Required density of fiberboard insulation

[brickie in oz]

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?

The sideways thrust was buttressed with the basalt blocks both sides of the arch.

[david s]

Re: Required density of fiberboard insulation

What did you do to counter the expansion of the arch tending to thrust the end walls out? or did you build your end walls inside the arch?
Dave

[brickie in oz]

Re: Required density of fiberboard insulation

What did you do to counter the expansion of the arch tending to thrust the end walls out?

For the outer arch.
With the thermal break that is incorporated in the build the outer arch barely gets warn to the touch, so far so good.


For the vault roof.
The chimney load (450Kgs) is supported on the front oven walls providing substantial downwards thrust, the back wall of the oven is tied in with the side walls to eliminate movement.

[david s]

Re: Required density of fiberboard insulation

OK should be right then. In kiln building the end walls of a vaulted kiln should be inside the vault otherwise steel bracing is required. Our ovens are small by comparison and do not reach high temps, so the amount of expansion is less, but it is still in the region of 0.5% which would be 5mm over a metre. It is for this reason that many kilns have their inner bricks dry stacked rather than mortared.

[WJW]

Re: Required density of fiberboard insulation

Is the expansion .5%...or is it .05%?

I thought in an earlier post you indicated the latter??

One other thing David, I've heard you say a couple of times that the end walls should be inside the sidewalls on a vaulted oven. Could you explain exactly what you mean by that and tell me why. Please keep in mind that I am a complete rookie and have no masonry experience when you are explaining. (In other words...dumb it down for me.)

Do you mean to say that the end walls should not be tied in with the side walls with interconnecting bricks?

Bill

[david s]

Re: Required density of fiberboard insulation

I was correct the first time 0.5% is about right, I wasn't thinking straight, probably my home brewed cider clouding judgement again, that's why I amended the post. It would be about 5mm over a length of around a metre, from ambient to 400 C.
What I mean by having the end walls inside the barrel arch rather than outside, is that the end walls should sit underneath the arch. Otherwise, when the arch expands it will tend to push the end walls out. The end walls should also be tied in to the side walls below the arch. in kiln building it is recommended to have steel bracing with adjustable steel rods and nuts to hold the kiln together if the end walls are outside the arch.
Dave

[david s]

Re: Required density of fiberboard insulation

The larger the radius of the arch in a barrel vault the greater the outward thrust on the side walls. This force is created by the mass. Thermal expansion increases the thrust as well. But there is also considerable force on the end walls from expansion. The longer the arch is the greater will be the amount of expansion. For a small oven it may not be a problem, but if your ach were two meters ten that would be about 1cm to deal with.

[WJW]

Re: Required density of fiberboard insulation

I understand what you're saying David..but with the end wall inside the vault, doesn't the end wall expand outward and tend to push the vault outward? I suppose if the vault is expanding as well that may not be a bad thing...but just asking.

Bill

[david s]

Re: Required density of fiberboard insulation

The end walls, arch and side walls expand the same amount so they don' t exert any extra force under the arch. the end walls should be tied in to the side walls at the corners, so there should not be a problem there. This is where the dome, being circular, has less problems with brick expansion.

[WJW]

Re: Required density of fiberboard insulation

I got the enclosure sidewalls attached, insulation in place, and the first few bricks down. Took Al's idea and went with the layer of blanket between the block walls and fiber board to allow some compressibility to deal with thermal expansion. This is going to be one well-insulated oven. When I'm not using it to cook, I'm going to keep my beer cold in it.

Photos below...if anyone sees anything that causes alarm bells to go off I'd love to hear it.

Rebar epoxied into slab.



Block walls laid in place and epoxied down so I could press against them when squeezing the insulation in place so that the fiberboard would actually be buttressing the firebrick sidewalls. (I'll fill the voids with concrete in a day or two.)



First few bricks going down. I intend to place the floor within the walls. I've seen it done both ways but it seemed that the floor would wedge up against the soldier course wall and take the load from the arch better. Any thoughts?



Detail of the side wall.


A buddy of mine is a concrete contractor...he loaned me his brick/block saw. The thing is a monster. It's MKDiamond's MK2000, 14 inch blade. The saw alone weighs 165lbs. I about killed myself getting it onto a table...but it sure goes through the bricks easy enough! I'm ready.

[david s]

Re: Required density of fiberboard insulation

Don't jam the floor bricks in too tight. Allow a tiny bit of room for them to expand without forcing out the walls.This is also one of the reasons the floor bricks are best laid without mortar. The other is that it is easier to replace a floor brick if you ever had to.

[WJW]

Re: Required density of fiberboard insulation

Got the floor down and am now planning the wall layout. The sidewalls will be a soldier course with a stretcher course on top of that.


That's a lot of 45 degree cuts...I definitely owe my buddy a few pizza's for the loan of his saw.



A basic idea of how the side wall and back wall will lay out. Time to go buy more bricks. (For anyone who's interested...that's a total of seventy-five bricks there.)


As always...if anyone is hearing alarm bells I'm all ears.

[brickie in oz]

Re: Required density of fiberboard insulation

As always...if anyone is hearing alarm bells I'm all ears.

The soldier course is pointless, they only do it on domes to gain extra height.

[WJW]

Re: Required density of fiberboard insulation

Why do you say it's pointless Al?

With the floor inside the oven and a soldier course that puts the top of the side walls at about nine inches off the floor. You think that's too high? Why?

I've cooked a total of two pizza's in a friend's oven so I know nothing about the practice other than what I've read. I have not done the calculations but, assuming a nine inch high wall, a relatively flat dome, and a 38 inch wide oven, I'd think I'd get a dome height of around eighteen inches in the center. Following the old 63% rule, that gives me a door height just under 12 inches. Those seem to be the dimensions that I've frequently heard are pretty widely accepted. What is it that you don't like about that?

Also, what wall height do you like and why. The two options I have seen most are either three stretcher courses placed on the floor (7.5 inch side wall), or a soldier course placed on the floor (9 inches). Does the extra 1.5 inches negatively affect performance in some way in your opinion, or is it simply a preference thing for you?

Bill

[brickie in oz]

Re: Required density of fiberboard insulation

Just lay stretcher courses, the domers do it to gain height, us vaulters just add extra courses.
Unless you are after the look of a soldier course, of course.