How to Cut and Install Pipe Insulation — Complete UK Guide

July 14, 2026

Introduction

Pipe insulation is only as good as its installation. The best-specified Armaflex EVO on a chilled water system will fail within months if the seams are not bonded, the joints are not adhered, or the fittings are left uninsulated.

This guide covers the correct techniques for cutting and installing the three main types of pipe insulation used in the UK — elastomeric foam (Armaflex, K-Flex), polyethylene foam (Climaflex, Tubolit), and mineral wool (Rocklap). It covers straight runs, bends, tees, valves, flanges, and the common mistakes that cause premature failure.

Tools and Materials

For elastomeric foam (Armaflex EVO, K-Flex ST):

Sharp insulation knife (a blunt knife compresses the foam and creates uneven cuts)
Armaflex 520 adhesive (or K-Flex adhesive for K-Flex products)
Tape measure and marker pen
Clean, dry cloth (for wiping pipe surface before installation)
Straight edge or mitre guide (for accurate cuts)

For polyethylene foam (Climaflex, Tubolit):

Sharp knife or scissors
Tape measure
Insulation tape (for securing joints on heating applications)

For mineral wool (Rocklap):

Sharp serrated knife or hacksaw
Tape measure and marker pen
Aluminium foil tape (for sealing joints)
Stainless steel banding (for securing cladding where required)

Before You Start

Check the pipe is clean and dry. Any moisture, grease, or debris on the pipe surface will prevent adhesive from bonding correctly and can cause corrosion under the insulation. Wipe the pipe with a clean, dry cloth before starting.

Check the pipe temperature. Armaflex 520 adhesive requires a minimum application temperature of +10°C. In cold conditions, warm the adhesive and the pipe surface before applying.

Select the correct bore size. The insulation bore must match the pipe outside diameter (OD), not the nominal bore. A 15mm nominal bore copper pipe has an 18mm OD — use insulation labelled as “15mm bore” which is sized for 15mm nominal bore copper pipe.

Plan the installation. Walk the pipe run before starting and identify all bends, tees, valves, and fittings. Plan how you will insulate each one before cutting any material.

Installing Elastomeric Foam — Straight Runs

Step 1 — Measure and cut to length. Measure the pipe run and cut the insulation to length with a sharp knife. Cut square ends for straight runs — a square cut ensures a tight butt joint with no gaps.

Step 2 — Apply adhesive to the longitudinal seam. Open the pre-slit seam and apply Armaflex 520 adhesive to both faces of the seam. Apply a thin, even coat. Allow to become tacky: approximately 3–5 minutes at 20°C, longer in cold or humid conditions. The adhesive is ready when it feels tacky to the touch but does not transfer to your finger.

Step 3 — Close the seam. Starting at one end, press the two faces of the seam firmly together along the full length. Work progressively along the seam. Press firmly with your fingers to ensure full contact.

Step 4 — Apply adhesive to butt joints. Apply adhesive to both end faces of adjacent lengths. Allow to become tacky. Press firmly together.

Step 5 — Check the installation. Run your finger along every seam and joint. Any area that springs open has not bonded — apply more adhesive and re-press.

For self-seal grades (Armaflex EVO Self-Seal): peel back the release paper and press the adhesive strip firmly along its full length. The full length must be bonded — not just the ends.

Installing Elastomeric Foam — Bends

Method 1: Mitre cuts (most common)

For 90° bends, cut two 45° mitres and join them together:

Cut the first section at 45° — place the insulation on a flat surface and cut diagonally across the end
Cut the second section at 45° in the opposite direction
Apply adhesive to both mitre faces
Allow to become tacky
Press firmly together, ensuring the two sections align correctly around the bend

For bends other than 90°, adjust the mitre angle accordingly. A 45° bend requires two 22.5° mitres.

Method 2: Pre-formed bends

Pre-formed 90° bends are available for standard pipe sizes. They eliminate the mitre joint and are preferable for critical applications (chilled water, refrigeration) where the vapour barrier must be maintained at the bend. Apply adhesive to the butt joints where the pre-formed bend meets the straight sections.

Method 3: Segmented bends (large diameter)

For large-diameter pipes where pre-formed bends are not available, cut multiple segments and join them together to form the bend. Each segment joint must be fully bonded.

Installing Elastomeric Foam — Tees

Tee junctions are the most challenging to insulate correctly. The branch insulation must be cut and fitted to the main pipe insulation without gaps.

Insulate the main pipe run first — leaving a gap at the tee junction
Cut the hole in the main pipe insulation — using a sharp knife, cut a hole the same diameter as the bore of the branch insulation
Cut the branch insulation — cut the end to fit snugly against the main pipe insulation, following the curve of the main insulation
Bond all joints — apply adhesive to all contact surfaces, allow to become tacky, and press firmly together
Check for gaps — inspect the tee junction carefully and apply additional adhesive to seal any gaps

Installing Elastomeric Foam — Valves and Flanges

Valves and flanges must be insulated to maintain the vapour barrier across the full pipe run. Leaving them uninsulated is one of the most common installation failures.

Option 1: Fabricated insulation sections
Cut sections of elastomeric foam sheet or tube to fit around the valve body. Bond all joints with adhesive. This requires skill and patience to achieve a neat, gap-free result.

Option 2: Purpose-made valve boxes
Purpose-made insulation valve boxes are available for standard valve types (gate valves, ball valves, butterfly valves). They provide a neat, professional finish and are quicker to install than fabricated sections. They are also removable for valve maintenance.

For flanges: cut a disc of elastomeric foam sheet to fit over each flange face. Bond to the adjacent pipe insulation. Cover the flange with a section of elastomeric foam tube or sheet, bonding all joints.

Installing Elastomeric Foam — Pipe Supports

Standard pipe clamps compress the insulation and create cold bridges — particularly problematic on cold pipe applications.

For cold pipes (chilled water, refrigeration, cold water): use insulated pipe supports with a thermal break. These incorporate a section of high-density insulation between the pipe and the support structure, maintaining the vapour barrier and preventing cold bridging.

For heating pipes: standard pipe clamps can be used, but position them to minimise compression of the insulation. Oversized clamps (sized for the insulated pipe OD) are preferable.

Installing Polyethylene Foam — Straight Runs

Polyethylene foam is simpler to install than elastomeric foam — no adhesive is required for heating applications.

Cut to length with a sharp knife or scissors
Open the pre-slit seam and slide onto the pipe
Push the seam closed — for heating pipes, no adhesive is required
Secure with insulation tape at joints and at regular intervals if required

For bends: cut 45° mitres and push together. Secure with insulation tape.

Note: polyethylene foam is not suitable for cold water pipes where condensation control is required — use elastomeric foam instead.

Installing Mineral Wool — Straight Runs

Select the correct pipe section (sized by pipe OD)
Open the hinged section and close around the pipe
Secure the longitudinal joint with the self-adhesive lap — press firmly along the full length
Secure butt joints with aluminium foil tape, overlapping both sections by at least 50mm
Press the tape firmly to ensure full adhesion

For bends: cut mitre sections and join with aluminium foil tape. Ensure all joints are fully sealed.

For fittings: fabricate sections from mineral wool sheet or use purpose-made fitting covers. Seal all joints with aluminium foil tape.

Common Installation Mistakes

Not allowing adhesive to become tacky
Armaflex 520 adhesive must be allowed to become tacky before the joint is closed. Closing the joint too soon results in a weak bond that will fail. Wait until the adhesive feels tacky to the touch but does not transfer to your finger.

Applying adhesive to only one face
Both faces of every joint must be coated with adhesive. Applying to one face only results in a weak bond.

Leaving fittings and valves uninsulated
Every fitting, valve, and flange must be insulated. Leaving them bare creates thermal bridges and vapour barrier failures on cold pipe applications.

Using a blunt knife
A blunt knife compresses the foam and creates uneven cuts that are difficult to bond. Use a sharp knife and replace the blade regularly.

Not checking the installation before commissioning
Walk the entire pipe run and check every seam, joint, and fitting before the system is commissioned. Once the system is running, any failure will be hidden inside the insulation.

Mixing adhesives between brands
Use Armaflex 520 for Armaflex products and K-Flex adhesive for K-Flex products. Do not mix adhesives between brands.

Applying insulation to a wet or cold pipe
The pipe must be clean, dry, and above +10°C before applying adhesive. Moisture on the pipe surface will prevent adhesive from bonding and can cause corrosion under the insulation.

Frequently Asked Questions

How do you cut pipe insulation for a bend?

For a 90° bend, cut two 45° mitres and join them together with adhesive. Place the insulation on a flat surface and cut diagonally across the end at 45°. Apply adhesive to both mitre faces, allow to become tacky, and press firmly together around the bend.

Do you need adhesive for pipe insulation?

For elastomeric foam (Armaflex, K-Flex) on cold pipe applications (chilled water, refrigeration, cold water), adhesive is essential — the vapour barrier must be continuous and unbroken. For polyethylene foam on heating applications, adhesive is not required but insulation tape can be used to secure joints.

How do you insulate a pipe valve?

Valves can be insulated using fabricated sections of elastomeric foam sheet or tube, bonded with adhesive. Purpose-made removable insulation valve boxes are also available for standard valve types and provide a neater, more professional finish.

What is the best knife for cutting pipe insulation?

A sharp, long-bladed knife is best for cutting elastomeric foam. A blunt knife compresses the foam and creates uneven cuts. Replace the blade regularly. For polyethylene foam, scissors or a sharp kitchen knife work well. For mineral wool, a serrated knife or hacksaw is required.

How do you insulate a pipe tee?

Insulate the main pipe run first, leaving a gap at the tee. Cut a hole in the main pipe insulation to accept the branch insulation. Cut the end of the branch insulation to fit snugly against the main pipe insulation. Bond all joints with adhesive and check carefully for gaps.

How long does Armaflex adhesive take to dry?

Armaflex 520 adhesive should be allowed to become tacky before closing the joint — approximately 3–5 minutes at 20°C. In cold or humid conditions, allow longer. Full cure takes approximately 24 hours.