Pdx 42" update

[david s]

You could try applying a stucco/render coat directly onto the blanket, but it is a very poor substrate to work to. A vermicrete layer as well as providing additional insulation, sets up firm and covers bumps in the blanket, particularly if the top of the oven has an extra layer which creates a step (as in post #261)

[Macrinehart]

Thank you Gulf and david s for the advice.

There is one more area I have questions on before going too much further. This pertains to accounting for material movement during heating and cooling, particularly where different materials are interacting around the base of the chimney.

Elsewhere I've read that the oven dome may move during heating and cooling. I assume that the insulation layer helps to allow this movement without cracking the vericrete or stucco layers of the dome.

I have also allowed about 1/2" gap between the dome and opening arch which will be sealed with fiberglass stove gasket material. It's pretty tight but should allow for some minor flexibility.

Over the interior and exterior arch I will have a Stainless steel transition to the chimney, and because this connects to both the interior and exterior arches, I have my doubts about the best way to attach this piece. I'm thinking of using fiberglass tadpole tape, as I think it should help provide a good seal with some flexibility to isolate interior dome movement from the exterior arch and the vermicrete and stucco layers.

Finally, I intend to surround the base of the ss chimney transition with vermicrete and stucco in the back the insulation till butt up to the ss transition piece. My plan is to leave only the front face of the exterior arch exposed.

I am a little concered that as these different components heat and cool and interact I could start having issues.

Can anyone see any yellow or red flags?

Thank you!

[david s]

The expansion at our service temperatures amounts to about 3-4 mm every time you reach pizza temperature.

[Gulf]

I think that you have all of it covered as for expansion of the actual dome.The CFB is doing that. However, you need to make a place for the steam to escape during curing if you haven't included that in your design. Also, there will need to be something that will performs as an expansion joint between any ss steel and any masonry including the vermicrete. As for actual attachment to the chimney, I would like to see the "transition piece that you are using. Usually, it is just a heat resistant gasket that is homemade. Where ss bolts are used for the actual anchoring, oversized holes through the ss a good practice to allow for expansion.

[Macrinehart]

Gulf - well noted on your advice. I do have a plan for a steam vent in vermicrete/stucco at the top of the dome. I've purchased a marine grade stainless steel mushroom cap which should provide sufficicient ventilation without letting anything in.

As for the transition piece, I had not planned to use any hardware for anchoring, although that is feasible. My thought was to use this fiberglass tadpole gasket (picture attached) with 2000F rated adhesive. This example features a 3/4" bulb diameter. I intended to glue the tail piece to the bricks and securing the bulb piece to the inside of the SS material ould provide both flexibility and a good seal. I suppose I could also provide some spacer material around the transition piece and use someting like this fiberglass gasket to seal that gap too.

Incidentally, I am working with a local fireplace guy on the transition piece. This guy has in business since the 70s and has accumulated a lot of vintage stove material over the years. He proposed salvaging a cast iron transition piece that integrates with a vintage stove model he has from his stock, but only if it fits.

I am a little skeptical about this idea. For one thing, cast iron is heavy, and I'm not sure if I want all that weight resting on the opening arch. Secondly, I don't know what that would do with respect to movement of the dome. Instead of being able to slide, I'd be afraid that lateral pressure would transfer through the cast iron and onto the opening arch.

Any thoughts? Am I good to be skeptical on this or is cast iron a good idea if the size works?

Thanks!

[mongota]

My challege is that I'm nearly to the edge of my hearth in the back, I have about 1.5" available now for both the vermicrete and stucco. Can I skip the vermicrete and go directly to stucco, or perhaps thin down the vermicrete layer? Or would I be better off adding an extra 1" of blanket around the entire oven and then going straight to stucco?

With those choices and those dimensional restrictions, I'd do a third layer of 1" blanket. Then stucco lath (expanded damond mesh). Then stucco.

When I built my oven I thought the lath would be a pain in the rear to apply but it was ridiculously easy and surprisingly fast. It made the subsequent stucco application worry free regarding my concerns over compressing the blanket while troweling on. I worried that I might compress the insulation while troweling it on, and the insulation expanding/rebounding and possibly cracking the stucco while the stucco layer was still plastic, before it had cured/hardened.

Congrats on your progress.