The present invention concerns a foldable duct (4) made of glass or stone fibres and binder agent having a foil (1) laminated thereon, on the inside and outside of the duct, wherein the foil (1) at least comprises:—an outer layer of aluminium (5),—an intermediate layer of a thermoplastic polymer (6) having a lower melt point than polyethylene, positioned closest to the outer layer of aluminium (5),—an intermediate layer of polyethylene (7), positioned between the intermediate layer of thermoplastic polymer (6), and—an inner layer of a thermoplastic polymer (8) having a lower melt point than polyethylene.