If you've spent any time browsing plastic pipe options, you've probably encountered two names: PEX and polybutylene (PB). Both are popular choices for domestic plumbing in the UK, both come in 15mm and 22mm sizes, and both work with push fit fittings — so what actually separates them? And more importantly, which one should you be reaching for?

In this guide we'll walk through the key differences, explain what the “barrier” part actually means (it's more important than it sounds), and help you pick the right pipe for the job — whether you're running a new hot and cold water system, replacing existing pipework, or tackling a central heating installation.

What Is PEX Barrier Pipe?

PEX stands for cross-linked polyethylene — polyethylene that's been treated to create molecular bonds between its polymer chains, giving the pipe greater strength, pressure resistance, and temperature tolerance than standard plastic pipe.

The barrier element refers to a thin oxygen-diffusion barrier layer built into the pipe wall — typically an EVOH (ethylene vinyl alcohol) layer sandwiched within the pipe structure. This barrier stops oxygen from permeating into the water inside, which matters enormously in closed-loop systems like central heating and underfloor heating. Without it, dissolved oxygen in the system water will gradually corrode metal components — pumps, heat exchangers, radiator valves — from the inside out.

In short: PEX barrier pipe is suitable for hot and cold water systems AND central heating. Standard plastic pipe without a barrier layer is not recommended for heating systems.

Our own-brand LavaTap PEX Barrier Pipe is WRAS approved for drinking water and rated for both hot and cold water service as well as central heating systems — so one pipe type covers pretty much everything you'll need in a domestic plumbing project.

What Is Polybutylene (PB) Barrier Pipe?

Polybutylene is a different polymer altogether — flexible thermoplastic that's been used in UK plumbing systems since the 1970s. It shares many of the same benefits as PEX: compatible with push fit fittings, suitable for hot and cold water, available in standard 15mm and 22mm sizes, and approved to BS 7291.

PB barrier pipe also includes that oxygen diffusion barrier, making it suitable for central heating applications too.

The main practical difference between PB and PEX is flexibility and pipe memory. PB is noticeably more flexible than PEX and lies flatter when uncoiled — it has less spring-back. If you're running long pipe runs through joists, ceilings, or tight spaces, that additional flexibility can make life significantly easier on site.

PEX, by comparison, is stiffer and retains more of its coiled shape when unrolled — which means more care is needed when routing it through awkward spaces, though it's by no means difficult to work with once you get a feel for it.

PEX vs PB: A Quick Side-by-Side

For most domestic plumbing jobs — first or second fix, hot and cold water, radiator connections — either pipe will do the job well. If you're running long unobstructed runs of pipe, the PEX's slight stiffness is a non-issue. If you're threading pipe through a house with lots of bends and obstacles, some installers prefer the PB for ease of routing.

15mm vs 22mm — Which Size Do I Need?

This is one of the most common questions in domestic plumbing, and the answer is fairly straightforward once you understand how a typical system is laid out.

15mm pipe is the standard size for branch connections — runs from a distribution point to individual outlets such as basins, taps, toilets, showers, and radiators. The vast majority of pipe used in a domestic installation is 15mm.

22mm pipe is used for main distribution runs — the primary pipework that carries water from the boiler or incoming mains supply to the distribution points throughout the property. Think of 22mm as the motorway and 15mm as the slip roads.

As a rough guide:

• Feeding an individual radiator or tap: 15mm
• Main flow and return from a boiler: 22mm
• Primary hot and cold feeds to a bathroom or kitchen: 22mm (sometimes 15mm on shorter runs)
• Branch to a single basin or toilet: 15mm

When in doubt, match the existing pipework size.

Is PEX Pipe Compatible with Push Fit Fittings?

Yes — and this is one of the biggest selling points of PEX (and PB) pipe over copper. Both materials are fully compatible with standard push fit fittings, including our LavaPlumb range, which covers couplers, elbows, tees, and stop ends in both 15mm and 22mm.

One important note: plastic pipe requires pipe inserts when used with push fit fittings. The insert (also called a liner or support sleeve) fits inside the pipe end before it's pushed into the fitting, reinforcing the pipe wall and ensuring a watertight seal. Without the insert, the fitting's grab ring can deform the pipe under pressure, leading to leaks.

Our kits include inserts, and they're available separately too:

• LavaPlumb 15mm Pipe Inserts — 100 Pack
• LavaPlumb 22mm Pipe Inserts — 50 Pack

Copper pipe does not require inserts — the rigid pipe wall handles the compression without support. So if you're connecting plastic pipe to copper fittings or mixing materials, always insert when the pipe going in is plastic.

Why Choose Plastic Pipe Over Copper?

Copper pipe has served domestic plumbing well for decades, but plastic has some genuine advantages that make it the go-to choice for many installers today:

No soldering required. Push fit connections are tool-free and don't need a blowtorch, flux, or solder — making installation faster and significantly more accessible for competent DIYers. A joint that would take a few minutes with copper and a blowtorch takes seconds with push fit.

Corrosion resistant. Plastic doesn't corrode, rust, or suffer from dezincification. In hard water areas particularly, plastic pipework simply doesn't degrade the way copper can over time.

Lighter and easier to handle. A 25m coil of 15mm PEX pipe is easy to carry and manoeuvre on site. Copper lengths are heavier, less flexible, and need cutting to size with more effort.

Quieter. Plastic pipe has much lower thermal conductivity than copper, which reduces expansion noise — the ticking and creaking you sometimes hear from copper pipework as it heats up and cools down.

Lower cost. Generally speaking, plastic pipe and push fit fittings come in at a lower overall cost than copper pipe with compression or soldered fittings — especially when you factor in labour time.

Our LavaTap PEX Barrier Pipe Range

Our PEX barrier pipe is manufactured to BS 7291 Class S and WRAS approved for use with drinking water. It's suitable for hot and cold water services and central heating systems, and it's fully compatible with standard 15mm and 22mm push fit fittings including our LavaPlumb range.

Available in multiple coil lengths so you can buy exactly what you need — no waste, no shortfall:

15mm PEX Barrier Pipe:

• 25m Coil — £22.99 — great for smaller jobs, repairs, or single-room installations
• 50m Coil — £44.99 — ideal for bathrooms, extensions, or multi-radiator runs
• 100m Coil — £59.99 — best value for full-house installations or trade use

22mm PEX Barrier Pipe:

• 25m Coil — £35.99 — covers most main distribution runs in a typical home
• 50m Coil — £59.99 — for larger properties or trade stock

We also stock polybutylene barrier pipe for those who prefer the extra flexibility:

• 15mm PB Barrier Pipe 25m — White, £25.99
• 15mm PB Barrier Pipe 25m — Grey, £25.99

Frequently Asked Questions

Can I mix PEX and copper pipe in the same system?

Yes — both 15mm and 22mm PEX pipe can be connected directly to copper pipe using standard push fit or compression fittings. This is useful for repairs, extensions, or hybrid installations where part of the system is already copper. When connecting to push fit, remember to use pipe inserts on the plastic side.

Does PEX pipe need to be clipped?

Yes — like copper pipe, PEX should be clipped at regular intervals to prevent sagging, particularly on horizontal runs. For 15mm pipe, clips should be spaced no more than 300mm apart on horizontal runs and 500mm on vertical ones. For 22mm, 450mm horizontal and 600mm vertical are typical guidelines. Always follow your fitting manufacturer's recommendations.

Can I use PEX pipe for underfloor heating?

PEX barrier pipe is commonly used for underfloor heating systems, though the pipe used in UFH manifold systems is typically a smaller diameter (usually 16mm or 20mm) than the standard 15mm and 22mm used for radiator and hot and cold water pipework. Check the specification of your underfloor heating system before ordering.

Is PEX pipe safe for drinking water?

Yes — our LavaTap PEX barrier pipe is WRAS approved, which means it has been independently tested and certified as safe for use with drinking water in the UK. Look for the WRAS mark on any plastic pipe intended for potable water use.

What's the maximum temperature PEX pipe can handle?

PEX barrier pipe is typically rated for continuous operation at temperatures up to 95°C, making it well suited to central heating systems where flow temperatures can be high. Always check the technical datasheet for the specific product you're using.

Do I need special tools to cut PEX pipe?

A standard plastic pipe cutter is all you need — no power tools required. Our LavaPlumb Plastic Pipe Cutter (6–32mm) cuts cleanly through PEX, PB, PVC, and PPR pipe and is compatible with all pipe sizes up to 32mm. A clean, square cut is essential for a leak-free push fit connection — a hacksaw tends to leave a rougher edge, so a dedicated pipe cutter is always worth the small investment.

Can I use push fit fittings on PB pipe?

Yes — polybutylene pipe is fully compatible with standard push fit fittings in 15mm and 22mm. As with PEX, always use pipe inserts to support the pipe wall inside the fitting.

Ready to Get Piping?

Whether you're a homeowner tackling a first fix, a plumber stocking up for a full installation, or somewhere in between, our PEX barrier pipe and push fit fittings have you covered.

Shop the range:

• 🔵 LavaTap 15mm PEX Barrier Pipe 25m — £22.99
• 🔵 LavaTap 15mm PEX Barrier Pipe 50m — £44.99
• 🔵 LavaTap 15mm PEX Barrier Pipe 100m — £59.99
• 🔵 LavaTap 22mm PEX Barrier Pipe 25m — £35.99
• 🔵 LavaTap 22mm PEX Barrier Pipe 50m — £59.99
• 🔵 LavaTap 15mm Polybutylene Barrier Pipe 25m (White) — £25.99
• 🔵 LavaTap 15mm Polybutylene Barrier Pipe 25m (Grey) — £25.99
• 🔧 LavaPlumb 15mm Push Fit Couplers 10 Pack — £12.99
• 🔧 LavaPlumb 22mm Push Fit Couplers 5 Pack — £9.99
• 🔧 LavaPlumb 15mm Push Fit Elbows 10 Pack — £12.99
• 🔧 LavaPlumb 22mm Push Fit Elbows 5 Pack — £9.99
• 🔧 LavaPlumb 15mm Push Fit Tees 10 Pack — £20.99
• 🔧 LavaPlumb 22mm Push Fit Tees 5 Pack — £13.99
• 🪛 LavaPlumb Plastic Pipe Cutter 6–32mm — £4.99

Got a question about which pipe or fittings are right for your project? Drop us a line at contact@lavawholesale.co.uk and we'll point you in the right direction.

All LavaTap PEX and polybutylene barrier pipe is WRAS approved and manufactured to BS 7291.