36" build in coastal VA

[nlinva]

Yes, Superwool was really nice to work with. It made it quite easy to keep a nice round dome shape, so now I'm much less worried about the final shape. I also see why you feel chickenwire or some other lath is not really needed.

As for the collar: yes, I have a permanent roof, and the storm collar at the top of the chimney will prevent water from sliding down the pipe. But we do get rainstorms from time to time that have sufficient wind that rain will get in from the side. So I want to be prepared for that just in case.

[david s]

It looks like you have a permanent roof over your oven in which case you won’t have any rain running down the outside of the pipe, so no real need to seal the collar against it. However if it’s all out in the weather then seal the collar against the flue pipe. I have a second collar sitting over the first one in my build and the imperfect fit becomes the escape for moisture and pressure.
You could also drill a few holes (carefully) through the sides of the collar that you can plug up when the oven is not in use.
Nice work with the blanket fitting by the way. Isn't Superwool a dream to work with?

[nlinva]

After some cleaning, and adding silicone caulk to the expansion joint between the vent arch and the front landing, it was time to start on the insulation.

In the planning stages, I had aimed for 4" of Superwool; I calculated this required 3 boxes worth. Those initial calculation may have been for a 39" oven -- I'm not sure. With a 36" oven, I had calculated that those 3 boxes would suffice for 5", so that's what I set out to do.

I used all the off-cuts for the top and edges of each layer, which helped prevent wastage. As it stands now, I have 3" inslulation (mostly superwool, but some CalSil) throughout, and 5" for the dome. I added an extra inch (making it 6" thee) for the top of the dome, since my dome height is close to 1" below where a true hemisphere would be.

On the whole, I found the Superwool pretty comfortable to build with. it wasn't too hard to keep the shape of the dome pretty smooth, and to fill in gaps with off-cuts.



I think it is now time to start the drying fires. Kind of nervous about it, to be honest, but will track down my infrared thermometer and start the first fire tomorrow.

I also added a bit of insulation to the top of the chimney transition. The plan here is to add a bit of vermicrete to make it slope down just a bit from the chimney pipe to the edges, with the collar resting on the vermicrete. I plan to put a high-temp silicone gasket between the collar and the chimney pipe. The question is what I should do between the collar and the vermicrete/render it rests on. I can integrate it into that, or I can rest it loosely on top. The advantage of the latter is that this could serve a purpose comparable to the breather cap on the dome. The disadvantage is that it is theoretically possible some tiny critters might creep in and make a home. What do people think?

[nlinva]

Thanks Gulf. Your build is actually one of the ones I look back at again and again. I do take david s 's point that the vermicrete adds another quantity of water to the system, and therefore overall time to completion. But I don't think I have anywhere near the skill level required to make a nice hemispheric dome covered in brick splits without having a pretty firm surface to work on, which is why I planned on the vermicrete to begin with.

I think I'll stick with my original plan of blankets, then drying fires, then a thinnish layer of vermicrete, then more time to dry and some more fires, and finally the final layers of render & brick splits. I'm in no desperate rush; so long as I can actually use the oven for Thanksgiving, it's no big deal if it is still "naked", with blankets only. :-)

[david s]

A further consideration is whether the oven is in the weather or has a roof over it. As yours has a roof over it, it is less likely to get a large quantity of moisture in the insulation layers, but the roof will impair drying from the sun and wind during construction.

[Gulf]

Just a little difference of opinion. I have shaped two domes with a "plywood arc". I doubt that I would have been able to shape blanket to fit a a perfect sphere. I also doubt that I would have been able to apply brick splits to a soft blanket. Vermicrete has been the go to method for me to achieve the final shape in both cases. In both cases a hard shell was applied over the vermicrete layer for a proper substrate to apply the brick splits. There are proven methods that are documented on this forum to release the water/steam from the shell.

[david s]

Because blanket is somewhat springy as well as uneven and lumpy, it is not a suitable substrate to render against. You can attempt it, but do you need a rendered layer if you are covering it all with the brick splits?
Chickenwire to secure the blanket layers also compresses the blanket, reducing its insulation capacity. That’s one reason I never do it. The other two are that it takes a lot of time and it is introducing a conductive material in a layer that you want to be insulative.

[nlinva]

Thanks for the feedback david s. You are right that I do not need the vermicrete for insulation -- the blankets will do that fine. I was thinking of it purely to provide a shell for the render, as you mention. I can definitely just do the brick veneer (and the render/mortar they rest on) directly onto the blanket. In that case, would it make sense to have chickenwire for the mortar to hold on to, or do you think I can still do it directly onto the blankets?

And yes, I am planning to have a plywood arc to test the final dome shape against.

[david s]

I’m not sure how thick you intend your blanket insulation or vermicrete insulation to be, but firing an uninsulated dome can lead to severe cracking because of the huge temperature differential between the inner and outer surfaces. If there’s heaps of insulation for moisture to travel through, it may not get a chance to reach the atmosphere. Maybe the cement rendered shelldrying fires, in your case might be better done with one or two layers of 1” blanket before proceeding. Because the primary function of a vermicrete layer over the blanket is to provide a firm substrate for a hard rendered shell to be applied over, in your case both a vermicrete and a hard rendered shell would be redundant and also adding a large quantity of free water which becomes problematic if locked in, if you are covering the whole thing with brick splits. Vermicrete requires a very large proportion of water, much of which is free water not converted in the hydration process. If it were mine I’d be laying the final brick splits against the blanket.
If you’re worried about the final shape, a simple arc cut in plywood can be used to check the final arc on the outside

[nlinva]

The next step is to apply the blanket insulation, followed by the vermicrete, and then the outer shell.

A question: I am uncertain about the best sequencing here. I was initially thinking of:

1. Place blankets
2. Drying fires (7-10 days)
3. Place vermicrete
4. More drying fires (a shorter sequence?)
5. Final render & application of brick veneer

Alternatively, I could do:

1. Place blankets
2. Place vermicrete
3. Drying fires
4. Final render & veneer

The inner oven has been complete for about 5 weeks now, so I don't think there is likely to be a ton of residual moisture in it. It thus might be more efficient to go ahead and put on the vermicrete right away, before the drying fires (the second sequence above). What do people think?

[nlinva]

I also mortared in place the bottom courses of the outer shell for the oven. These are two rows of old bricks, which reach up to match the level of the landing/workspace. For the remainder of the outer shell, I plan to use the same thickness as the bricks (about 2.25"), with about 1" for a vermicrete layer to shape the dome, about 0.5" of render, and then 0.75" for brick veneer, so that the entire outer surface looks to be made of brick.

[nlinva]

I decided to put a silicone gasket around the bottom of chimney anchor plate, I used the red high temp permatex silicone for that, as well as for the space between the oven and the vent arch. It is super-tacky stuff, so it is not as neat around the arch as I would have liked, but with a little bit of extra clean-up it will be fine.

[nlinva]

This weekend I built the decorative arch. I was quite nervous about it being a pretty thin (bricks are vertical), freestanding unit, so I also constructed a short brick outer chimney connected to the decorative arch. The chimney is separated from the inner chimney transition by 2 inches of the CalSil board, so the whole thing is pretty rigid and able to provide a good connection to the decorative arch.

The arch itself is separated from the front of the vent arch by a piece of CalSil board as well. I plan to cover the exposed part of the insulation (on the inside) with vermicrete, covered in turn with a thin layer of render.

I also went ahead made the keystone stick out proud, with a thin piece of brick behind it to help the arch structure. I'm quite happy with how it turned out.

[nlinva]

Yes, I will have a render that connects to the decorative arch, but I think it makes sense to fill that particular gap with a piece of brick to help the structural integrity of the arch.

[Giovanni Rossi]

Some guys have a final render that extends to the back of the decorative arch. Wasn't sure if that is in your plan.