Thermal vs acoustic
Thermal insulation prevents heat gain or loss between the air inside the duct and the surrounding space. Acoustic insulation reduces noise transmission — both from the air handling unit through the duct, and from the duct into the surrounding space. Many systems need both.
Internal lining vs external wrap
External wrap is the most common — typically Isover Ductwrap or Rockwool slabs applied over the outside of the ductwork. Internal lining is specified where acoustic attenuation is needed in the airstream. Each has its place; the project spec defines which.
Supply, extract and chilled air
Supply ducts need insulation to keep heating or cooling effective. Extract ducts often need acoustic insulation. Chilled air ducts need vapour-sealed insulation to prevent external condensation.
External rooftop duct
External ductwork needs an additional weatherproof finish over the insulation — aluminium cladding or PIB are common choices. Without it, the insulation absorbs water and stops working.