#21  
Old 01-25-2012, 10:49 PM
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Default Re: Required density of fiberboard insulation

"How much does firbrick expand?"

From ambient to 400 C approx. 0.5%
This doesn't sound like much but it happens every time you fire and therefore creates stresses that eventually reveal themselves as cracks somewhere. It happens to kilns too, but the minor cracks seem to cope with the expansion and contraction and seldom get much worse.

Last edited by david s; 01-27-2012 at 10:58 PM.
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  #22  
Old 01-25-2012, 10:55 PM
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Default Re: Required density of fiberboard insulation

Quote:
Originally Posted by WJW View Post
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?
The problem with a vaulted dome is the sideways thrust of the roof that wants to push the side walls outwards, in a pompeii oven the weight is all transferred down to the floor.

Take a look at my build and how I over come this problem.
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  #23  
Old 01-26-2012, 12:19 PM
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Default Re: Required density of fiberboard insulation

Quote:
If anyone has thoughts or concerns I'd love to hear them.
Bill,

Kudos to you for your thorough investigation and unique oven design. You are the only one I am aware of who will utilizing ceramic fiber board insulation. I think it is a superior choice for insulation that also acts as a suitable buttress. Your build should go quickly and easily, and I can't wait to see what kinds of delicious meals you pull out it.

The only recommendation I would make is to consider a thermal break for your entryway. Take a look at Tom Scarborough's vault. He did a killer job and his oven works great.

Great job so far,

John
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Old 01-26-2012, 06:21 PM
WJW WJW is offline
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Default Re: Required density of fiberboard insulation

Glad you like the way it's going John but the truth is that you and a few others deserve more credit than I for the design. That's the great thing about this board and the UK Pizza Oven forum....lots of very knowledgable folks very willing to offer the wealth of their experience and constructive criticism. I hope to become knowledgable enough to return the favor to someone else.

As far as the thermal break I definitely plan on employing one. From what I've read over the last two months since I started thinking about doing this, it is pretty clear that the masonry in the door area is a potentialy significant location for loss of heat energy. I have read a copule of dscussions and seen people talk about everything from a simple airspace, to a layer of fiber blanket. Both of those things have their downsides IMO. One thing I was considering (and which I have not seen employed) was to leave a 1.25 inch airspace at the transition between door and landing (right at the terminus of the fiberboard insulation). I'd size the void so that I could then drop splits of insulating firebrick into the void. The only hesitation I have is that I don't know how durable that stuff is and whether it would fragment and contaminate food coming across the landing transition???

My guess is that it would work out ok and that even if the bricks eventually fell apart, I'd try to constyruct things so you could lift out the old fire bricks and drop new ones in as needed over time.

Any thoughts??

Bill
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Old 01-26-2012, 06:44 PM
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Default Re: Required density of fiberboard insulation

Dont use ceramic blanket for the heat break as it crumbles too easily releasing the fibres.
I used insulating fire bricks and they work great, they are a bit soft but it would take ages for one to wear away significantly enough to need replacing.
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  #26  
Old 01-26-2012, 07:15 PM
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Default Re: Required density of fiberboard insulation

Leaving a small space not only reduces transfer of heat by conduction, but also provides an expansion joint to prevent the build up of stresses from heat expansion. If the space is then filled with a material that is not slightly elastic then this advantage is lost. Vermicrete or perhaps high temp silicon would be a better alternative IMO as these materials can compress.
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Old 01-26-2012, 09:48 PM
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Default Re: Required density of fiberboard insulation

I have thought about this so much my head hurts. IMO, each of the aforementioned insulation options will work fine (ceramic fiber blanket, insulating firebrick, vermicrete, ceramic rope) as long as it is protected. None of these, especially refractory caulk will last very long if they are directly exposed to flame and hot oven gasses.

A number of builders have been successful merely butting the entryway bricks gently against the inner arch with a gap behind them to house the insulation. This allows for the barest minimum of contact, and theoretically limiting the highest amount of heat loss possible. This configuration must include the floor, for as SCChris demonstrated, a huge heat loss can occur through the oven floor to the entryway floor unless properly insulated.
John

Last edited by GianniFocaccia; 01-26-2012 at 09:51 PM.
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  #28  
Old 01-26-2012, 11:05 PM
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Default Re: Required density of fiberboard insulation

You could cover the entry and the gap with a stainless steel sheet. I don't go to that much trouble and use a 10mm gap filled with 6:1 vermicrete (works fine)
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Old 01-27-2012, 10:50 PM
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Default Re: Required density of fiberboard insulation

Slip joints, especially for a barrel vault, are your best option. While not true thermal isolation, they do allow for the expansion and contraction of the oven and are a nominal thermal break.
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  #30  
Old 01-28-2012, 12:50 PM
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Default Re: Required density of fiberboard insulation

Hey Al,

You said to look at how you overcame the sideward thust on your build. I read the whole thread but must have missed something. How did you overcome the stresses?

Bill

P.S.: Fantastic build by the way.
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