What is a vapour barrier / vapour retarder, and why must it be continuous?

Q: Barrier or retarder?

Technically a retarder "slows" vapour, while barrier is usually used for materials that let very little through. In practice the two words are used interchangeably. Some building documents (for example US building codes) grade them by vapour permeance, but those grades are for building envelopes. For pipe and duct insulation, go by the values the manufacturer states and the project's requirements.

Q: How is vapour resistance measured?

Water vapour permeance — how much vapour passes through a sheet of material per area, per time, per vapour-pressure difference. A common unit is the perm ; the lower the value, the harder it is for vapour to pass. Tested to standards such as ASTM E96 . Permeability (e.g. perm-in) — expressed per thickness of material, used for homogeneous materials such as elastomeric foam. μ value (water vapour diffusion resistance factor) — used in European standards; how many times more the material resists vapour than still air. The higher, the more resistant. Manufacturers may use different units and test methods, so do not compare figures in different units directly.

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

Illustration of a cold pipe wrapped in insulation and an outer foil skin, with moisture entering the insulation through one torn gap in the foil.

In short

A vapour retarder (often simply called a vapour barrier) is a material or facing layer that reduces the passage of water vapour. On cold pipes and ducts it stops vapour from the hot, humid air diffusing through to the cold surface and condensing inside the insulation. Its performance depends on continuity — an unsealed joint, tear or puncture lets vapour in, however good the material itself is.

1The surrounding air holds water vapour; on a surface colder than its dew point, the vapour condenses into water
2Water collects on the pipe and drips onto the ceiling or floor
3Insulation sized for the pipe temperature and the site's temperature and humidity keeps its outer surface warmer than the dew point
4Seams must be sealed as the manufacturer specifies — through any gap, humid air can reach the pipe and condense under the insulation

Barrier or retarder?

Technically a retarder "slows" vapour, while barrier is usually used for materials that let very little through. In practice the two words are used interchangeably. Some building documents (for example US building codes) grade them by vapour permeance, but those grades are for building envelopes. For pipe and duct insulation, go by the values the manufacturer states and the project's requirements.

How is vapour resistance measured?

Water vapour permeance — how much vapour passes through a sheet of material per area, per time, per vapour-pressure difference. A common unit is the perm; the lower the value, the harder it is for vapour to pass. Tested to standards such as ASTM E96.
Permeability (e.g. perm-in) — expressed per thickness of material, used for homogeneous materials such as elastomeric foam.
μ value (water vapour diffusion resistance factor) — used in European standards; how many times more the material resists vapour than still air. The higher, the more resistant.

Manufacturers may use different units and test methods, so do not compare figures in different units directly.

Closed-cell materials vs fibrous with a facing

	Closed-cell elastomeric	Fibrous (glass/stone wool)
---	---	---
Where vapour resistance comes from	The material itself, which has high vapour resistance	The facing, e.g. foil-scrim-kraft (FRK) or a vapour-retarder jacket, because vapour passes easily through the fibres
Weak points	Slit seams and butt joints — glue them as the maker instructs	Every joint, tear, puncture and pin point must be taped to stay continuous
Examples	AEROFLEX, Armaflex Class 1, MAXFLEX FR	Micro Duct Wrap FRK, SCG PCI (on cold pipes it needs a vapour-retarding jacket as designed)

More in Closed-cell vs fibrous insulation.

Why must it be continuous?

A cold pipe constantly draws water vapour towards itself, because the vapour pressure on the hot, humid side is higher than at the cold pipe surface. If the vapour-retarding layer has even one opening, vapour enters there, condenses inside, the insulation gets wet and loses performance, and the damage can spread along the pipe unseen from outside.

Points that must be sealed continuously:

Longitudinal seams and butt joints of the insulation
Fittings, elbows, tees, valves and flanges — the vapour retarder on a fitting must join that of the adjacent straight pipe
Pin points, hangers and every penetration
Terminations where the insulation ends (seal the edge)

Use the tape or adhesive the maker recommends for that material, e.g. aluminium foil tape for foil facings. More methods in Sealing insulation joints.

Do not put unfaced fibrous insulation on cold pipes

Bare fibrous insulation on a cold pipe readily lets vapour in to condense inside. If fibrous insulation is used for cold service, it needs a facing or vapour-retarder jacket designed for that job.

Sources

ASTM E96 — Standard Test Methods for Water Vapor Transmission of Materials
ASHRAE Handbook — Fundamentals, chapter Insulation for Mechanical Systems (importance of vapour retarders on below-ambient systems)
National Insulation Association, Insulation Outlook — articles on vapour-retarder jacketing and condensation on cold systems
NAIMA / Insulation Institute — Moisture Control: Utilizing Vapor Retarders
Johns Manville — Answering your questions about vapor retarders, vapor barriers and facings
Manufacturer documents: AEROFLEX, ArmaFlex Class 1, MAXFLEX FR, Micro Duct Wrap FRK