They come as a "U" channel. To cut to length, use tin snips to cut the two flanges. With a utility knife, score the web, then flex it back and forth a couple of times until it snaps off, and you'll get a clean burr-free cut. No power tools required. A cordless drill works wonders for assembly with self-tapping, fine-thread, pan head sheet metal screws. I'm sure there has to be myriad youtube or manufacturer video tutorials that can help get you started. If I was doing an enclosure, I'd use steel studs.

When I write "cement board", I recommend 1/2" Durock or Wonderboard. I'm not a fan of Hardie fiber-cement tile backer boards in exterior installations.