Late to the party

[ragazzo impasto]

This is a wonderful site with so much information, thank you all the the wonderful resource. I watched a few YouTube videos about cob and brick ovens and got the bug and jumped right in. I have already built my stand prior to finding this site. I have a patio built out of 16 inch stepping stone set on gravel and sand. I took up one corner of the stones and poured a 5 1/2 inch pad to build my oven on then reset the stepping stones. I built the stand out of 16 gauge galvanized studs ever 6” and wrapped with hardy backer and poured a 3 1/2 inch table top. I did it this way Incase I ever wanted to move it. I now realize I was delusional. It’s going to weigh over 2000 pounds when all said and done. The stand is 52 inches round with a 30 inch extension for the entry. I am shooting for a 40 in high oven floor. The concrete top is 33” high.

I plan to build a 36” igloo style dome. I have already purchased a few things. 40 new medium fire bricks ($1.25) for the floor, 200 fire brick ($.30) seconds from a local manufacturer, 7 - 8.5” od round clay chimney liner ($11), 2” x 39.4x39.4” Simvac ceramic fiber insuulation board ($87) for the floor and Cera Materials refractory ceramic fiber insulation ($97) 6# 2300F 1” x 24” x 25’ for the dome - may need more to get 3” thick . And now after reading the FB site have lots of questions.

1. Will the Simvac ceramic board work under the floor?

2. Should the insulation extend past the wall of the oven.? Should I cut it at 45 inches 36+4.5+4.5 or should I extend it a couple inches past the wall?

3. How far should the insulation go into the entry arch?

4. The board is 2 inches thick but it seems the general consensus of the forum is it should be thicker, can I add a 1 inch layer of perolite concrete mix to the top of the board?

5. I plan on having a floating cooking floor over a sand/fireclay base, is 1/4 inch of mixed sufficient?

6. I plan on using sailors on the first course. Can I stack two or three courses without taper to achieve the same effect as a soldier course or would I still have concern with the outward pressure like the soldier course?

7. Has anyone slanted the venting towards the back of the oven with a couple 45 couplers? Will this cause significant concern with the draft?


Next step is to build a jig for the saw, an IT, and the arch forms. Thanks again for the wonderful site. I hope mine will turn out at least half as good as yours, you have all set the bar extremely high.

[ragazzo impasto]

http://www.simondfibertech.com/ceramic-fiber-board/

[Gulf]

Hi Ragazzo,

Welcome to the forum. I'll try and and hit on as much of the info I can which you have provided. But, I will have to do it in paragraphs lol.

Great score on the 30 cent firebrick seconds. As for the floor brick, some say that minimum (low) duty firebrick are best for the cooking floor. However, medium duty are what the Forno Brovo plans call for. So, you shoud be ok there.

I have a consern about the stand. I would have used an all masonry stand. (I tend to over build) The metal studs sound beefy enough. But, I would not depend on the Hardie backer for permanent lateral support. If you haven't, do retro some good diagaonal bracing inside the stand.

A 30" extention for the entry seems like a heck of a reach. Do lay this oven out on a full sized template before proceeding. You can do that on cardboard or poster paper. Include the id and od of the oven walls, insulation, foot print of the entry and the finished stucco or enclosure. You can lay that right on top of what you have done and see where you are.

40" is a bit low for a finished floor height. Most recommend around mid sternum of the primary user. That is about 49" for me. And, I'm a little on the "bench legged" side. A low cooking floor will requre some bending to operate. That's not good for an old back like mine. You can increase this height by elevating your insuslaion layer and putting a secondary insulation layer. Elevating the insulation layer helps palce the insulation above the structural hearth to help guard against moisture infiltration. Also, it allows gravity to help in removing it imo.

You are on the boarder line on the with 8.5" l clay flue liners. They have an ID of 6.5". I think that you will be ok, but don't increase the size of your oven above 36". "7 - 8.5" od round clay chimney liner ($11)" ? Are planning on a 14 foot tall flue?

Questions from your post.

1: I think that the ceramic board will be fine. However, I have never understood the conversion of the crush strength factor (Kg/cm2). I'm only familiar with (psi) I'm sure that all of our resident engineers do and will help out with that.

2: I felt better about mine by extending it out past the dome walls by a couple of inches.

3: If you have enough board, you can extend it out under the floor of the entry. However, since you have are doing a masonry flue, don't set the walls of the entry on the insulation. You can also, level off the entry floor birck with v or pecrete and save any extra ceramic fiber board for much needed door insulation.

4: Put the perlcrete under the board. You want the best insulator the closest to the oven. Also, You need extra height. 4" would be better.

5: If your finished floor insulation is level and your floor brick are uniform, that layer is not needed imo.

6: I'm not sure what you mean, but I don't recommend sailors. Just, lay one or two stretcher courses vertical and then start the curve.

7: Yes, it's been done. But, I would not do it with a masonry flue. Too much weight to disperse over the dome without interfering with the insulation imo.

I hope this helps

EDIT: You may also want to reconsider a tall masonry flue on a metal stand. Taht will probably be too much weight.

[ragazzo impasto]

Gulf thank you so much for the feedback. I started before seeing this sight and really see I should have waited on the stand. It is clear I’m not moving the oven, and although it does have lateral supports and I believe it is structurally sound, If I started over I would build it out of cinderblock.

i wasn’t very clear on the arch extension - it’s 30in wide which I’m not sure is wide enough. Front to back on the table is 57” also may be to short. I will lay it out on top of the stand as you suggest.

The flue is 7ft seemed was too good of a deal to pass up. I am looking for a deal on metal.

I’m 6’2” so it sounds like I need a taller oven floor.

Thanks again Gulf I really appreciate the input. It’s clear I have a lot to learn

[ragazzo impasto]

I’ve opened the box on the insulation and laid it out. As long as I stay to the outer edge of the brick and don’t extend fair into the entry arch, I can get by using the cutoff corners. But there isn’t enough room to extend it a couple inches out without buying another box. Can I achieve the same support by pouring 4 inches of pecrete under the insulation and 6 inches outside - in effect encasing the insulation sides and bottom in pecrete? If not, I will order more - in for a penny as they say.

The Simvac board is aluminosiicate refactory ceramic fiber board 2300 F, density 20-24 ib/ft3, compression strength at 25% deformation, 78 psi.

Bill

[Gulf]

Bill,

That compression strength sounds a bit low to me. The calcium silicate board that I used is is 5% at 100 psi. It's not quite as good of an insulator as ceramic fiber board. So, it is probably denser. I seem to remember that ceramic fiber board's crush strength was lower. I just don't remember it being that much lower. Again, maybe one of the engineers on board will jump in here. I think JRPizza used ceramic fiber board on his build, if I remember correctly. I also think that UtahBeehiver has answered some questions regarding ceramic fiber board.

[david s]

A 6"diam flue pipe (ID) is recommended for ovens up to 36" dial. Not sure of the ID of your clay pipe, It may not be big enough. You also need to insulate a clay chimney pipe or it will crack from the heat because the outside is in the cooler ambient air.

[Gulf]

I'm bumping this for other's opinion about the compression strength above in post # 5..

[ragazzo impasto]

Thanks Joe. I’ve been testing the opening height and agree It needs to raised. You mention elevating the insulation level and adding a second layer of insulation. How would you recommend doing this?

[Gulf]

One fairly easy way to elevate the insulation layer is to add a layer of concrete pavers on top of the hearth. You can do multiple layers to get the height that you want. For the second layer of insulation, you could then form and pour a layer of 5 to 1 vermicrete on top of the pavers (not so easy). A way to do both at the same time with the same substance is to add a layer of foam glass block. The foam glass will not wick water from the slab. It is also a very good secondary insulation. Take a look at UtahBeehiver 's build. Here is a post from his build thread where he is installing the foam glass.

[Gulf]

I was hoping that someone else would respond wth an opinion on the compression strength that your material quotes ("25% deformation, 78 psi.") I did a little looking around and continue to find that compression strength to be a lot lower than most under floor insulations that we use. It's primary use may have been for industrial wall insulation where compression strength is not very important.

[WarEagle90]

Not sure how I missed this conversation, but here is my take on the compressive strength of the ceramic fiber board. If you compare the Simvac product to the FB insulation board, the Simvac compressive strength is about half at 10% deformation. FB is 72 psi and the Simvac is 39 psi. Now to provide some perspective, if we assume the 36" oven weighs about 2000 lbs, its loading will only be around 6 psi. So I'm not sure the compressive strength really matters as far as the dome is concerned. However, I would not build a masonry or clay tile chimney on top of either of the two products. I would land the load bearing surfaces of the the vent either directly on the hearth slab (that is what I am doing) or on vermicrete or perlcrete if you want insulation under the vent structure. You can certainly use the Simvac board under the vent floor. Should you decide to build a traditional vent with double wall stainless flue pipe, then the Simvac product should work fine under the entire vent. I hope this is at least clear as mud. Let me know if you have questions.

[ragazzo impasto]

Dan thank you for the feedback, greatly appreciate it. I started the stand before finding the site and bought a few products. Glad I can make use of the insulation board although my wife is worried about it’s safety. I am going to use double wall flue pipe so I plan to put it under the vent arch.

I want to raise the stand/hearth as Joe suggest, so I will be pouring 6 inches of 5-1 recipe of pelite concrete tomorrow. Also considering foam glass like Utahbeehiver per Joe’s suggestion. I like that it is lightweight and provides a water barrier but it’s just something else for my wife to worry about. Looking for a source, only one so far is $18 a square foot.

Or or would I be better off just sticking to p/vcrete at 8 inches. Happy wife as they say.

For those of you that used p/vcrete, what did you use for a water barrier? Is that what the aluminum foil is used for I’ve seen on a few builds?

Bill