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Old 01-24-2012, 09:58 AM
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Default Re: Required density of fiberboard insulation

Bill,
You can clad with reinforced concrete. It is not recommended because Portland cement breaks down over time from thermal cycling and is not a long-term solution. I also think full firebricks would provide thermal consistency (both heat up and radiation) as well as structural integrity.

Maybe someone else has a different view?
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Old 01-24-2012, 11:13 AM
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Default Re: Required density of fiberboard insulation

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Originally Posted by WJW View Post
why can't I clad with reinforced concrete to take up those loads?
If you insulate between the fire brick and concrete I cant see a problem, I wouldnt put the concrete hard up against the fire brick Alan Scott style as it creates too much thermal mass and takes too much fuel and time to heat up.
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Old 01-24-2012, 12:34 PM
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Default Re: Required density of fiberboard insulation

Bill,
You can clad with 50%portland and 50% lime in the mix. Also for a barrel vault your end walls should be inside the vault not outside.
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Old 01-24-2012, 01:28 PM
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Default Re: Required density of fiberboard insulation

Sorry for the newbie questions...

So are you guys suggesting I skip the cladding altogether and just go with the insulation up against the brick. (I'm trying to keep my mass down, but concrete is mass as well. If I can skip the concrete then there's little net increase by using the full thickness firebrick?) If that's the case then I can see why you'd say to use the brick laying on it's flat side.

I also need to apologize for using the wrong terms. When I said "concrete" I really meant that I was going to use a mortar mixture with fireclay added. I was assuming (maybe incorrectly) that I could apply that over a rebar or heavy chicken wire cage and that would work and take up the sheer load. If you guys think I'm wrong on that count please let me know.

Got my insulation...








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Old 01-25-2012, 01:46 PM
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Default Re: Required density of fiberboard insulation

McGills carries a few high temperature products, including the ceramic blanket in 1-inch (HF11) and 2-inch (HF-12) thickness. If you're local to them, it's cheaper that FB blanket and has identical specs. Their online ordering system used to be a total mess, but they cleaned it up. But as a walk-in, you'll be fine and won't get your credit card charged six times when the online system won't process it correctly (happened to me). Customer service ain't Forno Bravo style, but they're still ok. Just don't expect them to be as patient with newbie questions and "will this work for me" questions.

Quote:
Originally Posted by GianniFocaccia View Post
Bill,

I have never used these guys, but there is a place right down the freeway from you in North Hollywood called McGills Warehouse that carries refractory products. I understand this is a true warehouse (with minimal-to-no customer service) that caters to professionals and has a $50 minimum.

McGills Warehouse Online Department Store,HF102 2" Ceramic Fiber Boards Density: 17.5lb/ft3 or 280kg/m3

John

Last edited by azatty; 01-25-2012 at 01:48 PM.
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Old 01-25-2012, 02:12 PM
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Default Re: Required density of fiberboard insulation

Bill,

The structural integrity of a vault (arch) is not very good if you havea full-thickness brick dome (vault) on top of a thin sidewall (pier). There used to be a better explanation of vault geometry - snoop around the Auroville site and you may be able to find it. You can start here:

Auroville Earth Institute

IMO, skimping on side bricks (laying them upright) to save on mass is like putting really skinny rear tires on a dragster to save weight. You really do need the sidewall (pier) strength to keep your oven strong. Refractory 'concrete' is better than no buttressing, but I feel a continuous firebrick sidewall acting as a heatsink is best. Virtually all pompeii-style ovens are 4.5"-thick (all the way around) and this is for a reason. Keep in mind that buttressing is still recommended for barrel vaults with full-firebrick deep sidewalls because of the massive side thrust.

I do, however like the idea of Insboard 19 as your hotface insulation around the base of your vault. This simply isn't practical on a Pompeii oven although I think one guy (SCChris) may have pulled this off.
John

John
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Old 01-25-2012, 02:28 PM
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Default Re: Required density of fiberboard insulation

Here is a picture.

http://www.fornobravo.com/forum/atta...d-soldiers.jpg

Chris

PS In the end I did butress the sides of the entry. Better over engineered then not.

Last edited by SCChris; 01-25-2012 at 04:40 PM.
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Old 01-25-2012, 07:35 PM
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Default Re: Required density of fiberboard insulation

Making progress.

First of all, much thanks to John, Al, and others for their patient advice to a rookie.

Based upon that advice I've decided to butress the side walls with my three inch thick insulating board as shown in the photo below. I've also decided to follow the advice of those who know more and lay all the bricks in the sidewalls and rear wall flat. My roof arches will be on edge. As such, all walls and the roof arches will be a full 4.5 inches thick. With the absence of cladding and the 4.5 inche walls I believe I'll have essentially the same thermal mass as a pompeii.

I've decided to lock the insulating boards in place by anchoring the six inch thick cinder blocks to the hearth slab by drilling holes into the hearth slab, epoxying rebar dowels in place which will protrude up into the voids of the cinder blocks. Fill voids with a bit of concrete and I should be golden.

My initial concern was the ability of the fiberboard to take the load of being sandwhiched between the firebricks and the sidewall cinderblocks. But if the fiber board is strong enough to take weight of an entire oven on top of it (pushing it down on the hearth slab), then it should certainly take the forces generated sideways by the arch.

This should be an extremely well insulated oven. I still have 50 square feet of fiber blanket I can use to insulate the roof arches. I'm thinking three layers at one inch thick each.

The interior of the oven will be 36 inches wide by approx 44 inches long depending on how I do my chiminey transition/door area.

If anyone has thoughts or concerns I'd love to hear them.



Bill
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Old 01-25-2012, 07:42 PM
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Default Re: Required density of fiberboard insulation

Does the insulation board have any give in it to allow for expansion?
If not I think I would be putting a layer of blanket between the board and the outer concrete blockwork.

Something might give or break if it cant move?
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Old 01-25-2012, 10:36 PM
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Default Re: Required density of fiberboard insulation

That's some food for thought.

The board does have some give..but I doubt it would spring back and there'd be an airspace between the bricks and the board...which would sort of defeat the whole idea of using the insulation board to butress the bricks.

Same problem with blanket between the blockwork and the boards. If I'm not pressing up hard against the bricks with the blockwork, am I really butressing anything? I suppose I could compact the blanket pretty tightly between the fiberboard and outer blockwork...to the point that when cool I'm still taking up some of the sheer load through the press of the brick to the board, to the blanket, to the blocks. When things heat up the blanket is squeezed tighter...but still can compress further and avoid breaking things. ???

How much does firbrick expand? Does anyone know if the insulation also expands to provide some "growing room"?

Why doesn't the expansion issue present problems for other builds. For example, in a pompei style you have firebricks exposed to significant heat yet attached to door/entry transitions that are significantly cooler. Why isn't everything falling apart at those transititions?

Is anyone else aware of an oven where the rigid insulating board is locked it tight next to firebrick? It seem hard to be believe that I'm the first guy to try this technique.

Bill
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