Underfloor Insulation Film for Floor Heating – When and How to Use It?

We are still preparing products.

But you can look at other categories.

Underfloor heating waterproofing membrane – when and how to use it?

When it comes to the composition of a floor with underfloor heating, most of the discussion naturally revolves around insulation boards, piping, spacing or the thickness of anhydrite or cement screed. The waterproofing membrane, however, remains a quiet hero, about which less is said – yet if someone omits it or uses it incorrectly, the consequences are often painful and expensive. From the experience of several projects, we know that precisely this detail separates a solid expansion-moisture behavior of the finished floor from one where after a year, moisture stains, cracks in the floor covering or even damaged insulation appear.

This article is devoted comprehensively to the waterproofing membrane under underfloor heating – from the physical nature of the moisture problem, through the typology of membranes, specific situations where their use is essential, to the practical laying procedure including correct joining and connection to edge strips. You will also find typical errors we encounter during installation, and answers to the questions our customers ask most often.

Why moisture under underfloor heating matters at all

Underfloor heating works with a system in which warm water circulates in pipes embedded in the screed mass. The screed evenly distributes heat upwards and heats the entire floor area. This beautiful physics has one subtlety: the effect of heat on the lower part of the structure intensifies the movement of moisture. The warm air above the insulation board and the relatively cold layer below it (ceiling structure, concrete foundation slab) create a temperature gradient that drives water vapor downward – precisely to the place where, without a proper barrier, moisture condenses.

Condensed moisture in the insulation layer not only reduces the thermal insulation properties of the material (wet polystyrene significantly loses its lambda value), but with long-term exposure, it can also degrade the screed layer from the bottom. In the case of anhydrite screeds, which are more sensitive to water than cement ones, repeated wetting can lead to a slow degradation of sulfate bonds and a loss of tensile strength.

A second, opposite direction of moisture movement also exists: rising moisture from the subfloor. This is typical for:

foundation slabs on the ground without quality building waterproofing,
older buildings with insufficient or missing waterproofing,
basement floors,
renovations, where the original screed is exposed and the structure "breathes" moisture.

In both cases, the waterproofing membrane is the layer that interrupts the transport of moisture and protects the insulation and screed layers from its destructive effects.

Types of membranes used under underfloor heating

There are several types of membranes on the market used in floor systems, and not all perform the same function. It is important to distinguish them, so you do not mix purposes or invest in an unsuitable product.

Polyethylene (PE) membrane

The classic PE membrane of thickness 0.1 to 0.2 mm is a basic waterproofing element that interrupts capillary rising moisture and forms a vapor barrier. It is used directly on the surface of the concrete or other load-bearing structure before laying the insulation boards. Its main advantage is cost-effectiveness, ease of processing and sufficient diffusion properties. The disadvantage is lower mechanical resistance – it easily punctures during the laying of insulation boards or when walking on them.

Textured separation membrane (PE with aluminum layer)

A more sophisticated alternative is Textured separation membrane – 0.1×1030 mm; AL – 50 m². This membrane combines the function of waterproofing and separation with a suppressed texturing grid that facilitates precise laying of the piping according to a predetermined spacing. The aluminum layer also improves the reflectivity of thermal radiation upwards, which marginally contributes to the efficiency of the entire system. This membrane is laid directly on the thermal insulation (not under it) and simultaneously performs the role of a separation and installation aid – the texturing really saves time when measuring spacing.

Vapor-open vs. vapor-tight membranes

With underfloor heating, it is important to distinguish whether you need a vapor-tight barrier (preventing vapor escape into the structure) or a vapor-open membrane (capable of discharging vapor in one direction). For most applications above ground, a standard PE membrane is sufficient, as it is sufficiently vapor-tight. In special cases – for example, in swimming pool areas, technical rooms with high humidity or when using very hygroscopic screed materials – a certified vapor-tight membrane with an Sd value (equivalent diffusion thickness of an air layer) above 100 m may be required.

Reflective Foils with Bubble Layer

These products are occasionally promoted as a substitute for thermal insulation, which is not true in most cases from the perspective of standards and heating system physics. They can supplement a thin layer of thermal insulation, but never fully replace it. They work well for waterproofing purposes, however, their thinness and low compressive strength disqualify them for most applications under anhydrite or cement screed.

When Waterproofing Foil is Necessary (and When It Is Not)

One of the most common questions from installers: "Do I need to always install the foil, or only sometimes?" The answer is more nuanced than it might seem.

Situations where the foil is absolutely mandatory

Ground floor (earth, gravel, concrete without waterproofing): If the slab does not have its own waterproofing meeting current standards, or if its condition is uncertain, foil under the insulation is a necessity. Rising moisture is a real threat that intensifies over time.
Basement areas: Moisture from the ground and surrounding terrain acts from multiple directions. Even if the walls are insulated, the floor must have its own barrier.
Renovations of older buildings: When the original screed is removed and concrete is exposed, it is usually found that the original waterproofing (if it exists at all) does not meet today's standards. Foil is the fastest and cheapest solution here.
Intermediate floor constructions above wet areas: If the heated floor lies above a bathroom, laundry room, or technical room below, a separation foil protects the structure from moisture from both sides.
Application of anhydrite screed: Anhydrite is sensitive to moisture and the foil here plays a critical role as a barrier against moisture from the substrate, which could degrade the screed.

Situations where the foil is recommended, but less critical

Intermediate floor constructions in standard living spaces where the substrate is dry and new.
Renovations with proven functional waterproofing of the building (documented project documentation, new building within 5 years).
System boards with an integrated foil layer on the bottom (some boards have a PE layer directly on the bottom).

Situations where the foil is not enough and more is needed

If the moisture load is extreme (swimming pools, showers, direct contact with the ground without drainage), a standard 0.1 mm PE foil is not sufficient. Here, a full waterproofing solution (coatings, waterproof membranes, penetrations) designed by an architect or waterproofing specialist is required. Underfloor heating is then an additional layer of the construction, which is designed only after the moisture protection is resolved.

Where exactly the foil is placed in the construction

This is the point where laymen and occasionally even experienced installers make mistakes – the foil is not always in just one place in the construction. It depends on what function it is supposed to fulfill.

Foil UNDER the insulation board (waterproofing)

This is the classic placement. The foil is laid directly on the surface of the load-bearing structure (concrete, old screed) before the thermal insulation is placed. Its purpose is to prevent rising moisture from the substrate from entering the insulation layer. The width of the rolls is chosen so that the overlaps of adjacent strips are at least 100–150 mm and all joints are sealed with waterproofing tape.

Foil ABOVE the insulation board (separation, grid)

Here, a separating grid foil is placed. In this case, it does not perform the primary waterproofing function, but protects the insulation board from the effects of cement slurry during the pouring of the screed (to prevent cement from penetrating between the boards and destroying their geometry) and at the same time, the grid allows to accurately measure and lay the pipe without the need for further measuring. It is therefore more of a separation and mounting aid.

Combined construction: two foils

In more demanding cases (basement, floor on the ground, anhydrite), both are used: PE foil under the insulation board and separating foil above it. The total cost of this solution is negligible compared to potential repair costs, so this combination is standard in most professional projects.

How to properly lay down the vapor barrier film – step by step

Laying the film is a physically simple task, but accuracy is important. Any mistake here – an uncovered seam, a hole from a screwdriver, a missing connection to the edge strip – can only become apparent after years, when the floor is already finished and no one wants to even imagine its repair.

1. Preparation of the base

Before laying the film, the base must be clean, dry and free of sharp protrusions. Protrusions on the concrete slab (sharp edges, wires, wedges) must be sanded or cut off, because piercing the film while walking on it is the most common mistake in practice. If the surface is significantly uneven, we recommend a continuous leveling compound before laying the film.

The moisture content of the base should be measured before laying – max. 4 % (CM measurement) for a cement slab, max. 0.5 % for an anhydrite slab. If the values are higher, you need to wait for the surface to dry or address the cause of the moisture structurally.

2. Laying the film

Unroll the film perpendicular to the main axis of the room. The first strip is laid with a minimum overhang of 100–150 mm at the walls (this overhang will later be covered by edge strips and the screed – do not cut it prematurely). Adjacent strips must overlap by at least 100 mm along the entire length, and we recommend 150–200 mm in case of increased moisture load. The film must be laid as evenly as possible – folds and air pockets underneath cause localized deformations of the insulation board under load.

3. Joining and sealing the seams

This is a critical point. The seams of the films must be sealed continuously, without gaps. For this purpose, a special metallic tape for joining films – 55 mm × 50 m is used. This tape is wide enough (55 mm) to cover the seam from both sides and has good adhesion to the PE surface. Ordinary adhesive tape from the hardware store is not suitable – it does not have the necessary long-term adhesion, it peels off due to the alkaline environment from the concrete and moisture.

Apply the tape to a dry, clean surface of the film. If the film is dusty or wet, the seam will not hold. In practice, it has proven effective to seal the seams while laying the strips, not after unrolling the entire area – this prevents the already laid strips from shifting.

4. Connecting to the edge expansion strip

The edge strip (PE foam expansion strip) is fixed along the entire perimeter of the room before laying the insulation board. The film must be led upwards and connected to the edge strip – either glued behind it, or pulled up along the wall so that it forms a continuous basin along the entire perimeter. This detail prevents moisture from the screed layer from seeping along the walls into the structure. The film overhang at the walls (at least 100 mm above the planned height of the screed) must be cut only after the screed has hardened and before installing the baseboard trims.

5. Laying the insulation board and separation film

After completing the vapor barrier film, the thermal insulation boards are laid. They are placed on tongue and groove (or with sealed joints) so that the joints of the boards are not continuous. If you use a separation embossed film on top of the insulation, its joints are also sealed with metallic tape. The embossed grid on the surface of the film then allows you to precisely determine the position of the pipe without measuring – simply follow the printed grid. More about the entire installation process can be found in the article Step-by-step installation of floor heating – from insulation to pouring.

Fixing the pipe and its relation to the film

Once the film is properly laid, the pipe laying begins. It is important here that the method of fixing the pipe does not damage the integrity of the film.

The most common fixing methods are:

Fixing strips: Pipe fixing strip 16–18 mm; 1 m / 100 is laid on the film and the pipe is clicked into it. The strips are fixed to the base (through the insulation) using screws or pins – there is a risk of piercing the film! When screwing through the embossed film and insulation, always make sure the screw does not pierce the vapor barrier film under the insulation board. If it does, immediately seal the perforation with a patch made of metallic tape.
Fixing arcs: Pipe fixing arc PEX 16–18 mm is used to guide the pipe in arcs and direction changes – without it, the pipe tends to jump out of the set course in bends. The arcs are driven into the insulation without the need for screwing, which minimizes the risk of damaging the film.
Clips: Pipe fixing clip 50 mm is an alternative to fixing strips for individual fixing of each arc without the need for a continuous strip. Again, when driving them into the insulation, you must check the depth to ensure the vapor barrier film under the board is not pierced.

A more detailed overview of fixing elements and their selection can be found in the article Fixing of floor heating pipes – clips, strips and arcs.

Concrete examples from practice

Case 1: Reconstruction of an apartment from 1975, panel building, 2nd floor

The customer wanted to replace the old cast iron radiator system with underfloor heating. After uncovering the original screed, it turned out that under it lay the original sandy layer and a mortar screed without any film. The concrete ceiling slab had minor cracks. Although the 2nd floor was not in direct contact with the ground, we decided to use a PE film 0.15 mm under the insulation due to the fact that the ceiling slab was clearly damp at the edges (condensation from the non-heated basement was detected). Result: after completion, the floors are dry, with no signs of moisture. If we had omitted the film, the risk was real.

Case 2: New construction, ground floor slab on the ground, drainage in order

The investor had a construction-verified waterproofing of the foundation slab (modified bitumen sheet, double layer). Despite that, we used a PE film 0.1 mm under the EPS 100 insulation for two reasons: first, the film was a cheap insurance layer; second, it protected the insulation from cement milk, which always slightly penetrates between the slabs during screed pouring. A dimpled separation film was laid over the insulation, which significantly accelerated the pipe laying (spacing 150 mm on an area of 220 m²).

Case 3: Basement with visible moisture from the outside

This situation was more complicated. The basement walls had efflorescence (salt bloom), which indicated active rising damp. We clearly explained to the investor that a standard PE film was not enough – before laying the floor heating system, it was necessary to apply crystalline injection into the walls and floor, penetration, and then a waterproofing coating. Floor heating was implemented only after verifying the dryness of the base with a CM measurement. This example illustrates that the film is the last line of defense, not a substitute for solving the moisture problem at its source.

Case 4: Wooden beam structure (older family house)

A less typical, but not rare case: the customer had a floor on wooden beams with a plank access hatch. With such a base, underfloor heating is installed differently – a dry system with routed boards or aluminum lamellas, not a wet system with a screed. Here, the film is equally important, but placed differently: a vapor-tight membrane is laid over the thermal insulation (between the beams), to protect the wood from moisture from the heating system. A standard waterproofing film under the insulation would not solve the problem here, it could even cause condensation within the wood structure.

Standards and regulations – what does the legislation say?

Slovak and European standards for underfloor heating (STN EN 1264-4 – Embedded surface heating and cooling systems, water-based: Installation) do not specifically prescribe a particular type of film, but clearly require that the floor composition meets the requirements for moisture protection in accordance with EN ISO 13788 (Thermal and moisture properties of building structures and elements – Internal surface temperature to prevent critical surface moisture and internal condensation). This practically implies the necessity of a vapor barrier whenever there is a risk of condensation in the structure.

STN 73 0540-4 (Thermal protection of buildings) also requires verification of condensation in structures at the design stage. If the design calculation shows a risk of condensation, a waterproofing film (or another vapor-tight element) is a mandatory part of the design. In practice, this means: always have a building physics calculation prepared or verified before deciding to omit the film.

Most common mistakes during film installation

From long-term practice, we know that these mistakes keep repeating:

Insufficient overlap of joints: 50 mm instead of the recommended 100–150 mm. The joint will peel when walking on the insulation and a moisture bridge will form.
Unsealed joints: Some installers think that the weight of the insulation board will "close" the joints. This is a mistake – cement milk from the screed is liquid and will capillary penetrate even the smallest gap.
Cutting the film excess too early at the walls: The excess film at the walls is cut off before pouring the screed, which causes the screed to flow under the edge strip and to the wall. Result: a moisture bridge along the entire perimeter of the room.
Puncturing the film with fixing elements: Screws from fixing strips, wedges – if they pass through the film, each hole is a potential path for moisture. A patch with metal tape is a quick and effective solution.
Inappropriate tape for joints: Using common brown adhesive tape or carpet tape. These tapes lose adhesion after 2–5 years, which is a problem for a structure designed for 30+ years of service life.
Film only on part of the area: In larger areas, it happens that the installer forgets to continue the film behind partitions or into corners. Moisture will always find the weakest point.

A more systematic look at mistakes during the installation of the entire system can be found in the article Common mistakes during pipe laying for underfloor heating and how to avoid them.

Special situations: floor cooling

Underfloor heating is increasingly combined with a cooling mode in summer – the system pumps cold water and cools the floor through the pipes. Here, the requirements for waterproofing become even stricter. A cold floor is a surface on which air moisture condenses during the summer period. This condensation penetrates the screed from top to bottom – the opposite direction to rising moisture. Without a separation film above the insulation, the condensate gets directly into the insulation and reduces its efficiency. The issue of floor composition for combined mode is described in more detail in the article Underfloor heating and cooling – what must the floor composition meet?

In cooling mode, it is also important that the underside of the floor covering is not vapor-tight (e.g., an epoxy laminate layer without a diffusion opening) – otherwise the condensate would have nowhere to go and would accumulate at the interface between the covering and the screed, causing delamination of adhesives and swelling of the covering.

Most frequently asked questions (FAQ)

Can I use a common construction film 0.1 mm from the hardware store instead of a special separation film?

For waterproofing under the insulation board, yes – a common PE film 0.1 mm is sufficient for this function, provided that the joints are sealed with the correct tape. For the function of a separation dimpled film above the insulation board, however, no – that common film does not contain a mounting grid or a reflective AL layer, which means you have to measure the pipe spacing manually and it is not part of an efficient thermal layer.

What happens if I completely omit the film under the underfloor heating?

It depends on the specific conditions. If the floor is on a higher floor above dry rooms and the building is new, you probably will not notice any immediate problem. For a floor on the ground or in a basement, however, it is a gamble – moisture from the base will gradually saturate the insulation, reduce its performance, and after years may cause damage to the screed layer or surface coverings. Repairing such a floor means completely uncovering the entire composition, which is several times more expensive than the cost of the film.

Do I have to use a metallic tape, or is ordinary adhesive tape enough?

For the short-term phase of construction (until the screed is poured), ordinary tape might hold. However, the waterproofing film is supposed to function for the entire lifetime of the building – 30, 50, or even more years. Common adhesive tapes based on acrylic or rubber adhesive gradually lose adhesion in an alkaline concrete environment and under temperature cycles of heating. A metallic tape with special adhesive is specifically designed for these conditions and its lifespan corresponds to that of the structure.

Is a film necessary even with a system board with an integrated PE bottom layer?

It depends on the specific system board. Some have a pressed PE layer on the bottom that performs the waterproofing function, others do not. If the board has a certified integrated PE layer with the correct Sd value and the manufacturer guarantees it in the technical documentation, a separate PE film under the board is not necessary. However, the joints between the boards must be sealed so that the film layer is continuous. If the system board does not guarantee this (e.g., by tongue-and-groove joining without gluing), we recommend using a PE film regardless. A comparison of system boards and dimpled films can be found in the article System board vs. dimpled film – what is better for your base?

How many m² of foil do I need for a 50 m² room?

Basic calculation: area + 10–15 % for overlaps and edge allowance at the walls. For a 50 m² room, this means 55–58 m². In a low-ceilinged room with long walls, the allowance may be higher. Specifically: Separation foil with grid 50 m² will cover a standard room with approximately 40–44 m² of clean area. For larger areas, order according to the floor plan with a 15 % allowance.

Where exactly does the foil end – is it cut off at the door or does it continue through the whole apartment?

The foil should be continuous throughout the space where the screed is poured – this means it runs smoothly through the door into the adjacent room, provided the adjacent room also has a screed layer. If the adjacent room has a different type of floor (e.g. wooden floor without a screed), the foil ends at the threshold and the joint is sealed with an edge strip. Cutting the foil in the middle of the room or without a vapour-tight connection to the adjacent structure creates a moisture bridge.

Conclusion: foil is a cheap insurance for an expensive investment

Floor heating represents an investment in the thousands of euros – for the piping, manifold, control system, installation costs, and finally for the surface covering itself. A hydro-insulation foil – whether a classic PE foil under the insulation board or a grid separation foil above it – makes up only a fraction of this amount, yet it plays an irreplaceable role in the overall reliability of the system.

From experience, we know that investors who decided to skip the foil in order to "save money" return to it during renovations with significantly higher costs. On the other hand, projects where foils were used correctly and in the correct position have been operating without any moisture problems for ten years of operation.

When planning the construction of your floor with underfloor heating, also read other articles in our Knowledge Centre – for example, How to choose an insulation board for underfloor heating – thickness, material and requirements or What pipe diameter and spacing do I need for underfloor heating? – and your system will be designed correctly from the ground up.

Do you have a question about this topic?

Having trouble deciding or dealing with a specific situation in your home? Write to us – we are happy to advise.