What Is the Difference?
Pipe insulation materials fall into two broad categories based on their cellular structure: closed-cell and open-cell. The distinction is fundamental to condensation control and determines which applications each material is suitable for.
Closed-cell insulation has a cellular structure in which each cell is sealed and does not connect to adjacent cells. This structure prevents moisture vapour from passing through the material, giving it a very low water vapour permeability. Closed-cell materials include elastomeric foam (Armaflex EVO, K-Flex ST), polyisocyanurate (PIR) and extruded polystyrene (XPS).
Open-cell insulation has a cellular structure in which the cells are interconnected, allowing air and moisture vapour to pass through the material freely. Open-cell materials include mineral wool, glass wool, open-cell polyurethane foam and most flexible foam pipe lagging sold for domestic use.
This article is part of the Condensation & Vapour Barrier Resource Centre — the complete UK technical reference for condensation control in pipe insulation systems.
Why It Matters for Cold Pipework
On hot pipework, the choice between closed-cell and open-cell insulation is primarily a question of thermal performance and fire classification. Moisture is not a significant concern because the pipe surface is above the ambient dew point — condensation cannot form.
On cold pipework, the situation is fundamentally different. The pipe surface is below the dew point of the surrounding air, meaning moisture vapour in the air is constantly trying to migrate towards the cold surface. The insulation must resist this migration or condensation will form within the insulation system.
Open-cell insulation cannot resist moisture vapour migration. On cold pipework, moisture passes through open-cell insulation, reaches the cold pipe surface, and condenses. The insulation becomes saturated, loses its thermal performance, and the pipe corrodes beneath it. This process is often invisible until significant damage has occurred.
Closed-cell elastomeric foam resists moisture vapour migration because the sealed cell structure blocks the diffusion pathway. This is why it is the standard material for chilled water, cold water and refrigerant pipework.
Water Vapour Permeability
The technical measure of a material's resistance to moisture vapour diffusion is the water vapour diffusion resistance factor, expressed as µ (mu). The higher the µ value, the greater the resistance to moisture vapour migration.
| Material | Typical µ Value | Suitable for Cold Pipework? |
|---|---|---|
| Armaflex EVO (elastomeric foam) | ≥ 7,000 | Yes |
| K-Flex ST (elastomeric foam) | ≥ 5,000 | Yes |
| Mineral wool | 1–2 | No (without additional VB) |
| Glass wool | 1–2 | No (without additional VB) |
| Polyethylene foam | 1,500–3,000 | Marginal — not for refrigerant |
| PIR (foil-faced) | Very high (foil facing) | Yes (if foil intact) |
When Open-Cell Insulation Is Acceptable
Open-cell insulation is entirely appropriate for heating pipework, where the pipe surface is above the ambient dew point and condensation is not a risk. Mineral wool and glass wool pipe sections are widely used on heating systems, boiler pipework and domestic hot water services.
Open-cell insulation can also be used on cold pipework if it is combined with a robust external vapour barrier — typically an aluminium foil facing or a PVC jacket — that prevents moisture from entering the insulation. However, any breach in the external vapour barrier will allow moisture ingress, and the consequences on cold pipework are severe. For this reason, closed-cell elastomeric foam is strongly preferred for cold and chilled water applications in most commercial and industrial settings.
Closed-Cell Insulation for Heat Pump Systems
For heat pump pipe insulation, closed-cell elastomeric foam is the standard specification for both heating circuits and refrigerant lines. Most heat pump manufacturers specify Armaflex EVO or an equivalent closed-cell elastomeric foam product. Using open-cell insulation on refrigerant lines will typically void the equipment warranty and may affect MCS certification.
BS 5422 and Material Selection
BS 5422 (current edition) does not mandate specific insulation materials but specifies minimum thermal performance requirements. For chilled water and refrigerant pipework, the standard implicitly requires a material with adequate vapour barrier properties, as open-cell materials cannot achieve the required condensation control at the specified thicknesses.
In practice, closed-cell elastomeric foam is the default specification for cold pipework in BS 5422-compliant installations.
Summary
- Use closed-cell elastomeric foam (Armaflex EVO, K-Flex ST) for all cold, chilled water and refrigerant pipework.
- Use mineral wool or glass wool for heating pipework where condensation is not a risk.
- Never use open-cell foam or polyethylene foam on refrigerant suction lines.
- If open-cell insulation must be used on cold pipework, ensure a robust, continuous external vapour barrier is installed and maintained.
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