Required density of fiberboard insulation

[GianniFocaccia]

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

[SCChris]

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.

[WJW]

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

[brickie in oz]

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?

[WJW]

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

[david s]

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.

[brickie in oz]

Re: Required density of fiberboard insulation

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.

[GianniFocaccia]

Re: Required density of fiberboard insulation

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

[WJW]

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

[brickie in oz]

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.

[david s]

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.

[GianniFocaccia]

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

[david s]

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)

[Tscarborough]

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.

[WJW]

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.