Why Cold Pipes Sweat
Condensation on cold pipes is not a sign of a leaking pipe — it is atmospheric moisture condensing on a surface that has fallen below the dew point of the surrounding air. The colder the pipe and the more humid the environment, the greater the condensation risk.
Left unaddressed, condensation causes corrosion, saturates building fabric, promotes mould growth and significantly increases energy consumption on chilled water and refrigeration systems.
The solution is correctly specified and installed pipe insulation with an effective vapour barrier.
Which Pipes Are at Risk?
Any pipework operating below the ambient dew point is at risk of condensation. In practice, this includes:
- Cold water services — typically 10–15°C, at risk in warm, humid environments
- Chilled water flow and return — typically 6–12°C, high condensation risk in most UK conditions
- Refrigerant suction lines — can operate at -10°C or below, very high condensation risk
- Ground source heat pump brine circuits — typically 0–10°C, significant condensation risk
- Air handling unit connections — often in humid plant room environments
Choosing the Right Insulation
The most effective insulation for condensation control on cold pipework is closed-cell elastomeric foam. Products such as Armaflex EVO and K-Flex ST are specifically designed for this application because:
- The closed-cell structure resists moisture vapour diffusion
- The material acts as its own integral vapour barrier
- It can be installed without additional vapour barrier wrapping on most applications
- It is available in a wide range of pipe sizes and wall thicknesses
Open-cell foam, mineral wool and fibreglass insulation are not suitable for cold pipework without additional vapour barrier protection, as they absorb moisture readily and lose thermal performance rapidly when wet.
Specifying the Correct Thickness
Insulation thickness on cold pipework must be sufficient to keep the outer surface of the insulation above the dew point of the surrounding air. BS 5422:2011 provides minimum thickness values for chilled water and refrigerant pipework based on pipe outside diameter and operating temperature.
As a general guide for UK conditions:
| Application | Pipe OD | Minimum Thickness |
|---|---|---|
| Chilled water (6°C flow) | 22mm | 25mm |
| Chilled water (6°C flow) | 35mm | 30mm |
| Cold water service (10°C) | 22mm | 19mm |
| Refrigerant suction (-5°C) | 22mm | 25mm |
Always verify final thickness against BS 5422 Table 3 for chilled water or the manufacturer's specification for refrigerant lines.
Installing Correctly
Correct installation is as important as correct specification. The most common cause of condensation on insulated cold pipework is installation failure rather than undersized insulation.
1. Glue every joint
Apply elastomeric foam adhesive to both faces of every longitudinal seam and butt joint. Allow to become touch-dry before pressing together. An unglued joint is a direct pathway for moisture to reach the cold pipe surface.
2. Insulate all fittings and valves
Uninsulated fittings are condensation points. Use pre-formed sections or fabricate covers from sheet insulation. Every fitting should be insulated to the same standard as the pipework.
3. Seal around pipe supports
Pipe supports that contact the cold pipe directly create thermal bridges. Use insulated pipe supports or ensure the insulation continues through the support without compression.
4. Avoid compression
Compressed insulation has reduced thermal resistance. Do not over-tighten pipe clips or allow insulation to be compressed by adjacent services.
5. Protect outdoor insulation
On external pipework, use UV-resistant insulation such as Armaflex HT Solar or Armaflex Tuffcoat, or apply an approved UV-resistant cladding. Standard elastomeric foam degrades rapidly when exposed to sunlight, compromising the vapour barrier.
High-Humidity Environments
In environments with consistently high relative humidity — commercial kitchens, swimming pools, laundries, or outdoor installations in exposed locations — standard BS 5422 thicknesses may be insufficient. In these situations:
- Increase insulation thickness beyond BS 5422 minimums
- Consider enhanced vapour barrier protection such as aluminium foil facing or PVC jacketing
- Consult the insulation manufacturer's technical team for site-specific guidance
Maintenance and Inspection
Even correctly installed insulation can develop condensation problems over time if the vapour barrier is compromised. Inspect cold pipework insulation annually for:
- Surface cracking or deterioration (particularly on outdoor installations)
- Damp patches or discolouration on the insulation surface
- Soft or saturated sections indicating moisture ingress
- Uninsulated fittings or gaps at pipe supports
Replace any damaged sections promptly. A small area of failed insulation can allow moisture to track along the pipe surface beneath the insulation, causing damage well beyond the visible failure point.
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