Duct thickness

Hello there I haven't been able to find answer to this question in forum. How thick the duct pipes should be? We can choose between 1,2 and 2 mm thickness.  Thank you in advance for your answer.

What is the diameter? Thickness without diameter doesn't make sense.

Sorry. We will be using 20 cm ducts (that's about 8").

Just for a comparison, a single wall 8" diameter piece of stainless chimney is 0.5mm thickness. Either of your thicknesses are a lot more than is typical for chimney pipe.

Hi Jelena;
With proper care during installation, you can use a very thin wall pipe.
Bedded on cob and carefully encased in cob, your duct gains strength.
Careful placement of your larger stones (mass) to protect your ducts also ensures their structural integrity.

Concerns about collapsing your ductwork are only during construction, wet cob can move with enough weight bearing on it.
Once dry, the mass is self-supporting.

Create your mass over several days, and let the cob filler start to dry before adding the next layer of cob-encased stone.