What Thickness Pipe Insulation Do I Need? — Complete UK Guide

Introduction

Choosing the wrong thickness of pipe insulation is one of the most common and costly mistakes in building services. Too thin and you lose heat, get condensation, or fail a building inspection. Too thick and you waste money and space.

The correct thickness depends on four things: what the pipe is carrying, what temperature it operates at, what the surrounding conditions are, and what insulation material you are using.

This guide walks you through exactly how to determine the right thickness for every common application — from domestic hot water pipes to chilled water systems and heat pump refrigerant lines.

Why Thickness Matters

Pipe insulation works by creating a layer of low-conductivity material between the pipe surface and the surrounding air. The thicker the insulation, the greater the thermal resistance — and the less heat is lost (or gained) through the pipe wall.

But thickness is not the only variable. The thermal conductivity (λ value) of the insulation material is equally important. A 25mm layer of phenolic foam (λ ≈ 0.022 W/m·K) provides significantly better thermal performance than 25mm of polyethylene foam (λ ≈ 0.040 W/m·K).

This is why BS5422 — the British Standard that governs pipe insulation thickness in the UK — specifies minimum thicknesses based on both the pipe size and the λ value of the insulation material, not just a single fixed thickness.

The Two Key Standards

BS5422:2009 (Amended 2014)
The primary standard for pipe insulation thickness in UK buildings. Sets minimum thicknesses for heating, hot water, chilled water, and cold water pipework based on pipe OD and insulation λ value. Referenced by Part L of the Building Regulations — compliance is a legal requirement for notifiable building work. See: BS5422 Explained — The Complete Guide.

Heat pump and refrigeration manufacturer specifications
For refrigerant lines on heat pumps and air conditioning systems, the equipment manufacturer specifies minimum insulation thicknesses in the installation manual. These must be followed as a minimum, in addition to BS5422 where applicable.

Step-by-Step: How to Find the Right Thickness

Step 1 — Identify your application

Step 2 — Measure the pipe outside diameter (OD)

BS5422 uses pipe outside diameter (OD), not nominal bore or internal diameter. Common copper pipe ODs:

• 15mm nominal bore = 18mm OD
• 22mm nominal bore = 22mm OD
• 28mm nominal bore = 28mm OD
• 35mm nominal bore = 35mm OD
• 42mm nominal bore = 42mm OD
• 54mm nominal bore = 54mm OD

Step 3 — Identify the λ value of your insulation

Step 4 — Look up the BS5422 table for your application

Cross-reference your pipe OD and λ value in the relevant BS5422 table to find the minimum wall thickness.

Step 5 — Select the nearest available thickness

Insulation is available in standard wall thicknesses: 9mm, 13mm, 19mm, 25mm, 32mm, 38mm, 50mm. Select the nearest standard thickness that meets or exceeds the BS5422 minimum.

Indicative Thicknesses by Application

These are indicative values based on published BS5422 data. Always verify against the current standard and your insulation manufacturer's data.

Domestic Hot Water (DHW) — 60°C, Ambient 20°C

Space Heating — LTHW 75°C, Ambient 20°C

Chilled Water — 10°C Flow, Ambient 25°C / 60% RH

Note: Polyethylene foam is not suitable for chilled water — it is open-cell and provides no vapour barrier.

Cold Mains Water — 10°C, Ambient 20°C / 60% RH

Note: For cold mains water in warm or humid environments (lofts, plant rooms, kitchens), use closed-cell elastomeric foam — not polyethylene foam. See: Cold Water Pipe Insulation — Complete Guide.

Does Material Choice Affect Thickness?

Yes — significantly. The lower the λ value of the insulation, the thinner the section needed to achieve the same thermal performance.

Example: 54mm OD (50mm nominal) heating pipe (LTHW 75°C)

• Kingspan Kooltherm (λ 0.022): approximately 19mm required. See: Phenolic Foam Pipe Insulation — Complete Guide
• Armaflex EVO (λ 0.033): approximately 32mm required
• Tubolit (λ 0.040): approximately 38mm required

Phenolic foam (Kooltherm) achieves compliance at nearly half the thickness of polyethylene foam. This matters where space around pipework is limited.

Common Thickness Mistakes

Using nominal bore instead of OD
BS5422 tables use pipe outside diameter. A 15mm nominal bore copper pipe has an 18mm OD — not 15mm.

Using the wrong λ value
The λ value must be at the mean service temperature, not at 0°C or 10°C.

Under-insulating chilled water
Chilled water condensation control thicknesses are often greater than heating thicknesses for the same pipe size. See: Chilled Water Pipe Insulation — Complete Guide.

Ignoring fittings and valves
BS5422 applies to the full pipe run including bends, tees, valves, and flanges. See: How to Cut and Install Pipe Insulation.

Using polyethylene foam on cold pipes
Polyethylene foam is open-cell and provides no vapour barrier. Not suitable for chilled water, cold mains water in warm environments, or refrigerant lines. See: Vapour Barriers for Pipe Insulation.

Frequently Asked Questions

What is the minimum pipe insulation thickness required by Building Regulations?

Part L of the Building Regulations requires pipe insulation to meet BS5422 minimum thicknesses. For domestic hot water on a 22mm OD pipe using standard elastomeric foam, the minimum is typically 25mm.

Is 13mm pipe insulation enough?

For domestic cold water pipes in a heated building (ambient 20°C, 60% RH), 13mm elastomeric foam is generally sufficient for condensation control. For heating or hot water pipework, 13mm is not sufficient — BS5422 requires 25mm or more for most pipe sizes.

Is 25mm pipe insulation enough for heating pipework?

For most domestic heating pipework up to 42mm OD using elastomeric foam or polyethylene foam, 25mm meets BS5422 requirements. For larger pipes or higher flow temperatures, 32mm or 38mm may be required. See: Pipe Insulation for Domestic Heating.

What thickness insulation do I need for a 22mm copper pipe?

For a 22mm OD copper pipe carrying domestic hot water (60°C), BS5422 requires 25mm wall thickness using standard elastomeric foam. For chilled water at 10°C, 19mm is typically sufficient for condensation control at 25°C / 60% RH ambient.

What thickness insulation do I need for a 15mm copper pipe?

A 15mm nominal bore copper pipe has an outside diameter of 18mm. For domestic hot water (60°C), BS5422 requires 25mm wall thickness. For cold water condensation control, 13mm is typically sufficient.

What thickness pipe insulation for a heat pump?

For heat pump refrigerant lines, follow the manufacturer's installation manual — typically 19–25mm on the suction line and 13–19mm on the discharge line using elastomeric foam. For heating flow/return pipework, follow BS5422 — typically 25mm for pipes up to 35mm OD.

Related Guides

• BS5422 Explained — The Complete Guide
• Armaflex vs K-Flex — Which Should You Choose?
• How to Prevent Condensation on Copper Pipes
• Pipe Insulation for Heat Pumps
• Mineral Wool Pipe Insulation — Complete Guide
• Phenolic Foam Pipe Insulation — Complete Guide (Kooltherm)
• Pipe Insulation for Domestic Heating
• Pipe Insulation for Cold Water Systems
• How to Cut and Install Pipe Insulation
• Foil-Faced vs Elastomeric Foam — Which Should You Choose?
• Pipe Insulation Glossary