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HVAC and ductwork

Why do air-conditioning ducts need insulation?

5 min read · Written by our team · Updated 26 Sept 2026

A rectangular air duct above a ceiling, half of it wrapped in foil-faced insulation, with red heat arrows rising toward the underside of the duct.
In short

AC ducts are insulated for three main reasons: to reduce the cooling lost before the air reaches the room, to prevent condensation that makes cold duct surfaces drip onto the ceiling, and to save energy because the unit no longer has to make up for lost cooling. If the insulation is lined inside the duct (duct liner), it can also absorb airflow and equipment noise travelling along the duct.

Duct cross-sections: duct wrap sits around the outside of the duct with a foil vapour-retarder facing; duct liner is fixed inside the duct with an airstream face designed for moving airDuct wrap 123Duct liner 45 Duct cross-sections: duct wrap sits around the outside of the duct with a foil vapour-retarder facing; duct liner is fixed inside the duct with an airstream face designed for moving airDuct wrap 123Duct liner 45
  1. 1Insulation wrapped around the outside reduces heat gain and condensation on a cold duct
  2. 2The outer foil facing is the vapour retarder: keep it continuous and tape the joints as the manufacturer specifies
  3. 3Cold air in the duct
  4. 4Insulation lining the inside helps with heat and reduces noise carried along the duct
  5. 5The face in contact with the air must be designed for moving air — check the manufacturer's documents for air velocity and test data

What happens if a duct is not insulated?

Supply ducts usually run through ceiling voids, under roofs or other unconditioned spaces, where the surrounding air is much hotter and more humid than the air inside. Without insulation:

  • Heat from outside flows into the duct, so air leaves the diffusers warmer than designed and rooms at the end of the run cool slowly or not enough
  • The duct surface is colder than the dew point of the surrounding air, so water vapour condenses on it and drips onto the ceiling, causing stains, swelling or mould
  • The AC runs longer to make up for the lost cooling, raising the electricity bill

The reverse applies to warm-air ducts (heating in some buildings): they lose heat outward on the same principle.

What does duct insulation help with?

IssueHow insulation helps
------
Heat gain/loss through the ductReduces heat transfer through the duct wall, so air arrives close to its design temperature
Condensation on cold ductsKeeps the outer surface of the insulation system above the dew point, when the thickness suits the conditions and the vapour retarder is continuous
EnergyReduces the load the unit has to make up
Noise in the ductOnly internal duct liner with a sound-absorbing surface — external wrap does very little for noise inside the duct

Why does dripping matter so much in Thailand?

Thailand is hot and humid for most of the year, so the dew point of the air is fairly high. A cold duct with no insulation — or with insulation but open joints — can easily sweat. The full principle is in What are dew point and condensation?

Insulation alone is not enough — it must be continuous

Glass-wool insulation on cold ducts needs a foil facing or vapour retarder with every joint sealed. Any opening lets water vapour in to condense inside the insulation and on the duct. See What is a vapour barrier and why must it be continuous?

How thick should it be?

There is no single thickness for every job. It depends on the air temperature in the duct, the temperature and humidity of the space the duct passes through, the insulation's thermal conductivity and the project's requirements. The HVAC designer sets it — or ask us with your site details.

Where to next

Sources

  • ASHRAE Handbook — Fundamentals, chapter Insulation for Mechanical Systems (condensation control and energy loss principles)
  • NAIMA / Insulation Institute — A Guide to Insulated HVAC Duct Systems (AH121)
  • Johns Manville — Duct Liner vs. Duct Wrap (acoustic and installation differences)
  • Manufacturer documents for Micro Duct Wrap FRK and Micro Duct Liner (Microfiber)

Related products

Categories and example products for this topic

Duct and HVAC insulation
  • Micro Duct Wrap FRK — Foil-faced glass wool wrapped outside ducts: saves energy and controls condensation
  • Micro Duct Liner PFL — Glass wool board lining ducts inside: insulates and absorbs airflow noise (NRC published)
  • Aeroflex ACOUSTEX — Fibre-free open-cell foam, 25 mm, lining ducts to absorb noise generated in them
  • Duct Excel ISODEC — Factory-insulated glass wool flexible duct, in two types by air velocity and pressure
See related products Help me choose

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