Separation and Aluminium Foil for Underfloor Heating: When and How to Use Them

Separation and Aluminum Foil for Underfloor Heating: When and How to Use Them

When people hear "underfloor heating," most immediately picture pipes or electric mats and poured concrete. Few, however, pay enough attention to two inconspicuous materials that determine whether the whole floor structure actually works properly – separation foil and aluminum foil. In practice, we see time and again that these two items are either left out entirely, swapped for one another, or used in the wrong order. The result? A warped floating floor, thermal bridges, moisture condensation in the insulation, or even cracked anhydrite screed. This article explains what each foil does, when each is mandatory or optional, how they are installed, and what to avoid.

Why Foils Are Needed at All – the Physical Basis

A heated floor structure is not a static stack of materials – it is a dynamic system that continuously expands and contracts. A concrete screed 65 mm thick over an area of 40 m² will elongate by several millimeters in each direction when heated by 10 °C. If this screed is directly bonded to the underlying insulation or even to the wall, the stress will crack it somewhere. And it will always crack where we don't want it – in the middle of the room, along a pipe, or at a doorway.

Besides thermal expansion, two other physical phenomena play a role here: capillary moisture transfer and water vapor diffusion. Fresh concrete or anhydrite contains a huge amount of water. Some of this water, if allowed to evaporate uncontrolled, could migrate downward – into the polystyrene insulation. EPS absorbs moisture and loses its insulating properties. At the same time, in a dry system without screed, where the flooring is laid directly onto boards, we must deal with vapor diffusion from the substrate.

Foils solve both problems at once: the separation foil separates the screed from the insulation and allows it to move freely, while the aluminum foil in a dry system acts as a heat reflector and heat distributor. Each therefore has a completely different primary purpose, although in some products these functions are combined.

Separation Foil: What It Is, What It Does, and When You Must Use It

Separation foil is essentially a polyethylene (PE) film, typically 0.10 to 0.20 mm thick. Its primary role is mechanical and physical separation between the fresh screed and the underlying layers. It is not a vapor barrier in the classic construction sense – although PE's diffusion properties are fairly good – but primarily a sliding layer and a barrier against cement laitance seeping into the pores of the polystyrene.

Specific Functions of Separation Foil

• Sliding layer (movement separation): The screed can freely expand without transferring shear forces to the insulation board or rubbing against it and tearing.
• Cement laitance barrier: Fresh concrete contains cement laitance – a very fluid component that, without the foil, would soak into the EPS and bond the screed to the insulation. Once hardened, this bond would prevent expansion.
• Protection of insulation from screed moisture: EPS is not hygroscopic like mineral wool, but long-term exposure to technical moisture from concrete can reduce its λ value (thermal resistance) and thus the performance of the whole system.
• Pipe protection: In systems where the pipe is laid directly on the foil (not in a system board), the foil prevents direct contact between the concrete and the substrate and allows micro-movement of the pipe during thermal expansion.

Using a separation foil is mandatory in every wet application – that is, whenever the structure includes a poured screed (concrete, anhydrite, lightweight screed). It is not optional, not a luxury. If a designer or builder proposes leaving out the foil on the grounds that "the system board has a smooth surface," they are only partly right – a smooth surface reduces friction, but does not protect against cement laitance or against the screed welding itself to the EPS.

A typical product for wet systems: Separation Foil 601001H – a standard PE foil designed to be laid directly under concrete or anhydrite screeds in wet underfloor heating.

Installing Separation Foil in Practice

On job sites, we most often see two mistakes: the foil is cut too short (not covering the edge expansion strip near the wall), or the strips are merely overlapped without bonding and get pushed apart while the screed is being poured. The correct procedure is as follows:

• The foil is laid over the finished system insulation board with the pipes already in place, and it must overlap the edge expansion strip and be adhered or extended at least 10 cm up the wall.
• Individual strips are overlapped by at least 10–15 cm, and the joints are taped (ideally with PE or aluminum tape resistant to the alkalinity of concrete).
• At corners and edges, the foil must be carefully shaped – it must not form air pockets or large folds, as a fold under the screed can cause local bumps.
• Screed pouring should follow immediately after the foil is laid – the foil should not be left exposed for long, as walking on it can shift it and open the joints.

Aluminum Foil: A Completely Different Principle, a Different Use

Aluminum foil looks similar at first glance – a thin sheet of material. But its physical functions are diametrically different from PE separation foil. Aluminum is an excellent heat conductor (λ ≈ 200 W/m·K) and also has high infrared reflectivity. These two properties make aluminum foil an indispensable part of dry underfloor heating systems.

In a dry system (without screed), the pipe is placed into grooves in a polystyrene board and covered directly with a floor board or panel. The problem is that the pipe releases heat mainly by radiation and convection from a round surface – not evenly across an area. Without aluminum foil, heat would rise directly upward in a narrow stream above the pipe, while the area between pipes remains cold. The result: visible warm stripes on the floor surface exactly above the pipes and cold stripes between them. This effect is both aesthetically unpleasant and thermally uncomfortable.

Aluminum foil for dry underfloor heating solves this problem by conducting heat from the pipe laterally into the surrounding area, dramatically increasing the effective heat-transfer surface. Instead of a hot spot above the pipe, an even, warm surface is created. This is known as lateral heat spreading.

When Is Aluminum Foil Mandatory and When Is It Optional?

In a wet system, aluminum foil is not necessary – the concrete screed itself acts as an excellent heat distributor. Concrete has only λ ≈ 1.6–2.0 W/m·K (compared to aluminum's 200 W/m·K), but its mass and cross-sectional area effectively ensure even heat distribution.

In a dry system, aluminum foil is practically mandatory. Here we're talking about systems where the pipe is laid into a specially shaped polystyrene board with grooves or channels, and instead of a screed, a chipboard, OSB, gypsum board, or other dry panel is installed directly. Practical examples:

• Renovations of old apartments where the ceiling cannot bear a wet screed
• Wooden load-bearing structures (log cabins, timber houses), where wet technology is not suitable
• Installations with an urgent completion deadline – dry systems are usually walkable within 24 hours
• Attic rooms with limited height for the overall floor structure

Specially designed for dry systems is, for example, Polystyrene for dry underfloor heating UHPD (STIROTERMAL DRY), whose surface has preformed channels for the pipes and space for laying the aluminum foil.

System Boards and Their Relationship to Foils

Modern insulating system boards for underfloor heating are designed to work together with foils. However, it is important to understand which type of board requires which foil.

Boards for Wet Systems (with Screed)

Insulating system board UHP55 (STIROTERMAL BASIC) is a typical board for wet systems. It has a smooth surface with an embossed grid for orienting pipe layout and integrated edge overlap. After the pipes are secured in the fixing clips, PE separation foil follows, and then the screed. Aluminum foil is not part of the standard procedure here.

Similarly, Insulating system board UHP51 (STIROTERMAL DUO 11) is a board with integrated PE foil on the top surface. Here, the manufacturer has already incorporated the PE foil directly into the product. The advantage is eliminating the step of laying separation foil and reducing the risk of installation error. Nevertheless, it is still good practice to tape the joints between boards, as the integrated foil does not automatically bond adjacent boards.

Boards for Dry Systems

STIROTERMAL DRY (UHPD) has preformed channels with a trapezoidal profile, into which the aluminum foil is laid, followed by the pipe; the foil is then wrapped around the pipe, and the whole assembly is covered with a floor board. The aluminum foil here performs a dual function: heat distributor on the upward side and a reflective barrier facing downward (reflecting thermal radiation back upward from the polystyrene).

For more on choosing the right type of board, see the article Comparing system boards: STIROTERMAL BASIC vs DUO vs DRY vs SOLO, and for insulation thicknesses, see What polystyrene thickness is needed under underfloor heating.

Technical Parameters of the Foils and What They Affect

Separation Foil – Key Parameters

Parameter	Typical Value	Why It Matters
Thickness	0.10 – 0.20 mm	Thinner foil tears more easily during laying, thicker foil is more resistant
Material	PE (polyethylene)	Resistance to concrete alkalinity, diffusion resistance
Roll width	1.0 – 2.0 m	Fewer joints = lower risk of error
Sd value (diffusion resistance)	50 – 100 m	Protects insulation from moisture in the screed

Aluminum Foil – Key Parameters

Parameter	Typical Value	Why It Matters
Aluminum layer thickness	0.05 – 0.15 mm	Thin aluminum is sufficient for heat conduction; thicker holds its shape better in the channel
Backing layer	PE or PP backing	Prevents tearing when bent, provides mechanical strength during installation
Thermal conductivity (λ)	≈ 200 W/m·K	Lateral spreading of heat from the pipe into the surrounding area
Surface emissivity	ε ≈ 0.03 – 0.05	Low emissivity = high reflectivity of IR radiation upward
Operating temperature	up to 90–120 °C	Sufficient margin for standard underfloor systems (max. 55 °C pipes)

Step by Step: Installation in a Wet System with Separation Foil

On job sites, I've seen how rushing leads to compromises that only show up a year or two later. Here is a procedure that actually works:

1. Substrate preparation: The floor slab or concrete screed must be clean, level (max. ±5 mm/2 m), and dry (max. 4% moisture for concrete). Unevenness must be filled in, not left on top of the EPS.
2. Edge expansion strip: An expansion strip at least 8 mm thick and at least as tall as the EPS thickness + screed thickness + 2 cm must be attached to every wall, pillar, and door frame. This strip must never be forgotten – it is the most common cause of screed cracking.
3. Laying the system insulation board: Boards are laid in a staggered (brick-like) pattern with overlapping joints. Joints are glued or fixed using integrated overlaps. More in the article Installing a system insulation board under underfloor heating step by step.
4. Laying the pipe: The pipe is laid according to the design layout (spiral or serpentine). The minimum bending radius for PEX 16×2 is 80–100 mm.
5. Laying the separation foil: Strips overlapped by 10–15 cm, joints taped, extended up the wall over the expansion strip.
6. Pressure test of the pipe: Before pouring the screed, the system is filled with water and subjected to a pressure test of at least 6 bar for 24 hours.
7. Pouring the screed: For anhydrite, ambient temperature must be at least +5 °C; for concrete, also at least +5 °C. The pipe must be under pressure (min. 3 bar) during pouring. The screed is poured from the farthest corner toward the door.
8. Initial heating: Heating start-up should occur at least 28 days after pouring concrete (21 days for anhydrite), gradually starting at 25 °C with a daily increase of 5 °C up to the required operating temperature.

Step by Step: Installation in a Dry System with Aluminum Foil

A dry system is faster, but requires even greater precision when laying the aluminum foil. If the foil is not well pressed against the pipe along its entire length, thermal contact is poor and the heat distribution effect is lost.

1. Substrate preparation and laying UHPD boards: Same procedure as for wet systems, just without the need for the same level of flatness (tolerance ±8 mm/2 m).
2. Laying aluminum foil into the channels: The foil is placed into the preformed channels so that it fits closely against the sides and bottom of the channel. Preformed foil or foil fixed with adhesive strips holds its shape better.
3. Laying the pipe: The pipe is pushed into the channel over the aluminum foil, and the foil is folded around the pipe so the pipe achieves maximum thermal contact with the aluminum layer.
4. Connecting the aluminum foil between channels: On flat surface sections, the aluminum foil is also laid between the channels to cover the entire EPS surface. Some manufacturers supply wider rolls for this purpose. Joints are taped.
5. Laying the floor boards: OSB V100 min. 22 mm, gypsum fiberboard min. 25 mm, or cement fiberboard min. 20 mm. The boards are laid with staggered joints and screwed into the EPS through the aluminum.
6. Floor covering: Laid 24 hours after the floor panels are fixed.

Common Mistakes in Practice and Their Consequences

Over years of working with customers and installers, the same mistakes keep recurring. Some show up immediately, others only after months of operation.

Mistake #1: Omitting Separation Foil in a Wet System

"If the board is smooth, you don't need foil" – we hear this argument fairly often. The result: the concrete screed bonds to the EPS board, and after the first heating cycle, when the screed starts to expand, either the screed cracks or the EPS detaches from the substrate. In addition, cement laitance penetrates the EPS and locally reduces its insulating properties.

Mistake #2: Swapping Aluminum and PE Foil

Aluminum foil in a wet system under concrete is problematic – concrete reacts alkaline with aluminum and can damage the aluminum layer. PE separation foil in a dry system instead of aluminum does not perform the function of a heat distributor, and the floor will have cold stripes between the pipes.

Mistake #3: Insufficient Overlap and Bonding of Separation Foil Joints

Joints that are not taped will open when the screed is poured. The concrete pushes the foil into the gap, bonds with the EPS, and creates exactly the problem we were trying to avoid. Checking the joints before pouring is mandatory, not optional.

Mistake #4: Insufficient Wall Extension of the Foil

Foil extended only 3–4 cm up the wall is not enough. The expansion movement of the screed (several mm) will pull it away from contact with the strip, creating a thermal bridge and a moisture entry point at the edge of the insulation. The minimum is 10 cm above the surface of the final screed.

Mistake #5: Aluminum Foil Without Contact with the Pipe

In dry systems, we see installers place the foil into the channel "loosely" and slide the pipe in without wrapping the foil around it. The thermal resistance between the pipe and the foil is then high, and heat distribution works poorly. The foil must be in direct contact with the entire circumference of the pipe.

More mistakes are discussed in detail in the article Common mistakes when installing polystyrene and system boards under underfloor heating.

Combining Foils: When Are Both Needed?

There are situations where both foils – aluminum and PE separation – are used in a single structure. A typical case is a hybrid system where, over a dry substrate (e.g., in an attic on wooden beams), STIROTERMAL DRY with aluminum foil and pipes is laid first, and instead of a floor board, a thin self-leveling compound (30–40 mm) is still poured on top. In this case, the aluminum foil in the channels ensures heat distribution, and a PE separation foil is added on top of it so that the self-leveling compound does not bond to the aluminum and can expand freely.

This combination is less common, but we are seeing it more and more in renovations. You can learn more about the differences between wet and dry systems in the article Dry vs. wet underfloor heating: which polystyrene and foil are suitable.

Quantity, Consumption Calculation, and Practical Ordering Tips

One of the questions customers ask most often when calling is: how much foil do I need? The answer is relatively simple, but mistakes are made when the extra allowance is forgotten.

For PE separation foil: Room area (m²) + 15% for overlaps and wall extension. Example: a 30 m² room, perimeter 22 running meters, wall extension height 0.2 m → 30 + 30×0.15 + 22×0.2 = 30 + 4.5 + 4.4 = ~39 m². Round up to the nearest full roll.

For aluminum foil in a dry system: You need to cover the entire board surface – both the channels (where the aluminum must cover the entire pipe circumference) and the gaps between channels. Consumption is therefore always greater than the room area alone. Practical tip: calculate the room area, multiply by a factor of 1.3, and you'll have a good margin.

Storing foils: Foils must not be stored in direct sunlight (UV degrades PE) or in freezing conditions (aluminum foil becomes brittle). The ideal storage temperature is 10–25 °C.

Frequently Asked Questions (FAQ)

Can I use regular construction PE foil from a hardware store instead of a special separation foil under a concrete screed?

Technically, yes, as long as the foil meets the minimum thickness of 0.10 mm and is made of polyethylene (PE). The problem, however, is that generic "construction" foils from hardware stores often have unspecified or unreliable thickness, may be made from recycled material with lower rigidity, and their joints bond less well. In an underfloor heating system, where repairs after the screed has been poured are extremely costly, it really pays to use a proven product such as Separation Foil 601001H with known parameters and guaranteed thickness.

What happens if aluminum foil is completely omitted in a dry system?

The system will work, but much less efficiently. There will be measurable temperature differences on the surface of the floor board between the area above the pipe and the area between pipes – the difference can be 4–8 °C. With wood flooring, this can cause uneven stresses; with laminate flooring, ring-shaped deformations. Aluminum foil is not just a comfort feature – it's a functional part of the system that ensures its designed thermal output.

Can aluminum foil be used in a wet system (under concrete) to achieve better heat distribution?

No, this use is not recommended. Concrete and anhydrite have a strongly alkaline pH (9–13), which attacks the aluminum layer. Aluminum reacts with the alkaline environment, producing hydrogen and degrading. Besides the chemical instability, aluminum foil under a screed would not achieve a significant effect anyway, since the concrete screed itself distributes heat evenly enough. In a wet system, the correct choice is exclusively PE separation foil.

Do I need to tape the joints of separation foil with special tape, or is regular adhesive tape enough?

Regular office or painter's tape is not enough. You need tape that can withstand the alkaline environment of concrete, temperatures of 40–60 °C during the first heating cycle, and mechanical stress during screed pouring. Special PE tapes for construction foils or aluminum tapes with acrylic adhesive are ideal. Some separation foil manufacturers supply suitable tape with their products – always check compatibility.

My project drawing shows a "vapor barrier under the screed" – is this the same as separation foil?

Not entirely – the terms overlap partially but are not identical. A vapor barrier is primarily designed to prevent water vapor diffusion and has a defined diffusion resistance (Sd value). Separation foil performs a mechanical separation function, although PE material also has good diffusion resistance (Sd approx. 50–100 m), meaning it partly fulfills vapor barrier functions as well. In the context of underfloor heating above a heated space, a standard separation foil is sufficient. If the underfloor heating is above an unheated floor or basement, the project may require a true vapor barrier with a defined Sd value – in that case, follow the design documentation.

Where exactly should the separation foil end at a door frame?

The door frame is the most critical point in the entire foil installation. The foil must run across the entire opening under the frame and be extended on both sides of the expansion strip (on the room side and the hallway side) if both spaces have underfloor heating. If only a cold hallway is on one side, the foil is terminated behind the frame and sealed in. The frame must never be embedded directly in the screed – there must always be an expansion gap with a strip between the frame and the screed, otherwise cracking is guaranteed within two heating seasons.

Conclusion: Inconspicuous Foils Determine the Outcome of the Whole System

Separation and aluminum foil are two of the cheapest items in the entire underfloor heating installation. Nevertheless, their correct or incorrect use determines whether the whole system runs trouble-free for years, or whether cracks appear in the screed, cold spots appear on the floor, or moisture builds up in the insulation within a few months. Remember: PE separation foil belongs in every wet system, always; aluminum foil is the foundation of every dry system. Don't mix them up, don't skimp on them, and don't be lazy about taping the joints. The result is underfloor heating that works exactly as designed – evenly, efficiently, and without unpleasant surprises.

If you're deciding between a wet and dry system, or can't decide on the type of system board, check out our other articles in the Knowledge Center: How to choose the right insulating system board for underfloor heating, Comparing system boards: STIROTERMAL BASIC vs DUO vs DRY vs SOLO, or Why the floor above the insulation board cracks or warps.

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