Barrel Vault Design in NC

[Gulf]

Your on the right track. The vent we used is called a gearbox breather cap. If you will attach a 1/2" threaded pvc connector at the apex of the dome's render, you will be able to attach a small breather cap later. Until you get the vent cap that you want, you will be able to plug it. I had a very large breather cap on mine to begin with. I later bushinged it down to recieve a 3/8th inch male threaded cap. I like the smaller cap. It is not as noticeable. It is made of a black plastic type material. I'm sure that it could be spray painted any color. There are plenty of other options out there for a vent cap. A simple goose neck shaped piece of tubing will relieve the pressure and keep the rain out.

[david s]

If you've not already cast that top section for your flue gallery, don't forget to consider creating some volume at its base so smoke can collect there and make a smooth transition to the pipe. This is what I referred to in my previous post about an inverted funnel. You can easily shape some polystyrene to form around to create this form.

[Ron-NC]

David, the cast is done using an 8" cardboard tube (diameter of the chimney pipe) and is curing. Hopefully the 8" diameter will be enough for an adequate draw. As I finished wiring in the insulation, I am becoming a bit nervous about stucco on the back wall as it is such a large vertical surface. I doubled up on the chicken wire and will use stainless needles on the first layer (3/4" thick) thinking this may form a nice solid surface to proceed with two additional stucco coats. The surface behind the wire is firm (rockwool insulation) but of course it is not attached into the oven bricks. Since this backing is not as solid as a typical stucco backing, I was wondering if there was any further way to make the stucco more substantial.

Gulf - thanks for the breather link - I will put in a threaded pipe behind the chimney pipe.

[Ron-NC]

After a week and a half, the chimney form was pulled and a small paper fire started to see how the chimney opening pulls smoke. Most of the smoke pulls through the opening even without the chimney. Unfortunately the back of the lower portion of the poured chimney base was way to thin at the bottom and will no doubt chip away until where it becomes thicker, further up. Since there is Rockwool behind this area, I will let it go for a while to see how much chips off and then use some fireplace cement to cover over the area. There is no hurry as the Rockwool can take the extreme heat and will get coated over with soot over time. If anyone knows of a better product to use than fireplace cement let me know.

After the form was pulled, there is now a need in the brick arch to repoint the bricks that were hidden under the form. This will be done after the curing fires are completed so as to not slow this part down.

This was also a good time to frame up a box in the pergola rafters through which the chimney will fit. You can see the framing in one of the pics.

Another important consideration is the chicken wire. Since I am using stucco to protect the exterior, two layers chicken wire and some 9 gauge wire were used to form a good base to embed the first layer of stucco. The 9 gauge wire was a suggestion from a stucco pro who stated the this would be a good idea to raise a layer of chicken wire above the insulation. This will allow the stucco to get behind the wire which will add the necessary strength. The first layer will be at least 3/4" reinforced with steel needles, and will take at least 600 lbs of material. I will get as much quartz sand today as my van can carry!

[Ron-NC]

Started the second fire while wiring down the chicken wire - got the oven up the about 500 F. on the side walls and let it slowly cool down overnight. No problems so far. There is rain today so I covered up the oven and will continue curing tomorrow. This will go on for at least seven days to drive all the moisture out of the home brew which was used between the two layers of bricks and over the top of the oven. It takes a while for the heat to absorb into these layers so I expect some long, steady hot fires over the next several days.

[Ron-NC]

The chimney was mounted and another fire started. Since the last fire, upwards of 400 F did not seem to get all layers really heated through, yesterdays fire was full out, clearing out the entire oven. The oven was 900+ F. for several hours. During the high heat, I continued wiring down the chicken wire and noticed warm moisture at the top of the oven insulation, which is what I was hoping would happen. This now gives me the sense that all layers of masonry are drying out.

Using high heat this early works in my build due to the many layers of mass, plus all the insulation will slowly bring the temperature down after firing. Also, the back wall is dry stacked as is the floor, so any heat caused movement will not impact these areas. Finally, Davids earlier recommendation to put the back and front walls under the vault will also allow expansion of the main vault without impacting the other walls. So far so good.

After the coals burned down and the oven cooled to 500+ F, a plywood door form was used to loosely cover the oven over night. Twelve hours later the oven was 480 F. The "darkened" plywood was removed and will cool down for a couple more hours before I start another fire at high heat.

[david s]

As you have a roof over your oven to keep out the rain, you'd be better off making any render layer over the insulation porous rather than waterproof. Proprietary cement renders are partially waterproof so a better option is something like a 4:1:1 sand, cement, lime render.which will remain porous and slightly flexible.. Start using the oven for cooking now and forget about the outer layer until it's perfectly dry. Use some sheet plastic thrown over the oven to test if moisture still condenses under it if you're unsure. You will find the performance of your oven will improve for some time. Ie faster heat up, less fuel used and better heat retention.

[Ron-NC]

Thanks - great idea throwing plastic to see if moisture exists. I did run the oven full tilt today, 900+ F for eight hours. Unlike the day before, there was no visible moisture anywhere. The oven was shut down after dinner - Pizza! I used two type of flour for a comparison and the caputo 00 is definitely preferred from a taste and work-ability perspective. Also had to make a temporary peel until the real one arrives next week.

The oven was covered with 4 inches of insulation as a temporary door which should keep the over night heat in much better than yesterday.

[Ron-NC]

Quick update on oven performance:
Day 1 900+ F making pizzas; cooled down to 750 F a couple hours later
Day 2 650 F
Day 3 500 F
Day 5 220 F

The performance should improve once a thick layer of stucco seals in the insulation.

The Roxul rockwool is performing well. The oven was covered during days 3 - 5 due to rain, and at day 5, there was no moisture on the plastic covering the oven. At hi heat there was some heat leaching through the front fire break, near the chimney, which was then stuffed with more insulation and will be covered with home brew around this area as the heat from the poured chimney base is very high. The poured chimney base has held up well with the extreme heat - no cracks yet.

[Ron-NC]

In between waiting for cement to dry, the door was made. Earlier this year some stainless steel sheet metal was scavenged from old grills and cut to the shape of the door. The insulation in the door is two pieces of Roxul rockwool board leftover from the oven insulation. Each piece is 2 inches thick for a total of 4 inches of door insulation. The toughest part was cutting and shaping the edge pieces. A tin snip was used to cut slits about 1/2 inch around all the pieces, so the edging could be bent and fit around the front and back pieces. The edging was connected to the front and back with stainless steel pop rivets - never used rivets before but they were fairly easy to work with making the connections. The last step will be the handles but I need some bolts to connect what will be wooden handles. A 12" thermometer will also be inserted to monitor the oven temperature over the course of several days.

[Ron-NC]

The last big step now is applying the stucco finish. The plan is to use multiple layers ending up with a minimum of 2 inches of thickness. The final layer will be a pure lime finish coat and this layer will carry the reddish brown color. The lime layer, although more difficult to cure will provide a more watertight finish as the lime will breath, slowly evaporating water as it gets wet.

The first layer is the thickest to fill in the uneven voids around the two layers of chicken wire. This mix was 1:1:1.25:4 where slightly more lime was used. Stainless steel needles were also added to provide additional support with the chicken wire.

The next layer followed the same formula with slightly more sand as each layer will be successively less rich in cement which is a common stucco technique to reduce cracks. Poly fibers were added to the mix to provide additional support for the stucco. This layer also started filling in uneven areas to better shape the curves to a uniform arch shape.

Each layer was cured for at least seven days before the next layer was added. The stucco was also kept damp for the first five days to slowly cure the cement. Old blankets were wetted down at least four times a day to keep the moisture on the stucco. These first two layers together formed the "scratch" coat where each layer was scratched with a trowel to provide better adhesion for the next layer. A total of 1,300 pounds of dry material was mixed which works out to a minimum 1 inch thickness - much thicker along the bottom areas. The next "brown" coat will go on after Christmas as the weather is cooperating by staying above 40 degrees F.

[Ron-NC]

It's been a while but all is now finished. All told the vault has four layers of lime / Portland stucco followed with 7 coats of lime wash containing 20% lime (same lime as used in the stucco). The use of lime throughout the stucco process has provided a more flexible (breathable) surface which allows any absorbed water to evaporate quickly. The lime will continue to cure over the next several months creating a stronger finish.

Some tile was added to the front for looks and the bottom oven door was built with some pressure treated wood. The area under the oven stays dry but not sure what this area will be used for yet. Probably storing all the utensils for bread and pizza making.

Performance of the oven is quite good. The inside surface soot clears in about 1 and 1/2 hours and the fire is fed for another 1 and 1/2 hours to saturate the bricks. A thick gasket was attached to the insulated hearth door to better seal the oven.

This past weekend oven temps in Fahrenheit were recorded as follows:
Day 1 pizza - 900 after letting the coals burn down a bit; could have cooked pizzas for three hours at these high temps (only needed one hour this time)
Day 2 bread 610 (had to open the door for two hours and wipe down the floor twiceOv to cool down the oven)
Day 3 baby back ribs 400 (had to open the door for two more hours to cool down the oven)
Day 4 260