Double-layer inner duct wall. The inner duct wall is covered with a fibre glass insulation. The insulation is covered with a double-layer outer duct.
The insulation reduces the heat gain or loss resulting from a temperature difference between the air flowing in the duct and the surrounding air.
The outer duct acts as a vapour barrier to prevent condensation to enter into the insulation. Condensation can occur on the outside of a duct carrying air at lower temperatures than the surrounding air.