So besides the building regs and not leaving your fingerprints stuck to the flue, what are the advantages to running a double wall insulated flue over a single wall? Here they stack up something called "bricks aloevere" (clay bricks with honeycomb outer layer and a large central hole) into a big clay tube and slap on some refactory cement with no liner. As installed at our last place by the local approved artisan for the previous tennant :/i see you are in france there is a section on twinwall in the uk building regs however they seem more bothered in letting 475 degree c single wall pipe running through a ceiling into a bedroom or something
Itll only be exposed to the elements for the last 8ft or so...