How to choose duct insulation

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

Three short duct pieces in a row: a rectangular duct insulated outside, a rectangular duct lined inside, and a round flexible insulated duct.

In short

Start with the operating conditions: is the duct carrying cold or warm air, and how hot and humid is the space it runs through? Then choose the form (external duct wrap, internal duct liner or factory-insulated flexible duct) by whether you need to reduce noise in the duct. Next, check that the facing/vapour retarder can be sealed continuously and that the fire and smoke test results meet the project's requirements. Thickness should be set by the designer for the real conditions — no single value fits every job.

1Insulation wrapped around the outside reduces heat gain and condensation on a cold duct
2The outer foil facing is the vapour retarder: keep it continuous and tape the joints as the manufacturer specifies
3Cold air in the duct
4Insulation lining the inside helps with heat and reduces noise carried along the duct
5The face in contact with the air must be designed for moving air — check the manufacturer's documents for air velocity and test data

1. Know the operating conditions

Information to have before choosing:

Air temperature in the duct (cold supply, return, warm air)
Where the duct runs: in a conditioned room, ceiling void, under the roof or outdoors
Temperature and relative humidity of that space — the more humid, the greater the condensation risk
Air velocity in the duct (critical for internal liner)
Project requirements: drawings, specification, referenced standards

These set the thickness needed, which the system designer can calculate. Do not copy a number from another job. More in What does insulation thickness depend on?

2. Choose the form: wrap, liner or flexible duct

Form	Suits	Example
---	---	---
External wrap (duct wrap)	Installed metal ducts needing thermal and condensation control, noise in the duct not a priority	Micro Duct Wrap FRK
Internal liner (duct liner)	Thermal insulation plus reducing airflow/equipment noise in the duct	Micro Duct Liner PFL, AEROFLEX ACOUSTEX
Insulated flexible duct	Connections from the main duct to diffusers	Duct Excel ISODEC

Details in Duct wrap vs duct liner and Flexible duct.

3. The facing and vapour retarder must be continuous

On cold ducts, fibrous insulation (glass wool, stone wool) lets water vapour through, so it relies on a vapour-retarding facing such as foil-scrim-kraft (FRK). Check:

The facing's water vapour permeance in the maker's documents — the lower the value, the harder it is for vapour to pass
The tape and joint-sealing method the maker recommends, e.g. aluminium foil tape
Whether the facing is tough enough to resist scratches during installation and service

See the principle in What is a vapour barrier and why must it be continuous? and Sealing insulation joints.

4. The project's fire and smoke requirements

Many projects require materials in air systems to meet flame-spread and smoke criteria, e.g. referring to NFPA 90A with ASTM E84 / UL 723 test results, or other standards such as BS 476. Compare the maker's test results with what the specification asks for. A test result belongs to the product under the tested conditions — it does not certify the whole system. If unsure, consult the designer or ask us.

5. Other considerations

Outdoor ducts — need protection from sun, rain and mechanical damage as the maker recommends
Accessories — adhesive, pins and tape compatible with the insulation (Accessories)
Space and weight — insulation thickness takes up ceiling space; allow for it at design stage

A common mistake

The right insulation installed without continuity — gaps at joints, untaped pins, or bands pulled so tight the insulation is crushed — still leaves a duct that drips. See Why do AC pipes drip?

Sources

ASHRAE Handbook — Fundamentals, chapter Insulation for Mechanical Systems
NAIMA / Insulation Institute — A Guide to Insulated HVAC Duct Systems (AH121)
NFPA 90A and ASTM E84 / UL 723 (flame-spread and smoke requirements for materials in air systems)
UL 181 — Factory-Made Air Ducts and Air Connectors
ASTM E96 — Water Vapor Transmission of Materials
Manufacturer documents: Micro Duct Wrap FRK, Micro Duct Liner (Microfiber), ACOUSTEX (Aeroflex), ISODEC (Duct Excel)