exhaust pipe gas condensing boiler / thermal bridge

Hello, In our new building we have insulation between the rafters, so the roof structure is a cold space. The exhaust pipe of the gas heating goes from the roof structure through the insulation and upper floor into the technical room on the ground floor, where the gas condensing boiler is located. The pipe is double-walled and made of metal on the outside (cold) and in my opinion is a thermal bridge, because the roof structure is cold, but the room is warm...3/4 of the pipe is made of metal and one part is made of plastic. Does the pipe have to be completely made of plastic inside? Otherwise condensation will always form there, right? A casing around the pipe will be made in the room. Regards Body

Hi

A clear no to both questions.
You should discuss coverings of the exhaust pipe with your chimney sweep.

Olli

Where is my flaw in reasoning? We had water on the outside of the pipe, in the room... the pipe is cold in the room. I consider ceramic or plastic the right material in this case but not metal.
Best regards body