PEX vs multilayer pipe - differences and usage
PEX vs multilayer pipe - why this choice is one of the most important decisions when it comes to heating and water distribution
When someone asks what pipe to use for underfloor heating, radiator circuits or water distribution, they very quickly come across two competing technologies - pure PEX pipe (cross-linked polyethylene) and multilayer pipe with an aluminum insert (often referred to as PEX-AL-PEX or PERT-AL-PEX). Both belong to the group of plastic pipe systems, both can be purchased in coils and in rods, and both are connected using compression or threaded fittings - and precisely because of this, they are often confused or chosen "at a glance", without the designer or installer being aware of the differences that will become apparent after a few years in the form of leaks, cracking plaster above the distribution or expansion problems.
In this article, we will examine what both types of pipe are made of, what physical properties they have, how their behavior differs in the event of temperature changes, where pure PEX can be used and where, on the contrary, multilayer pipe is clearly the better choice, and finally also what to pay attention to during installation and storage. If you are also dealing with the selection of pipe diameter, we recommend supplementing your knowledge with the articles What pipe diameter do I need according to the type of distribution and What pressure and temperature can PEX pipe withstand (PN6, PN10).
What is PEX pipe - material, production and types of crosslinking
PEX is an abbreviation for "cross-linked polyethylene". Ordinary polyethylene (for example, that from which plastic shopping bags are made) has a linear molecular structure - polymer chains lie next to each other but are not firmly connected to each other. When heated, such a material softens and loses strength. The crosslinking process (crosslinking) creates transverse chemical bonds between individual chains, resulting in a three-dimensional network. The result is a material that retains its mechanical strength, resistance to flow under pressure (so-called creep) and long-term durability of about 50 years or more even at high temperatures (60-95 °C).
According to the crosslinking method, we distinguish three basic types of PEX pipe:
- PEX-a - crosslinking by the peroxide method (Engel process) takes place already in the melted state of the material before extrusion. The highest degree of crosslinking is achieved (over 85 %), the pipe has the best flexibility, the highest resistance to cracking under stress and so-called "shape memory" - after mild heating it can partially return to its original shape, which is an advantage for example when repairing a leak.
- PEX-b - crosslinking by the silane method takes place only after extrusion, by the action of moisture and a catalyst. It is cheaper to produce, the pipe is slightly harder and less flexible, the degree of crosslinking is usually 65-70 %.
- PEX-c - crosslinking by electron beam irradiation after extrusion. The resulting quality is between PEX-a and PEX-b, the pipe is somewhat more brittle at low temperatures.
In the range at atria.sk you will find for example PEX Pipe - 20x2; PN6-T=+90° C, PN10-T=+60° C - 200m or a smaller size PEX Pipe - 17x2; PN6-T=+90° C, PN10-T=+60° C - 120m, both made by the PEX-a technology, which is considered today in professional practice as a quality standard for underfloor heating and water distribution.
Oxygen barrier - why it is essential for PEX pipe
Pure polyethylene is partially permeable to oxygen - O2 molecules can diffuse through the pipe wall into the heating water. In a closed heating circuit with metal components (circulation pumps, steel radiators, heat exchangers), this causes corrosion and the formation of sludge. Therefore, an external barrier layer of EVOH (ethylene-vinyl alcohol) is applied to PEX pipes intended for heating, which reduces oxygen diffusion to a value specified by the DIN 4726 standard (max. 0.32 mg/l per day at 40 °C). This layer is extremely thin on PEX pipes (in the order of tenths of a millimeter) and must be part of the wall structure - without it, it is a pipe suitable only for potable water distribution, not for a closed heating circuit.
What is a multilayer pipe - construction with a welded aluminum insert
Multilayer pipe (in practice it is also called "layered", "composite" or by the abbreviation PEX-AL-PEX or PERT-AL-PEX) solves the problem of oxygen diffusion and thermal expansion completely differently - by a physical aluminum barrier embedded directly in the wall of the pipe. The wall structure typically has five layers:
- inner layer - cross-linked polyethylene (PEX) or temperature-resistant PE-RT (polyethylene with increased temperature resistance),
- adhesive (adhesive) layer,
- middle layer - longitudinally welded aluminum foil (typically 0.2-0.4 mm),
- second adhesive layer,
- outer protective layer - again PEX or PE-RT.
The aluminum layer serves three functions at the same time: it is a 100% oxygen barrier (no additional diffusion through EVOH is necessary), significantly reduces the linear thermal expansion of the pipe and at the same time gives the pipe the ability to "hold the shape" after bending - similarly to when bending copper or aluminum pipe by hand, the multilayer pipe will remain in the shape into which it is formed, without the need to fix it with anything.
Graphic comparison of the wall structure of both types of pipe
At atria.sk you will mainly encounter high-quality PEX-a pipe in this category, which is widely used for underfloor heating thanks to the barrier layer and high flexibility - for example PEX Pipe - 17x2; PN6-T=+90° C, PN10-T=+60° C - 240m or a larger coil PEX Pipe - 17x2; PN6-T=+90° C, PN10-T=+60° C - 600m for larger areas, where it is more cost-effective to buy fewer larger coils instead of several small ones.
Physical properties - where the real differences are
Thermal linear expansion
This is probably the most important practical difference between the two materials. Pure PEX has a linear expansion coefficient of approximately 0.15-0.20 mm per meter of length per 1 °C temperature change. In the case of multilayer pipe with an aluminum insert, this value is significantly lower due to the metal inside the wall - typically 0.025-0.03 mm/m·K, i.e. approximately 5-7 times less.
In practice, this means: a ten-meter section of pure PEX pipe will extend by approximately 75-100 mm when heated from 20 °C to 70 °C (i.e. by 50 °C). An equally long section of multilayer pipe will extend by only 12-15 mm. When the distribution is laid in grooves in the masonry, where the pipe is firmly covered with mortar, or when risers are anchored at several points, this is a crucial difference - the higher expansion of PEX requires a more thorough solution to compensation (expansion loops, compensators, free placement in a protective sleeve) and neglecting this detail will result in cracking of the plaster, pushing the pipe out of the groove or stressing the fittings.
Graphic comparison of thermal expansion
Flexibility, shape memory and installation convenience
Clean PEX pipe is more flexible and after bending it tends to partially return to its original straight shape - therefore, when laying floor loops, it is fixed with clamps, system strips or clips into the system board, otherwise it would "pull out" from the intended loop shape. This property is, however, also an advantage when overcoming tight radii and when working in larger areas, where the pipe is unwound directly from the reel and laid in a meander or spiral.
Multi-layer pipe, on the other hand, behaves like soft metal - bend it with your hands (for smaller diameters) or with a bender, and it remains precisely in the given shape without the need for further fixation. This is a big advantage when running distribution lines in an apartment under plaster, around corners, through partitions - the installer shapes the route exactly according to the need and does not have to deal with "springing back" of the material. The disadvantage is that repeated bending at the same place (for example, when correcting a shaping error) can lead to a kink in the aluminum layer, which will be visible as a white break in the wall and that section will have to be cut out - with clean PEX this risk practically does not exist.
Table - clear comparison of properties
| Property | Clean PEX pipe | Multi-layer (PEX-AL-PEX) |
|---|---|---|
| Oxygen barrier | thin EVOH layer | intact aluminum foil |
| Thermal expansion | high (0.15-0.2 mm/m·K) | low (0.025-0.03 mm/m·K) |
| Shape memory | yes, tries to straighten | holds shape after bending |
| Risk of kinking | practically zero | real when bending repeatedly |
| Delivery | reels and rods | reels and rods |
| Typical pressure/temperature | PN6/90°C, PN10/60°C | PN10/95°C commonly |
| Price per meter | usually lower | usually higher |
| UV/frost resistance during storage | good, but limited | slightly worse with repeated freezing |
A detailed list of maximum pressures and temperatures for various combinations can be found in a separate article "What pressure and temperature can PEX pipe withstand (PN6, PN10)" - the values PN6/T90°C and PN10/T60°C mentioned directly in product names, for example in PEX Pipe - 25x2.3; PN6-T=+90° C, PN10-T=+60° C - 200m, mean that at a lower pressure of 6 bars the pipe can withstand a temperature of up to 90 °C, while at a higher pressure of 10 bars the maximum continuous temperature is 60 °C - this is a common way manufacturers of PEX indicate the combined pressure-temperature resistance of the material.
Where to use clean PEX pipe and where multi-layer - practical recommendations
Floor heating
For floor heating, clean PEX pipe is the clear choice in the vast majority of cases, most often in the size 16x2 or 17x2 mm. The reason is simple - the pipe is laid into a system board with protrusions or fixed with clips directly onto the reinforcement mesh, so the shape memory of the material does not interfere here, on the contrary, its flexibility allows easy routing in tight meanders and spirals without the risk of kinking. At the same time, it is cast into a concrete screed, so the high thermal expansion is practically not noticeable in practice - the pipe is "trapped" in the mass of the screed and has nowhere to move in terms of expansion, its expansion is reflected more in microscopic stresses in the concrete, which are considered when planning expansion joints in the floor. For a typical family house, about 80-120 m of pipe is used for one floor heating circuit, so it is worth buying larger reels such as 240 m or 600 m variants, where the price per meter is more favorable and at the same time the number of joints in the floor is minimized (ideally, one loop should be run without any joints in the screed).
Water and heating distribution in walls, risers, radiator connections
On the other hand, multi-layer pipe is preferred in the majority of professional implementations, mainly due to its low thermal expansion and ability to hold shape. When routing distribution in a groove in masonry or in a gypsum board partition, the installer bends the pipe precisely along the route, fixes it with clamps at defined intervals (usually 50-80 cm for horizontal routing) and does not have to deal with large expansion loops, which would otherwise take up space. For long risers in multi-storey buildings, low expansion is key - a long vertical section of clean PEX would extend so much during heating system startup that it would be necessary to install compensators on each floor, which is not needed with multi-layer pipe or is significantly reduced.
Connections to distributors and short sections
For short, straight sections (for example, connecting a boiler to a heating distributor at a distance of 1-2 meters), clean PEX pipe of a larger diameter, for example PEX Pipe - 20x2, can also be used without any problem - expansion on such a short section is negligible and the advantage is the lower price and easier handling directly from the reel without the need for shaping.
Schematic of a typical floor heating system with a distributor
How the installation and joining of both types of pipe differ
Similar families of fittings are used in practice for both types of pipe - crimping (crimping a ring or collar with hydraulic pliers), threaded (mechanical tightening with a sealing ring, convenient for repairs without the need for special tools) or expansion systems (expanding the end of the pipe with an expander and then pulling it back onto the fitting thanks to the shape memory of the material, typical mainly for PEX-a). The exact step-by-step procedure, including recommended insertion depths and tightening torques, can be found in the article "Installation of PEX and multi-layer pipe step by step", here we will mention only the key differences.
Before fitting a fitting on multi-layer pipe, it is necessary to remove the sharp edge from the cut and in many cases also to calibrate and slightly expand the end with a calibration tool, because the aluminum layer slightly changes the geometry of the cut. For clean PEX pipe, on the other hand, the aforementioned expansion method is often used (for example, the PPSU fitting system), which is fast, does not require O-rings or sealing and uses the shape memory of PEX - the material contracts back onto the fitting after expansion and creates a permanent, very reliable connection. This method does not work with multi-layer pipe, because the aluminum layer prevents the required contraction of the material.
Recommended types of couplings and fittings for individual diameters and materials are discussed in detail in the article "What couplings and fittings to use on PEX pipe" - the general rule is not to mix fittings from different manufacturer systems and always use a calibrator/tool recommended directly by the pipe manufacturer.
Installation procedure - simplified scheme
Storage, handling and common mistakes in practice
Both types of pipe are supplied in coils (for smaller diameters, typically up to 25-32 mm) and in straight lengths (for larger diameters or where a perfectly straight run is required, for example in visible pipe runs). When storing coils, it is important not to stack them on the edge, not to fold them and to protect them from direct sunlight and freezing - cold PEX and multilayer pipe become more brittle and microcracks may occur when unwinding from the coil at low temperatures (below 0 °C), which may only become apparent after several years of operation as a leak at the bend. We therefore recommend allowing the coil to acclimatize in a temperature-controlled room for at least several hours before installation. More detailed guidelines can be found in the article "Storage and handling of pipes in coils and straight lengths."
Among the dozens of realizations we commonly encounter in practice, the following are among the most frequent mistakes:
- using pure PEX pipe for a long vertical run without compensating for linear expansion - the result is pipe pushing out of the clamps and cracking plaster at the point where the pipe passes through the floor,
- repeated bending of multilayer pipe at the same location when "adjusting" the route - the aluminium layer will crack and a permanent weakening of the pipe wall will occur at that location,
- fitting installation without calibration of the end of the multilayer pipe - the edge after the aluminium layer will damage the sealing O-ring,
- placing PEX pipe into floor screed with a joint in the middle of the loop instead of running it in one piece - in the case of a hidden leak, the location of the fault is then difficult to find,
- storing coils in direct sunlight outdoors during summer, when the surface temperature of the coil can exceed 50-60 °C and the material deforms.
If a problem does occur, i.e. the pipe cracks or leaks, the diagnostic and repair procedure is described in the article "Cracked or leaking pipe - causes and solutions", where we also explain how to distinguish a fault caused by material fatigue from an installation error.
Price comparison and overall economics of the solution
Pure PEX pipe is generally cheaper per meter than multilayer pipe, the difference is in the order of tens of percent depending on the diameter and manufacturer. In large-scale applications, such as floor heating with a total requirement of 300-800 m of pipe for a typical family home, this saving is noticeable in the overall budget - and since pure PEX is also technically more suitable for floor heating, the choice is clear from both perspectives.
On the other hand, for wall runs, you need to consider not only the price of the pipe itself, but also the price of labor and the risk of future repairs - the higher purchase price of multilayer pipe is usually offset in most cases by easier and faster installation (fewer shaped parts, fewer fittings due to the material's ability to hold shape) and lower risk of problems with expansion after years of operation. Experienced installers therefore often combine both materials in one project when calculating a job - multilayer for wall runs and vertical runs, and pure PEX for floor loops - and this combination has proven to be the most economical and reliable in practice.
Frequently asked questions about PEX and multilayer pipe
Can PEX pipe be used for cold and hot water supply in an apartment?
Yes, PEX pipe with the appropriate certificate for potable water (hygienic certificate) is commonly used for water supply. It is important to ensure that the pipe intended for water supply has the correct certification, as not every PEX pipe with an oxygen barrier for heating is automatically suitable for potable water, and vice versa.
Can I combine pure PEX pipe and multilayer pipe in one system?
Yes, it is common to use multilayer pipe for wall runs and vertical runs and pure PEX for floor loops in one heating system. The connection is made via a manifold or transition coupling - it is important to use compatible fittings for the given diameter and material on both sides of the joint.
Why does multilayer pipe not "return" to its original shape after bending, unlike PEX?
This is due to the aluminium layer inside the wall, which behaves like a thin metal sheet - once the material's elastic limit is exceeded, it remains permanently deformed (similar to bending a wire), whereas pure PEX retains some elasticity even after bending and tends to return closer to its original straight shape.
Is it necessary to address thermal expansion compensation for multilayer pipe?
For short and medium sections, it is usually not necessary, which is why this pipe is preferred for wall runs. For very long straight runs (tens of meters, for example in industrial halls), it is still advisable to consider expansion elements, although to a much lesser extent than with pure PEX.
What diameter of PEX pipe is most commonly used for floor heating in a family home?
The most common sizes are 16x2 and 17x2 mm, and sometimes 20x2 mm is used for larger areas or larger loop spacing. The exact choice depends on the room's heat loss, the length of the loop and the required flow - a detailed procedure can be found in the article "What pipe diameter do I need according to the type of distribution."
Will PEX pipe withstand the pressure in a typical home heating system?
Yes, a typical home heating system operates at a working pressure of 1.5-2.5 bar and a temperature of up to 70-75 °C, which is well below the PN6/90°C or PN10/60°C values that PEX pipe can safely handle in the long term. The pipe also has a reserve for short-term pressure tests during installation, which are commonly performed at 1.3 times the operating pressure.
Summary - how to decide
If you are dealing with floor heating, large-area loops embedded in a concrete screed or need a cost-effective solution for direct short sections, choose pure PEX pipe - its flexibility, compatibility with expansion fittings and lower price are a clear advantage here. On the other hand, if you are dealing with water or heating distribution in walls, vertical runs in a multi-storey building or any route where it is important to precisely and permanently shape the pipe without the risk of expansion problems, multilayer pipe with an aluminium insert will save you trouble and future complaints. Most professional realizations ultimately combine both systems where it makes technical and economic sense - and it is precisely this combination of knowledge that distinguishes a well-designed installation from one that will start causing problems in a few years. The full range of PEX pipes in various diameters and lengths can be found in the category Pipes - heating, water, floor.
Do you have a question about this topic?
Not sure how to decide or dealing with a specific situation in your home? Write to us - we are happy to help.