35mm Pipe Insulation Guide: Sizes, Materials & Wall Thickness

35mm Pipe Insulation — Complete UK Guide

35mm pipe insulation is specified for 35mm copper pipe, which has an outside diameter (OD) of 35mm. It is a standard commercial heating pipe size, widely used in larger domestic heating systems, light commercial heating mains and primary circuits. This guide covers material selection, wall thickness, BS 5422 compliance and installation for 35mm pipework.

Shop 35mm pipe insulation: Browse the full 35mm collection

Is 35mm Copper Pipe OD or Nominal?

In the UK, copper pipe is measured by its outside diameter (OD). 35mm copper pipe has an OD of exactly 35mm. This means you need pipe insulation with a 35mm bore — the internal diameter of the insulation must match the OD of the pipe.

This is different from steel pipe, which is measured by nominal bore (NB) — a historical designation that does not correspond to the actual pipe OD. If you are insulating steel pipework, check the actual OD before ordering. Read our guide: Copper Pipe Size vs Insulation Size

What is 35mm Copper Pipe Used For?

35mm copper pipe is used in:

• Larger domestic heating systems — primary flow and return circuits in properties with multiple zones or high heat demand
• Light commercial heating mains — LTHW primary circuits in small commercial buildings
• Hot water primary circuits — connections between boilers, cylinders and distribution pipework
• Underfloor heating manifold connections

35mm Pipe Insulation Materials

Armaflex EVO — Elastomeric Foam

Armaflex EVO is the most widely specified pipe insulation for 35mm heating and hot water pipework. It is a closed-cell nitrile rubber foam with a built-in vapour barrier, Class O fire rating and Microban antimicrobial protection.

• Thermal conductivity: λ ≤ 0.033 W/mK at 0°C
• Operating temperature: -50°C to +105°C
• Fire rating: Class O (BS 476), B-s2,d0 (EN 13501)
• Available in standard (open slit) and self-seal versions
• Available in 9mm, 13mm, 19mm, 25mm and 32mm wall thickness for 35mm bore

Best for: Domestic and light commercial heating, hot water pipework, condensation control

Kingspan Kooltherm — Phenolic Foam

Kingspan Kooltherm is the highest-performing pipe insulation for 35mm commercial heating pipework. With λ = 0.025 W/mK, it achieves BS 5422 compliance at a thinner wall than elastomeric foam, saving space in congested plant rooms and risers.

• Thermal conductivity: λ = 0.025 W/mK (aged value)
• Operating temperature: -180°C to +110°C
• Fire rating: Class O (BS 476), B-s1,d0 (EN 13501)
• Rigid, foil-faced sections
• Available in 20mm, 25mm, 30mm, 40mm and 50mm wall thickness for 35mm bore

Best for: Commercial heating BS 5422 compliance, space-constrained plant rooms, district heating

Rockwool Rocklap — Mineral Wool

Rockwool Rocklap is specified for high-temperature heating pipework and fire-rated applications on 35mm pipework.

• Fire rating: A1 non-combustible (Euroclass A1)
• Operating temperature: up to 750°C continuous
• Available in 25mm, 30mm, 40mm and 50mm wall thickness for 35mm bore

Best for: High-temperature heating mains, steam pipework, fire-rated applications

Wall Thickness Selection for 35mm Pipe Insulation

The correct wall thickness for 35mm pipe insulation depends on the application:

BS 5422 Requirements for 35mm Pipe Insulation

Under BS 5422, the minimum insulation thickness for 35mm OD pipework (33.7mm OD band) on a standard LTHW heating system is:

• Kingspan Kooltherm (λ = 0.025) — 25mm wall
• Armaflex EVO (λ = 0.033) — 32mm wall
• Rockwool Rocklap (λ = 0.034) — 30–40mm wall

Phenolic insulation achieves compliance at a thinner wall because of its lower lambda value. Read our full guide: BS 5422 Pipe Insulation Standards Guide

How to Fit 35mm Pipe Insulation

Armaflex EVO (Standard)

1. Cut the insulation to length with a sharp knife — cut square for butt joints
2. Open the slit and fit around the 35mm pipe
3. Apply Armaflex 520 adhesive to both faces of the longitudinal seam and all butt joints
4. Allow to become touch-dry, then press seams firmly together
5. For bends, cut mitres at 45° and glue all faces

Armaflex EVO (Self Seal)

1. Cut to length and fit around the pipe
2. Peel the backing strip and press the self-adhesive closure firmly together
3. Apply adhesive to all butt joints

Read our full installation guide: Armaflex Installation Guide

Shop 35mm Pipe Insulation by Wall Thickness

• 9mm wall — basic domestic frost protection
• 13mm wall — standard domestic heating
• 19mm wall — commercial HVAC and unheated spaces
• 25mm wall — BS 5422 commercial heating (Kooltherm) / condensation control
• 30mm wall — high temperature and industrial
• 40mm wall — steam and process pipework

Related Pipe Size Guides

• 28mm Pipe Insulation Guide
• 22mm Pipe Insulation Guide
• 15mm Pipe Insulation Guide
• Pipe Insulation by Pipe Size — Complete Sizing Guide
• How to Measure Pipe Insulation Size
• Pipe Insulation Wall Thickness Guide
• BS 5422 Pipe Insulation Standards Guide

Frequently Asked Questions

What size insulation do I need for 35mm copper pipe?

You need pipe insulation with a 35mm bore — the internal diameter of the insulation must match the outside diameter of the pipe. 35mm copper pipe has an OD of 35mm, so 35mm bore insulation is the correct fit.

What wall thickness do I need for 35mm pipe insulation?

For domestic heating in a heated space, 13mm wall is the standard minimum. For commercial heating BS 5422 compliance, 25mm Kooltherm or 32mm Armaflex is typically required. For high-temperature applications, 30mm or 40mm mineral wool is specified.

Can I use Armaflex on 35mm heating pipes?

Yes. Armaflex EVO is suitable for 35mm heating and hot water pipework up to +105°C. For commercial BS 5422 compliance, 32mm wall Armaflex or 25mm wall Kooltherm is typically required for 35mm OD pipework.

Is 35mm pipe insulation the same as 35mm bore insulation?

Yes — when we refer to 35mm pipe insulation, we mean insulation with a 35mm bore (internal diameter) designed to fit 35mm OD copper pipe.