Where to Start
Choose the path that matches your situation.
🔍
I need to understand condensation
Start with the theory — dew point, why pipes sweat, and what drives condensation risk.
Dew Point Explained →🛡️
I need to specify a vapour barrier
Understand μ values, material selection, and how to maintain barrier integrity on site.
Vapour Barriers Guide →🔧
I have a condensation problem to fix
Diagnose why insulation is getting wet and how to repair a failed vapour barrier.
Why Insulation Gets Wet →❄️
I'm insulating a specific cold system
Jump straight to the application guide for chilled water, refrigerant, or cold water pipework.
Browse Applications →What Causes Condensation on Pipes?
Condensation forms when the surface temperature of a pipe falls below the dew point of the surrounding air. On cold pipework — chilled water systems, refrigerant lines, cold water supplies — the pipe surface is often significantly colder than the ambient air, creating a persistent condensation risk.
The three variables that determine condensation risk are:
- Pipe temperature — the colder the pipe, the higher the risk
- Ambient air temperature — warmer air holds more moisture
- Relative humidity — higher humidity raises the dew point closer to the pipe surface temperature
Pipe insulation addresses this by raising the outer surface temperature of the insulation above the dew point. But insulation alone is not enough — the vapour barrier must remain intact to prevent humid air from migrating through the insulation to the cold pipe surface, where it will condense internally and cause waterlogging, corrosion, and mould.
This is why closed-cell insulation materials with high μ values — such as Armaflex EVO (μ ≥ 10,000) and K-Flex ST (μ ≥ 7,000) — are the standard specification for cold pipe applications.
Dew Point Explained
The dew point is the temperature at which air becomes saturated with water vapour and condensation begins to form. It is determined by the combination of air temperature and relative humidity — not by the pipe temperature itself.
For example, at 20°C and 60% RH, the dew point is approximately 12°C. Any pipe surface below 12°C will attract condensation. A chilled water flow pipe at 6°C is well below this threshold — without adequate insulation and a continuous vapour barrier, condensation is inevitable.
Understanding dew point is the foundation of correct insulation thickness specification. BS 5422 provides the reference method for calculating minimum insulation thickness to prevent surface condensation on cold pipes in the UK.
Read: Dew Point Explained for Pipe Insulation →Why Vapour Barriers Matter
A vapour barrier (also called a vapour retarder or vapour check) is a layer of material that resists the passage of water vapour. In pipe insulation, it is the outer surface of the insulation itself — or an additional facing — that prevents humid air from migrating inward to the cold pipe surface.
The performance of a vapour barrier is measured by the water vapour diffusion resistance factor (μ). The higher the μ value, the more resistant the material is to vapour transmission:
| Material | μ Value | Suitable for Cold Pipes? |
|---|---|---|
| Armaflex EVO (elastomeric foam) | ≥ 10,000 | ✅ Yes — preferred for refrigeration & chilled water |
| K-Flex ST (elastomeric foam) | ≥ 7,000 | ✅ Yes — suitable for most cold pipe applications |
| Armaflex AF (older grade) | ≥ 7,000 | ✅ Yes |
| Mineral wool (glass/rock) | 1–2 | ⚠️ Only with additional vapour barrier facing |
| Phenolic foam | ~100–200 | ⚠️ Limited — requires facing and careful jointing |
Critically, a vapour barrier is only as good as its weakest point. Every joint, cut, and penetration must be sealed with the correct adhesive or tape. A single unsealed joint can allow enough vapour ingress to waterlog the insulation over time.
Read: Vapour Barriers for Pipe Insulation — Complete Guide →Common Installation Failures
Most condensation problems on insulated pipework are not caused by wrong material selection — they are caused by installation errors that compromise the vapour barrier. The most common failures are:
Unsealed longitudinal joints
Slit tube insulation must be glued along the full length of the seam. Gaps allow vapour ingress at the highest-risk point.
Butt joints not glued
Every end-to-end joint must be fully coated with adhesive on both faces before closing. Dry joints are a common source of failure.
Wrong adhesive
Using general-purpose contact adhesive instead of Armaflex 520 or K-Flex adhesive degrades the vapour barrier at every joint.
Mechanical damage after installation
Cuts, abrasions, and cable ties pulled too tight can breach the outer skin of elastomeric foam, creating vapour pathways.
Open-cell insulation on cold pipes
Using mineral wool or open-cell foam without an additional vapour barrier facing on cold pipework will result in waterlogging.
Insufficient insulation thickness
Under-specified thickness means the outer surface temperature of the insulation falls below the dew point, causing surface condensation even with an intact vapour barrier.
Choose Your Guide
Nine technical guides covering every aspect of condensation control and vapour barrier specification.
Theory
Dew Point Explained for Pipe Insulation
What dew point is, how to calculate it, and why it is the critical design parameter for cold pipe insulation.
Read guide →Prevention
How to Prevent Condensation on Cold Pipes
A practical step-by-step guide to selecting the right insulation, specifying correct thickness, and ensuring vapour barrier continuity.
Read guide →Application
Condensation on Refrigerant Pipework
Specific guidance for refrigerant suction and liquid lines — the highest-risk condensation application in commercial buildings.
Read guide →Material Selection
Closed-Cell vs Open-Cell Pipe Insulation
Why closed-cell elastomeric foam is the correct choice for cold pipe applications and when open-cell materials are appropriate.
Read guide →Diagnosis
Why Pipe Insulation Gets Wet
How to diagnose whether wet insulation is caused by condensation, a plumbing leak, or installation failure — and what to do about it.
Read guide →Repair
How to Repair Failed Vapour Barriers
Step-by-step repair procedures for damaged or failed vapour barriers on cold pipework, including when to strip and replace.
Read guide →Specification
Vapour Barriers for Pipe Insulation — Complete Guide
The definitive reference for vapour barrier specification — μ values, material comparison, jointing, and maintenance.
Read guide →Application
Chilled Water Pipe Insulation Guide
Insulation specification for chilled water flow and return pipework in HVAC and commercial cooling systems.
Read guide →Application
Cold Water Pipe Insulation Guide
Insulation requirements for cold water supply pipework — condensation prevention, frost protection, and compliance.
Read guide →Real-World Applications
Condensation control requirements vary by system type. Jump to the application most relevant to your project.
❄️ Chilled Water Systems
Flow temperatures typically 6–12°C. High condensation risk in humid plant rooms. Closed-cell elastomeric foam with μ ≥ 7,000 required. Thickness to BS 5422 Table 3.
Chilled Water Guide →🧊 Cold Water Supply
Mains cold water at 10–15°C. Condensation risk in warm, humid environments such as commercial kitchens and plant rooms. Closed-cell foam or foam with vapour barrier facing.
Cold Water Guide →🔬 Refrigerant Pipework
Suction lines at −5°C to −30°C. Extreme condensation risk. Armaflex EVO (μ ≥ 10,000) is the preferred specification. All joints must be fully glued with Armaflex 520 adhesive.
Refrigerant Pipework Guide →🌡️ Heat Pump Pipework
Source-side pipework on air source heat pumps operates below ambient in heating mode. Outdoor exposure adds UV and weather resistance requirements alongside vapour control.
Heat Pump Hub →🏭 HVAC & AHU Pipework
Cooling coil connections and chilled water distribution in air handling units. Confined spaces and complex geometry require flexible closed-cell insulation with meticulous jointing.
Prevention Guide →🏗️ Commercial Plant Rooms
High ambient temperatures and humidity in plant rooms create severe condensation conditions. All cold pipework must be fully insulated with no gaps at supports, hangers, or valves.
Diagnosis Guide →Quick Specification Reference
Material selection guide by application. Thickness must be confirmed to BS 5422 for the specific pipe size and operating temperature.
| Application | Pipe Temp Range | Recommended Material | Min μ Value | Vapour Barrier Required? |
|---|---|---|---|---|
| Refrigerant suction lines | −30°C to −5°C | Armaflex EVO | ≥ 10,000 | ✅ Integral — all joints fully glued |
| Chilled water flow | 6°C to 12°C | Armaflex EVO / K-Flex ST | ≥ 7,000 | ✅ Integral — all joints fully glued |
| Cold water supply | 10°C to 15°C | Armaflex EVO / K-Flex ST | ≥ 7,000 | ✅ Recommended in humid environments |
| Heat pump source-side (outdoor) | −5°C to +10°C | Armaflex Tuffcoat / HT Solar | ≥ 7,000 | ✅ Integral + UV-resistant outer skin |
| HVAC cooling coil connections | 6°C to 15°C | Armaflex EVO / K-Flex ST | ≥ 7,000 | ✅ Integral — meticulous jointing essential |
Insulation thickness must be confirmed to BS 5422 for the specific pipe OD and operating temperature. See BS 5422 Resource Centre →
Recommended Products
The products most commonly specified for condensation control on cold pipework.
🏆
Armaflex EVO Pipe Insulation
μ ≥ 10,000. The premium closed-cell elastomeric foam for refrigeration and chilled water applications.
Shop Armaflex EVO⭐
K-Flex ST Pipe Insulation
μ ≥ 7,000. High-performance closed-cell foam suitable for chilled water and cold water applications.
Shop K-Flex ST🔗
Armaflex 520 Adhesive
The correct adhesive for sealing Armaflex joints and maintaining vapour barrier integrity. Do not substitute.
Shop Armaflex Adhesive🛡️
Vapour Barrier Tape
For sealing joints on foil-faced insulation and repairing minor vapour barrier damage on site.
Shop Insulation TapeFrequently Asked Questions
What is the difference between a vapour barrier and a vapour retarder?
The terms are often used interchangeably in the UK. Technically, a vapour barrier provides near-total resistance to vapour transmission (μ ≥ 10,000), while a vapour retarder slows but does not stop vapour movement. For cold pipe applications, a true vapour barrier material such as Armaflex EVO is always preferred.
Can I use mineral wool insulation on cold pipes?
Mineral wool has a μ value of 1–2, meaning it offers virtually no resistance to vapour transmission. It should only be used on cold pipes when combined with a separate, continuous vapour barrier facing — such as an all-service jacket or foil-scrim-kraft facing — with all joints sealed. In practice, closed-cell elastomeric foam is almost always the better choice for cold pipe applications.
How do I know if my insulation has a condensation problem?
Signs include visible surface condensation or dripping from insulated pipework, soft or waterlogged insulation when pressed, discolouration or mould on the outer surface, and corrosion on pipe supports or adjacent steelwork. If the insulation feels wet internally when cut open, the vapour barrier has failed and the insulation should be replaced.
Is Armaflex EVO better than K-Flex ST for vapour barrier performance?
Yes. Armaflex EVO has μ ≥ 10,000 compared to K-Flex ST's μ ≥ 7,000 — a 43% stronger vapour barrier. For demanding chilled water and refrigeration applications, Armaflex EVO's superior vapour barrier provides a meaningful margin of safety over the installation's 20+ year service life.
What adhesive should I use to bond elastomeric foam joints?
Use Armaflex 520 adhesive for Armaflex products and K-Flex adhesive for K-Flex products. Do not mix adhesives between brands and do not use general-purpose contact adhesives — these degrade the vapour barrier at every joint and will cause long-term failure.
How thick does pipe insulation need to be to prevent condensation?
Thickness depends on pipe OD, pipe operating temperature, ambient temperature, relative humidity, and insulation material. BS 5422 provides the reference tables for the UK. As a general guide, a 22mm OD chilled water pipe at 6°C in a 25°C/60% RH environment typically requires 25–32mm of elastomeric foam insulation.
Can I repair a failed vapour barrier without replacing all the insulation?
Minor surface damage can be repaired with Armaflex adhesive or vapour barrier tape if the insulation itself is still dry. If the insulation has absorbed moisture, it must be replaced — wet insulation cannot be dried out effectively in situ and will continue to cause problems.
Does BS 5422 cover condensation control?
Yes. BS 5422:2009+A1:2011 includes tables for minimum insulation thickness to prevent surface condensation on cold pipes. It specifies thickness based on pipe size, operating temperature, and ambient conditions. It is the primary reference standard for cold pipe insulation specification in the UK.
What is the dew point and why does it matter for pipe insulation?
The dew point is the temperature at which air becomes saturated and condensation begins to form. If the outer surface of your pipe insulation is below the dew point of the surrounding air, condensation will form on the surface. If the vapour barrier is compromised, condensation will form inside the insulation on the pipe surface. Correct insulation thickness keeps the outer surface above the dew point; a continuous vapour barrier prevents internal condensation.
Do pipe supports and hangers need to be insulated?
Yes. Pipe supports and hangers are a common source of cold bridging on chilled water and refrigerant pipework. Any uninsulated metal contact point between the cold pipe and the structure will cause localised condensation and corrosion. Insulated pipe supports or insert blocks should be used on all cold pipe applications.
Is condensation on cold pipes a compliance issue?
In commercial buildings, condensation on cold pipework can indicate non-compliance with BS 5422 insulation thickness requirements and may be flagged during building services commissioning or M&E inspections. In food production and pharmaceutical environments, condensation on pipework is a hygiene and regulatory issue that must be addressed immediately.
How long should correctly installed pipe insulation last on cold pipes?
Correctly specified and installed closed-cell elastomeric foam insulation on cold pipes should last 20–25 years or more. The most common cause of premature failure is vapour barrier compromise at joints, not material degradation. Annual visual inspection of joints, seams, and supports is recommended to catch early-stage failures before moisture ingress becomes extensive.