Barrel Vault Design in NC

[Ron-NC]

Chicken wire was then used to cover the insulation using tapcon screws and wire to pull it tight. I don't think I had the correct drill bit for the screws as it was nearly impossible to drill the holes into the slab concrete, but I did find some alternative areas in the base to get some leverage to pull the wire tight. There is a bit more to do today to finish the front and some additional chicken wire will be used in the back to better accept the stucco on this vertical surface.

I have about 4 cubic ft. of perlite leftover which will be used today, in a 7:1 ratio applied over the vault area only to smooth out the vault shape and provide a base for the stucco finish. I may first use a 10:1 ratio to fill in any voids in the insulation area before a richer, thin coat is used over the vault surface. I would probably skip this entire step if not for having some leftover Perlite, as the Roxul insulation is plenty thick at seven inches total around most areas (except for the front which will be thinner in some spots due to the chimney).

The front and back will stay open when the oven is slowly cured a week from now.

I am thinking about placing some sort of steam vent behind the chimney but not sure if this is necessary or how yet to do that. Any advice would be great.

[Ron-NC]

The insulation was placed in two layers, alternating horizontal and vertical direction to better cover the seems. Random pieces were cut and stuffed into places as necessary to fill all voids.

[Ron-NC]

The insulation used is standard Roxul wall insulation from any big box store. The material has some firmness but is still flexible enough to form around the vault. This worked well in a barrel vault application with a combination of vertical walls and curves. A utility knife was used to score a line for measuring the cut pieces and a thin kitchen serrated blade knife used to saw through the material, which made a nice clean edge - very easy to work with.

[Ron-NC]

Here is a pic of the poured chimney insert. The cardboard tube was a bit more toward the front than what I had envisioned; hopefully there is enough thickness in the front to avoid any issues. The concrete will cure for a full week before I move the forms, as the weather has turned cool, so the cure will be slow. While the cure is happening, in three days the insulation will be placed on the vault.

[Ron-NC]

While the chimney arch is curing, the form for the chimney insert was built. One inch of insulation was placed between the concrete and inner arch as part of the heat break for the front of the oven. A cardboard tube (not shown) will be cut and set in the form for the flue area. A chimney anchor plate will then sit on top of the poured keystone / chimney insert. Stainless steel bolts will also be inserted into the cement to firmly attach the anchor plate to the form. The thickness of the cement will be 1 1/2" at the thinnest point in the front and back and getting much thicker as you move to the sides. The concrete mix will be 1:1:1:4 with stainless steel needles mixed in for reinforcement. A small amount of Polyester cement fibers will also be mixed in. Some time has to spent today reinforcing the form in advance of the pour.

[Ron-NC]

The form was built for the chimney arch and the bricks mortared in place. My lack of masonry skills again kicked in as the arch is a bit crooked. Hopefully using two bricks side by side should be enough to support the chimney structure, avoiding any further buttressing. The arch thickness measures 8" X 8". I will let this sit for three days before proceeding with the poured keystone / chimney insert.

[david s]

I've really pushed the boundaries with my flue gallery design because I want to keep the weight down as much as possible. In my design the flue gallery sits directly onto the dome, so I don't want a heavy heat sink there. My heat break/expansion joint is between the flue gallery and the outer decorative arch.
The flue gallery weighs only 10 kgs, but is rich with stainless steel reinforcing. The buttresses on the sides provide extra strength allowing the rest of it to be thinner and therefore lighter. I haven't had any failures . I coat the inside with black tile grout, cosmetic only.
I think one of the most important things is to have some volume at the base of the pipe. i.e. the flue gallery should be like an inverted funnel. This is essay to do when casting but quite difficult to form with brick.
Also, make the hole that the pipe fits into a slightly loose fit. You don't want the expanding steel pipe placing undue stress on the surrounding refractory as it will expand first being more conductive.

[Ron-NC]

David, forgot to ask, what type of material did you use to coat the inside of the chimney base. Thanks again.

[Ron-NC]

thanks - I like the idea of setting the chimney on the oven inner arch, especially since it saves space toward the front of the base. The anchor plate should be here in a couple days so I can evaluate it's use then. It appears from the specs that the anchor plate is only slightly bigger then the chimney itself - 11.25" plate vs. 10" chimney, so using the plate should not add much to the depth. The loss of depth may be more a function of how thick the cement around the plate needs to be. How much castable was required for the above assembly you made?

[david s]

One of the problems in accommodating a an anchor plate is that it ends up drastically increasing the depth of your entry which makes the oven more difficult to work. Having a deep tunnel to work past before getting anywhere near the oven door is a drawback for front flued ovens. You may be able to trim down the anchor plate to help reduce this. My solution is a little different, I've set the flue pipe slightly behind the oven door which ends up making the depth of the entry the same as the width of the flue pipe. Pic attached to show my explanation. I don't use an anchor plate either, my flu fits into the hole in the flue gallery. I think you should use castable refractory rather than home-brew to make your casting.

[Ron-NC]

Quite a bit of time has been spent trying to figure out the chimney arch. Since I am pouring the keystone / chimney base, I am a bit concerned with how thick the front / back cement wall should be to make sure it supports the heavy chimney. I just ordered the chimney anchor plate which measures 11.25" x 11.25. The anchor plate will be bolted to the poured cement base. I would like to minimize thickness of the front / back cement wall as much as possible as any additional thickness will extend beyond the lower bricks. I am looking for any advise as to whether or not I need to extend the front / back cement wall beyond the anchor plate measurement of 11.25 inches. The attached pics show what I am thinking. The drawing below has an assumed cement width of 12", yielding a 2 inch overhang, but if this could be reduced to just beyond the anchor plate dimension, I think it may look nicer. I am particularly looking to hear from anyone who has used home brew and what you think the minimum thickness should be for acceptable strength.

[Ron-NC]

Before the chimney arch is built, a base was constructed with solid clay bricks. Next the arch form will be constructed in such a way as to also accommodate the poured concrete discussed earlier. The home brew mix will be 1:1:1:4 -slightly more rich in cement than the 1:1:1:5 mix I have usually been using.

A side view has been included to better see how the heat break insulation fits into the chimney arch.

[Ron-NC]

Rain day today so I'll finish catching up my progress. Here are a number of pics of how the insulation heat break was constructed. I scavenged some stainless steel off an old grill and cut it to shape to wrap around a piece of Roxul Insulation which is two inches thick. This insulation piece will sit next to the hearth bricks. In order to make a good fit a piece of wood was cut to the shape of the insulation and the metal then pounded around the wood. Since this was used metal, I cleaned it with Bar Keepers Friend, mixing a paste, applying the paste to the metal, and lots of scrubbing. It cleaned up nicely and this will no doubt be done annually to keep the rust at bay.

The last pic shows the location of the heat break and two cut to size one inch thick pieces of ceramic fiber on either side of the hearth heat break. I would have preferred to use fiber ceramic all around as it is more rigid, but ran out of material.

This approach of wrapping metal around insulation will be followed for the remainder oh the heat break pieces between the chimney and inner arches.

[Ron-NC]

With the back wall in, I decided to move next to the chimney arch. A rough layout is presented here to show what I am thinking about doing. I have an 8 inch inside diameter triple insulated stove pipe to be placed on the inner arch edge, and the chimney arch. The second pic has an orange box drawn in to show where the stove pipe will go. The plan is to use clay bricks for most of the arches, and then pour a home brew stove pipe insert (essentially a very large keystone), into which the stove pipe will rest. This should allow the heavy stove pipe to rest on a more stable platform than using bricks alone. I am thinking if I were to build the entire chimney arch out of bricks, the arch would need to be wider to accommodate the stove pipe which has a 10 inch outside diameter. Any thoughts on this approach would be appreciated.

Another departure is the heat break. As can be seen in the picture, there will be a 2 inch heat break between the inner / chimney archs filled in with Roxul insulation, capped with some stainless steel sheets wrapped around the insulation. This is another reason I am thinking of pouring a chimney insert, as my approach is using an 8 inch wide chimney arch rather than the more traditional 12 - 13.5 inch chimney arch. The total wide of the chimney area is:

8 inch chimney arch
2 inch insulation gap
2 inch inner arch flat area
1 inch chimney insert additional width
or 13 available inches for the chimney insert where the chimney will sit. As noted, the insert will extend out 1 inch (maybe 1 1/2 inches) to add a bit of thickness to the insert unit.

I will provide pics of the form once put in place, but if anyone notices anything that is problematic, let me know.

[Ron-NC]

Next, I moved to the back wall. I am taking david's advise and building the back wall inside the vault walls to account for expansion, which could have pushed out the back wall if built flush with the vault. Each brick at the end of each row was cut to fit as close a possible to the vault arch. The bricks were then removed and laid out on my work surface to for easy access during installation. The first row was mortared to get things level (sort of level that is) and all remaining layers were dry fitted. A thick coat of home brew was then applied around the perimeter to fill in any gaps which occurred due to less than precise cuts to fit the pieces in.

I do have another question about the back wall gaps. The bricks are dry fitted so there are small gaps where the bricks sit on top of each other. Should I put a coating of home brew on the back to cover the gaps? I would think the small gaps would have no impact since the smoke will be pulled to the front toward the chimney. Just want to make sure I don't miss something here. Seven inches of Roxul insulation will be applied behind the back wall so I don't think any heat will escape.