What Is Dew Point?
Dew point is the temperature at which moisture in the air begins to condense into liquid water. When a surface — such as a cold pipe or the outer face of pipe insulation — falls below the dew point temperature of the surrounding air, condensation forms.
For pipe insulation, dew point is not an abstract concept. It is the critical threshold that determines whether your insulation system will perform correctly or fail prematurely.
Why Dew Point Matters for Pipe Insulation
On cold pipework — including chilled water systems, cold water services, refrigerant lines and ground source heat pump circuits — the pipe surface temperature is often well below the ambient air temperature. If the insulation is undersized, incorrectly installed, or has a compromised vapour barrier, the pipe surface can fall below the dew point of the surrounding air.
The result is condensation forming on or within the insulation. Over time, this leads to:
- Saturated insulation with significantly reduced thermal performance
- Corrosion of the pipe and fittings beneath the insulation
- Mould growth on the insulation surface
- Structural damage to surrounding building fabric
- Increased energy consumption as the system works harder to maintain temperature
How Dew Point Is Calculated
Dew point depends on two variables: ambient air temperature and relative humidity. The higher the relative humidity, the closer the dew point is to the ambient temperature.
| Ambient Temp | Relative Humidity | Approximate Dew Point |
|---|---|---|
| 20°C | 50% | 9°C |
| 20°C | 70% | 14°C |
| 20°C | 85% | 17°C |
| 25°C | 60% | 16°C |
| 25°C | 80% | 21°C |
| 30°C | 70% | 23°C |
For a chilled water pipe operating at 6°C in a plant room at 25°C and 70% relative humidity, the dew point is approximately 19°C. The pipe surface is well below the dew point, meaning condensation will form unless the insulation system prevents the pipe surface temperature from being reached at the insulation outer face.
The Role of Insulation Thickness
Insulation does not simply reduce heat transfer — on cold pipework, it raises the temperature of the outer surface of the insulation above the dew point of the surrounding air. If the insulation is thick enough, the outer surface remains above dew point and no condensation forms.
This is why BS 5422 specifies minimum insulation thicknesses for chilled water and refrigerant pipework. The standard accounts for typical operating temperatures and ambient conditions to ensure the outer surface of correctly specified insulation remains above dew point under normal conditions.
Undersizing insulation — even by a few millimetres — can push the outer surface temperature below dew point, particularly in humid environments or during summer peak conditions.
The Role of the Vapour Barrier
Closed-cell elastomeric foam insulation such as Armaflex EVO and K-Flex ST acts as its own vapour barrier. The closed-cell structure resists moisture diffusion, preventing water vapour from migrating through the insulation to the cold pipe surface.
This is why vapour barriers for pipe insulation are not optional on cold and chilled water systems — they are a fundamental part of the insulation system's ability to control condensation.
Any breach in the vapour barrier — an unglued joint, an uninsulated fitting, a puncture — creates a pathway for moisture to reach the cold surface. Condensation then forms inside the insulation where it cannot be seen, causing progressive deterioration.
Dew Point and BS 5422
BS 5422:2011 includes specific thickness requirements for chilled water and refrigerant pipework that are calculated to prevent condensation under defined ambient conditions. Compliance with BS 5422 does not guarantee condensation control in all environments — particularly in high-humidity locations such as commercial kitchens, swimming pools or outdoor installations — but it provides the correct baseline for most UK applications.
Where ambient conditions exceed the assumptions in BS 5422, additional insulation thickness or enhanced vapour barrier protection may be required. Consult the insulation manufacturer's technical data for guidance on high-humidity applications.
Practical Implications for Installers
Understanding dew point has direct practical consequences for installation:
- Specify by outside diameter, not nominal bore — incorrect sizing creates gaps that allow condensation pathways.
- Glue every joint — unsealed butt joints and longitudinal seams are the most common cause of vapour barrier failure.
- Insulate all fittings and valves — uninsulated fittings are condensation points regardless of how well the pipework is insulated.
- Check ambient conditions — in high-humidity environments, increase insulation thickness beyond BS 5422 minimums.
- Inspect annually on outdoor installations — UV degradation and mechanical damage can compromise the vapour barrier over time.
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