ᐅ exhaust pipe of gas condensing boiler / thermal bridge

Hello,
In our newly built house, we have insulation installed between the rafters, so the attic space is unheated. The exhaust pipe for the gas heating system runs from the attic through the insulation and the upper floor to the utility room on the ground floor, where the gas condensing boiler is located.
The pipe is double-walled, with a metal outer layer (cold), and in my opinion, it acts as a thermal bridge because the attic is cold but the room is warm... Three-quarters of the pipe is metal, and a section is made of plastic.

Does the entire pipe need to be made of plastic inside the living spaces?

Otherwise, won't condensation always form there?
A boxed enclosure will be built around the pipe in the room.

Best regards,
Body

Hello

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

Olli

Where is my mistake in thinking?
We had water on the outside of the pipe, inside the room... the pipe is cold inside the room. In this case, I believe ceramic or plastic would be the right material, but not metal.

Regards, body