Foil-Faced vs Elastomeric Foam Pipe Insulation — Which Should You Choose?

Introduction

When specifying pipe insulation for cold pipe applications — chilled water, refrigeration, cold mains water — two types of insulation provide an effective vapour barrier: foil-faced insulation (typically phenolic foam with an aluminium foil facing, such as Kingspan Kooltherm) and closed-cell elastomeric foam (Armaflex EVO, K-Flex ST).

Both work. Both are widely specified. But they work differently, have different strengths and weaknesses, and suit different applications. This guide explains the key differences and tells you which to choose for every common application.

How Each Type Provides a Vapour Barrier

Closed-cell elastomeric foam — integral vapour barrier

In Armaflex EVO and K-Flex ST, the vapour barrier is integral to the bulk material. The closed-cell structure of the foam means water vapour cannot pass through the material itself. The outer skin of the foam acts as the vapour barrier.

This means the vapour barrier is present everywhere the foam is present — including at bends, tees, and fittings. The only points where the vapour barrier can be compromised are at joints between sections, which must be bonded with adhesive.

Foil-faced insulation — separate vapour barrier

In Kingspan Kooltherm and similar foil-faced products, the vapour barrier is provided by the aluminium foil facing, which has an extremely high μ value (effectively impermeable). The insulation core (phenolic foam) has a relatively low μ value on its own.

This means the vapour barrier is only present where the foil facing is intact. At every joint — butt joints, bends, tees, fittings — the foil facing must be lapped and sealed with aluminium foil tape to maintain the vapour barrier. Any breach in the foil, however small, creates a vapour barrier failure.

Head-to-Head Comparison

When to Choose Elastomeric Foam (Armaflex EVO)

Chilled water systems
Armaflex EVO is the standard specification for chilled water pipe insulation in the UK. Its integral vapour barrier is more forgiving of minor installation imperfections than foil-faced insulation. For chilled water, the reliability of the integral vapour barrier outweighs the thin-wall advantage of phenolic foam in most cases.

Refrigeration and heat pump pipework
The flexible nature of elastomeric foam makes it easier to install on the complex pipework geometry of refrigeration and heat pump systems. The integral vapour barrier is essential on refrigerant suction lines operating below ambient temperature.

Cold water pipes
For cold mains water condensation control, Armaflex EVO is the standard specification. The 13–25mm wall thicknesses required are readily available in elastomeric foam.

High-humidity environments (pool plant rooms, kitchens)
Armaflex EVO’s Microban antimicrobial protection is a significant advantage in high-humidity environments. The integral vapour barrier also provides more reliable performance in conditions where installation quality may be variable.

Outdoor applications
Armaflex Tuffcoat (pre-jacketed Armaflex EVO) is available for outdoor and underground applications. Foil-faced phenolic foam requires additional weatherproof cladding for outdoor use.

When to Choose Foil-Faced Insulation (Kooltherm)

Space-constrained commercial heating installations
The primary advantage of Kooltherm is its thin-wall performance. On a 54mm OD heating pipe, Kooltherm requires approximately 20mm vs 32mm for Armaflex EVO. In congested ceiling voids and plant rooms where insulation clashes with structure and other services, this difference is significant.

Large-diameter commercial pipework
On pipes above 76mm OD, the difference in wall thickness between Kooltherm and elastomeric foam becomes increasingly pronounced. For large-diameter heating mains in commercial buildings, Kooltherm’s thin-wall performance can make the difference between a workable and unworkable installation.

Part L compliance on larger pipes
Where the specification requires the minimum possible insulation bulk for Part L compliance, Kooltherm achieves compliance at thinner wall thicknesses than any other material.

The Installation Quality Factor

The most important practical difference between the two types is the consequence of installation imperfections.

Elastomeric foam: if a seam is not perfectly bonded, the vapour barrier is locally compromised but the closed-cell foam on either side of the seam still provides significant resistance to water vapour transmission. The failure is gradual.

Foil-faced insulation: if the foil tape at a joint is not perfectly applied — a small gap, a lifted edge, a pinhole — the vapour barrier is completely absent at that point. Water vapour passes straight through the gap to the insulation core and condenses. The failure is immediate and localised.

This does not mean foil-faced insulation is inferior — when correctly installed, its foil facing provides a stronger vapour barrier than elastomeric foam. But it does mean that installation quality is more critical, and that foil-faced insulation is less forgiving of the minor imperfections that occur in real-world installations.

Can You Mix the Two Types?

Different pipe runs in the same installation can use different insulation types. However, mixing types on the same pipe run is not recommended — the two types use different jointing methods, have different wall thicknesses, and mixing them creates a complex joint at the transition point that is a potential vapour barrier weak point.

Frequently Asked Questions

Is foil-faced insulation better than elastomeric foam?

Neither is universally better — they suit different applications. Foil-faced phenolic foam (Kooltherm) achieves thinner wall thicknesses and has a stronger foil vapour barrier when intact. Elastomeric foam (Armaflex EVO) has an integral vapour barrier that is more forgiving of installation imperfections and is better suited to cold pipe applications, outdoor use, and high-humidity environments.

Which has a better vapour barrier — Armaflex or Kooltherm?

Kooltherm’s aluminium foil facing has a higher μ value (> 100,000) than Armaflex EVO (μ ≥ 10,000). However, the foil facing is only effective when intact — any breach creates a complete vapour barrier failure. Armaflex EVO’s integral vapour barrier is more robust in practice because it is present throughout the bulk of the material, not just at the surface.

Can foil-faced insulation be used on chilled water pipes?

Yes, but the foil facing must be maintained intact at all joints and fittings with aluminium foil tape. For most chilled water applications, closed-cell elastomeric foam (Armaflex EVO) is simpler and more reliable.

Why is Kooltherm thinner than Armaflex?

Because Kooltherm has a lower thermal conductivity (λ ≈ 0.022 vs 0.033 W/m·K for Armaflex EVO). A lower λ value means less material is needed to achieve the same thermal resistance.

Can I use foil-faced insulation outdoors?

Not without additional weatherproofing. The aluminium foil facing is not sufficient for outdoor exposure. Protect with aluminium cladding or a weatherproof outer jacket. For outdoor applications, Armaflex Tuffcoat is simpler and more reliable.

Related Guides

• Vapour Barriers for Pipe Insulation
• Phenolic Foam Pipe Insulation — Complete Guide (Kooltherm)
• Armaflex vs K-Flex — Which Should You Choose?
• Chilled Water Pipe Insulation — Complete Guide
• What Thickness Pipe Insulation Do I Need?

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