Introduction
The vapour barrier is the most misunderstood and most frequently compromised element of pipe insulation. Get it right, and your insulation will perform for 20+ years. Get it wrong, and you will have dripping pipes, waterlogged insulation, corrosion, and mould — often hidden inside ceiling voids where the damage compounds for years before it becomes visible.
This guide explains exactly what a vapour barrier is, why it is critical on cold pipe applications, which insulation materials provide one, and how to maintain its integrity during installation.
What Is a Vapour Barrier?
A vapour barrier (also called a vapour retarder or vapour check) is a layer of material that resists the passage of water vapour. In the context of pipe insulation, it is a layer on the outer surface of the insulation that prevents water vapour from the surrounding air from migrating through the insulation to the cold pipe surface.
The performance of a vapour barrier is measured by the water vapour diffusion resistance factor (μ). The higher the μ value, the more resistant the material is to water vapour transmission.
| Material | μ Value |
|---|---|
| Armaflex EVO (elastomeric foam) | ≥ 10,000 |
| K-Flex ST (elastomeric foam) | ≥ 7,000 |
| Armaflex AF (older grade) | ≥ 7,000 |
| Kingspan Kooltherm (phenolic foam, foil-faced) | ≥ 100,000 (foil facing) |
| Polyethylene foam (Climaflex, Tubolit) | ~1,000–2,000 (no effective barrier) |
| Mineral wool | ~1 (no vapour barrier) |
| Aluminium foil | > 1,000,000 |
Why Is a Vapour Barrier Needed on Cold Pipes?
On a hot pipe, the pipe surface is warmer than the surrounding air. Water vapour does not condense on a warm surface — it condenses on cold surfaces. So, on a heating pipe, there is no risk of condensation and no need for a vapour barrier.
On a cold pipe — chilled water, refrigerant suction lines, cold mains water in warm environments — the pipe surface is colder than the surrounding air. Water vapour in the air will condense on the cold pipe surface if it can reach it.
If the vapour barrier is absent, insufficient, or breached:
- Water vapour migrates through the insulation towards the cold pipe
- At some point within the insulation, the temperature drops below the dew point
- Water vapour condenses at that point — inside the insulation
- The insulation becomes saturated with liquid water
- Thermal performance collapses — the insulation is now conducting heat rather than resisting it
- Condensation accelerates, corrosion begins on the pipe surface, and mould grows in the saturated insulation
This process is invisible from the outside until the insulation is completely saturated and water begins to drip from the outer surface — by which time significant damage has already occurred.
Which Insulation Materials Have a Vapour Barrier?
Closed-cell elastomeric foam — integral vapour barrier
Armaflex EVO and K-Flex ST are closed-cell foams. The cells are sealed, which means water vapour cannot pass through the bulk of the material. The outer skin of the foam acts as the vapour barrier. This is the key advantage of closed-cell elastomeric foam for cold pipe applications — the vapour barrier is built into the material. There is no separate vapour barrier layer to install, damage, or forget.
Phenolic foam with foil facing — separate vapour barrier
Kingspan Kooltherm pipe insulation is phenolic foam with an aluminium foil facing. The foil facing is the vapour barrier — it has an extremely high μ value (effectively impermeable to water vapour). However, the foil facing is a separate layer that must be maintained intact at all joints and fittings. Any breach in the foil — a cut, a tear, an unbonded joint — creates a vapour barrier failure.
Polyethylene foam — no effective vapour barrier
Polyethylene foam (Climaflex, Tubolit) has a μ value of ~1,000–2,000 — far too low to provide effective vapour barrier performance on cold pipe applications. Water vapour passes through it readily. It must not be used on chilled water, refrigerant lines, or cold mains water pipes in warm environments.
Mineral wool — no vapour barrier
Mineral wool has a μ value of approximately 1 — it offers essentially no resistance to water vapour transmission. It is completely unsuitable for cold pipe applications. It is appropriate only for high-temperature heating and steam pipework where condensation is not a risk.
The Dew Point: Understanding the Risk
The dew point is the temperature at which water vapour in the air begins to condense into liquid water. It depends on the air temperature and relative humidity.
| Ambient Temperature | Relative Humidity | Dew Point |
|---|---|---|
| 20°C | 50% | 9°C |
| 20°C | 60% | 12°C |
| 20°C | 70% | 14°C |
| 25°C | 60% | 17°C |
| 25°C | 70% | 19°C |
| 25°C | 80% | 21°C |
| 30°C | 70% | 24°C |
A chilled water pipe at 10°C in a plant room at 25°C / 60% RH has a dew point of 17°C. The pipe surface at 10°C is well below the dew point — condensation will form on any uninsulated surface. The insulation must be thick enough for its outer surface temperature to remain above 17°C. If the insulation is too thin, the outer surface will be below 17°C and condensation will form on the outside of the insulation, which is also a failure.
Maintaining Vapour Barrier Integrity: The Critical Installation Details
The most common cause of vapour barrier failure is not the wrong material — it is poor installation. A correctly specified closed-cell elastomeric foam installation will fail if the vapour barrier is breached at joints, fittings, or penetrations.
Longitudinal seams
The pre-slit seam on the elastomeric foam tube is the primary vapour barrier joint. Apply Armaflex 520 adhesive to both faces of the seam, allow to become tacky (approximately 3–5 minutes at 20°C), then press firmly together along the full length. On self-seal grades, press the factory-applied adhesive strip firmly along its entire length — not just at the ends.
Butt joints
Apply adhesive to both end faces, allow to become tacky, then press firmly together. Stagger butt joints where possible to avoid a continuous joint line along the pipe run.
Bends
Mitre-cut joints on bends must be fully bonded. Pre-formed 90° bends eliminate the need for a mitre joint and are preferable for critical applications.
Tees
Tee junctions are the most difficult to insulate with a continuous vapour barrier. The branch insulation must be cut and fitted carefully to the main pipe insulation, with all joints fully bonded. No gaps.
Valves and flanges
Valves, flanges, and strainers must be insulated with purpose-made valve boxes or carefully fabricated insulation sections. All joints must be bonded. A valve left uninsulated creates a breach in the vapour barrier — water vapour will condense on the valve body and migrate into the adjacent insulation.
Pipe supports and hangers
Where the pipe passes through a support or hanger, the insulation must continue through or around the support without gaps. Standard pipe clamps compress the insulation and create cold bridges — use insulated pipe supports with a thermal break for cold pipe applications.
Penetrations through walls and floors
Where insulated pipework passes through a wall or floor, the insulation must continue uninterrupted through the penetration. The penetration must be sealed to prevent air movement that could carry moisture to the pipe surface.
Vapour Barrier for Phenolic Foam Insulation
Phenolic foam (Kingspan Kooltherm) has an aluminium foil facing that acts as the vapour barrier. The foil is highly effective — but it must be maintained intact at every joint.
At butt joints, the foil facing must be lapped and sealed with aluminium foil tape, then pressed firmly to both surfaces to ensure an airtight seal. At bends and fittings, the foil facing must be carefully cut and fitted, with all edges sealed with aluminium foil tape. At pipe supports, the foil facing must not be punctured — use insulated pipe supports or protect the foil at the support location.
Any breach in the foil facing — however small — creates a vapour barrier failure. Water vapour will migrate through the breach, reach the cold phenolic foam, and condense within the insulation.
Vapour Barrier Inspection
There is no simple field test for vapour barrier integrity. The best approach is a thorough visual inspection before the system is commissioned:
- Walk the entire pipe run and check every seam, joint, and fitting
- Look for unbonded seams — the foam will spring open slightly if the adhesive has not bonded
- Check all butt joints are fully bonded — press the joint and check for any movement
- Check all fittings and valves are fully insulated with no gaps
- Check all pipe supports — ensure the insulation continues through the support without compression
Once the chilled water system is running, any vapour barrier failure will become apparent within days or weeks, as condensation forms at the point of failure. Early inspection and repair is far less costly than replacing saturated insulation after months of operation.
Frequently Asked Questions
Does all pipe insulation have a vapour barrier?
No. Only closed-cell insulation materials provide an effective vapour barrier. Mineral wool has no vapour barrier. Polyethylene foam has a very low μ value and is not suitable for cold pipe applications. Closed-cell elastomeric foam (Armaflex EVO, K-Flex ST) has an integral vapour barrier. Phenolic foam (Kooltherm) has a separate aluminium foil vapour barrier.
What is the μ value, and why does it matter?
The μ value (water vapour diffusion resistance factor) measures how resistant a material is to water vapour transmission. A higher μ value means a more effective vapour barrier. Armaflex EVO has μ ≥ 10,000 — significantly higher than K-Flex ST (μ ≥ 7,000) and far higher than polyethylene foam (~1,000–2,000).
Do I need a vapour barrier on heating pipes?
No. Heating pipes are warmer than the surrounding air, so condensation cannot form on their surfaces. A vapour barrier is only required on cold pipe applications — chilled water, refrigerant lines, and cold mains water in warm or humid environments.
What happens if the vapour barrier is breached?
Water vapour migrates through the breach to the cold pipe surface, where it condenses. The insulation becomes saturated with liquid water, loses its thermal performance, and the condensation problem accelerates. Corrosion begins on the pipe surface, and mould grows in the saturated insulation.
Can I repair a damaged vapour barrier?
Yes — if caught early. For elastomeric foam, apply Armaflex 520 adhesive to the damaged area and press firmly. For foil-faced phenolic foam, apply aluminium foil tape over the damaged area, ensuring the tape is firmly bonded and the repair is airtight.
Is Armaflex EVO better than K-Flex ST for vapour barrier performance?
Yes. Armaflex EVO has μ ≥ 10,000 compared to K-Flex ST’s μ ≥ 7,000 — a 43% stronger vapour barrier. For demanding chilled-water and refrigeration applications, Armaflex EVO’s superior vapour barrier provides a meaningful margin of safety over the installation's 20+ year service life.
What adhesive should I use to bond elastomeric foam joints?
Armaflex 520 adhesive for Armaflex products. K-Flex adhesive for K-Flex products. Do not mix adhesives between brands. Do not use general-purpose contact adhesives.
Related Guides
- How to Prevent Condensation on Copper Pipes
- Chilled Water Pipe Insulation — Complete Guide
- Armaflex vs K-Flex — Which Should You Choose?
- What Thickness Pipe Insulation Do I Need?
- Foil-Faced Insulation: When and Why You Need a Vapour Barrier
0 comments