Insulation: to vermiculite or not to vermiculite?

[PiRoDa]

I am in the process of designing my own oven. As I'm a bit reluctant of building the dome myself, I was looking into pre-cast domes. I found a company nearby that can supply me with a precast dome. However, their instructions read as follow:

- After wetting the joints between the various pieces that make up the dome, spread 5 cm of Refrax on the outside. If you want to have greater thermal inertia, it is recommended to cast a layer of Refrax all over the dome of a thickness of up to 6 cm. In larger ovens, a thin wire mesh must be added to Refrax.
- After the layer of Refrax, thermally insulate the oven by placing multiple layers of ceramic fibre on the dome. We recommend an initial 2/4 cm layer of ceramic fiber followed by a 10/15 cm layer of vermiculite, expanded clay or rock wool for high temperatures.

I do understand the fact that you want to have good thermal inertia, but I don't really understand why you cannot just add 5cm of ceramic fibre instead of 2cm + 10cm vermiculite? If I need to do that, my other dimensions will become rather massive. Any thoughts?

[UtahBeehiver]

You do not get thermal mass (aka ability to store heat) from ceramic fiber insulation only insulation. You do not mentioned how thick the dome is. 50mm is typically a min. thickness. So they are correct that adding refrax (assuming this is a dense refractory material) will give you more thermal mass. Vcrete on top of the ceramic blanket will also improve insulation but not the thermal mass. There have been several cast domes done recently. Nick JC out of the UK did a really nice and documented build.

[Timothy_W]

Good ability to maintain the temperature inside the oven is a major factor. If the oven cools down quickly, it will take a lot of resources to heat it up. It is not beneficial in terms of both money and time costs.

[david s]

If you intend on building an igloo style oven with an outer cement rendered shell, then that layer needs a firm substrate to work against. It is possible to render directly over the blanket, but the surface is a bit lumpy and springy. The vermicrete layer evens out the bumps as well as providing a firm substrate to work over.
If you plan on building a dog kennel style, the space between the enclosure and the blanket is usually filled with loose perlite.