Condensation on Pipes: Causes, Prevention and the Right Insulation

June 7, 2026

Condensation forms on cold pipework when warm, moist air contacts a surface below the dew point. Left untreated, it causes corrosion, mould growth, water damage and energy loss. The right pipe insulation prevents condensation by creating a thermal barrier that keeps the pipe surface above the dew point temperature.

What Causes Condensation on Pipes?

Condensation occurs when the surface temperature of a pipe falls below the dew point of the surrounding air. The dew point depends on the air temperature and relative humidity. In a typical UK building, chilled water pipes (operating at 6–12°C), refrigeration pipework and cold water supply pipes are all vulnerable to condensation, particularly in warm, humid plant rooms or during summer months.

The Risks of Condensation on Pipework

Corrosion — persistent moisture on metal pipe surfaces causes rust and corrosion, shortening pipe life
Mould growth — damp insulation and pipe surfaces promote mould, which can affect indoor air quality
Water damage — condensation dripping from pipework can damage ceilings, floors and equipment below
Energy loss — wet insulation loses its thermal performance, increasing heat gain on cold systems
Insulation failure — open-cell insulation saturated with condensation must be replaced

Why Closed-Cell Insulation is Essential

Not all pipe insulation prevents condensation. Open-cell foam allows water vapour to pass through and reach the cold pipe surface. Only closed-cell insulation — such as Armaflex elastomeric rubber — provides the low water vapour permeability needed to reliably prevent condensation on cold pipework.

The key measure is the water vapour resistance factor (µ). Armaflex has a µ value exceeding 7,000, meaning water vapour transmission through the insulation wall is negligible. Standard polyethylene foam has a µ value of around 2,000–3,000 — adequate for hot pipes but insufficient for cold pipework in humid conditions.

Choosing the Right Wall Thickness

The required insulation thickness for condensation control depends on the pipe temperature, ambient temperature and relative humidity. Thicker insulation keeps the outer surface of the insulation above the dew point, preventing condensation on the insulation surface as well as on the pipe. In standard UK commercial conditions, 19mm wall is typically the minimum for chilled water pipework. In high-humidity environments or at lower pipe temperatures, 25mm or 32mm wall may be required.

Sealing Joints — The Critical Step

Even the best insulation will fail to prevent condensation if joints are not properly sealed. Every unsealed seam or butt joint is a gap in the vapour barrier where moist air can reach the cold pipe surface. All longitudinal seams and butt joints must be sealed with Armaflex 520 adhesive or Armaflex tape. Read our Armaflex adhesive guide for correct application technique.

Condensation on Specific Pipe Types

Chilled Water Pipes

Chilled water systems are the most common source of condensation problems in commercial buildings. Flow temperatures of 6–12°C are well below the dew point in most occupied spaces. Armaflex Class O at 19–25mm wall thickness is the standard specification.

Refrigeration Pipework

Refrigeration suction lines operate at very low temperatures and require closed-cell insulation with a high vapour resistance. See our refrigeration pipe insulation collection for suitable products.

Cold Water Supply Pipes

Cold water supply pipes in warm, humid spaces such as plant rooms and basements can suffer from condensation. Closed-cell insulation prevents moisture reaching the pipe surface and also protects against mould growth on cold water pipes.

Condensate Pipes

Boiler condensate pipes running externally are vulnerable to freezing rather than condensation, but the solution is the same — closed-cell pipe insulation. Read our dedicated condensate pipe insulation guide for full details.

Frequently Asked Questions

Does standard foam pipe insulation prevent condensation?
Standard polyethylene foam (such as Climaflex) has a lower vapour resistance than Armaflex elastomeric rubber. For hot water pipes, it is generally adequate. For cold pipework in humid conditions, closed-cell Armaflex is the correct choice.

How do I know if my pipe insulation is preventing condensation?
If condensation is forming on the outer surface of the insulation, the insulation is too thin or the vapour resistance is insufficient. If condensation is forming beneath the insulation on the pipe surface, the insulation has failed or joints are unsealed.

Can I use Rockwool on cold water pipes?
Rockwool has very low vapour resistance and is not suitable for cold pipework without a vapour barrier facing. Even with a facing, the risk of condensation at joints is higher than with Armaflex. See our Armaflex vs Rockwool guide for a full comparison.