Common mistakes when installing polystyrene and system boards for underfloor heating
Common mistakes when installing polystyrene and system boards under underfloor heating
Over the years of practice in the field of underfloor heating, we have seen just about everything. An investment in a quality underfloor heating system can go to waste if seemingly minor, but actually fundamental mistakes are made when installing the insulation. Paradoxically, most of these are not the result of ignorance or negligence in the classic sense – they are rather mistakes born of underestimation, of haste, or of someone doing something "the usual way" without stopping to think whether it actually works in that particular situation. This article is an attempt at a systematic overview of what most commonly goes wrong, why it goes wrong, and how to prevent it.
If you're still planning your choice of insulation materials, we recommend first reading the article How to choose the right system insulation board for underfloor heating and Comparison of system boards: STIROTERMAL BASIC vs DUO vs DRY vs SOLO. Here we will focus exclusively on the installation process itself and the mistakes that occur during it.
1. The substrate is not level or clean – and that's not a "minor detail"
The first and one of the most common mistakes begins even before the boards are unpacked. The floor screed or concrete slab onto which the insulation is laid is not sufficiently level, dry, and free of debris. While for classic ceramic tiling a substrate flatness deviation of 3–5 mm over a 2 m straightedge might still be acceptable, for system insulation boards with profiled studs this is a problem.
If the substrate is not level, the boards "rock" on raised spots and an air layer forms beneath them in depressed spots. This has several consequences: mechanical instability of the entire structure (especially with a dry system), unevenly distributed load when walking on it, and in the case of a wet anhydrite or cement screed, there is a risk that the board will deform or shift under the pressure of the screed being poured, which can create thermal bridges or damage the piping.
Practical example: We did a job in a new building where the concreting company handed over a floor with a deviation of up to 8 mm over 2 meters. The person laying the polystyrene "solved" this by simply forcing the boards into place. At first glance everything looked fine. After the anhydrite was poured and heating started, waves and several cracks appeared on the surface – exactly in the places where the boards had shifted under the uneven substrate.
Solution: Before installation, always measure the flatness of the substrate with a long straightedge (at least 2 m). Maximum tolerance: 3 mm over 2 m. If the unevenness is greater, a leveling screed or local grinding of high spots is needed. The substrate must be dry (moisture measurement – for anhydrite max. 0.5 CM%, for cement screed max. 2 CM%) and free of dust, oil, and residues of other materials.
2. Wrong choice of insulation thickness and type for the given structure
This is a mistake that cannot be fixed without dismantling the entire floor. The thickness of the insulation under underfloor heating is not a matter of "whatever fits," but a calculated requirement arising from building physics and the STN EN 1264 standard. Many builders think "some polystyrene" will do, or use whatever is left over from the facade or roof.
Basic rule: insulation thickness depends on what is below the floor. Above unheated spaces (basement, garage, exterior), the minimum required thickness of EPS 150 is usually 100–120 mm. Above heated rooms (between apartments), 30–50 mm is sufficient. Above soil (without a basement), it is generally 80–100 mm. If you use too thin a layer, heat from the loops will travel more downward than upward – the system works inefficiently, energy consumption rises, and in some cases the room below may even overheat.
Another dimension of the problem is confusing system boards intended for wet systems with boards for dry systems, and vice versa. Polystyrene for dry underfloor heating UHPD (STIROTERMAL DRY) has profiled grooves adapted for laying pipes without pouring – it is a system with low structural height, where special distribution plates and the wearing layer are placed directly above the pipe. If you use this type in a wet system and pour anhydrite over it, you gain nothing extra – on the contrary, you may have problems with the weight and rigidity of the whole structure.
This topic is covered in more detail in the article What thickness of polystyrene is needed under underfloor heating and also Dry vs wet underfloor heating: which polystyrene and foil are suitable.
3. Boards are not properly bonded – joints line up with each other
This is a classic mistake made by a bricklayer laying bricks "in line" – but with system boards it has even more serious consequences. If you lay the boards so that the joints of adjacent rows align (i.e., cross at a single point), you create a line of reduced rigidity through which the screed can crack while setting and shrinking. These cracks then propagate all the way to the wearing layer.
Proper bonding of boards means that each subsequent row is offset by at least half the length of the board (a bond like in bricklaying). This applies both to boards with straight edges and to tongue-and-groove boards.
In addition, installers sometimes leave open gaps between boards – places where the boards are not tightly against each other, but there is a gap of 3–10 mm between them. These gaps are thermal bridges. In practice, this means the screed or pouring compound flows into them, and although this may appear to "fill" the gap at first glance, local stress and cracking can occur during setting. Correct method: boards are laid tightly against each other. For tongue-and-groove boards, the tongue is aligned into the groove and the connection is pressed together by hand without using a hammer.
4. Missing or improperly fitted expansion strip
Without an edge expansion strip around the entire perimeter of the room, it simply doesn't work. And yet this mistake is made very regularly. The expansion strip made of foamed polyethylene (PE foam, 8–10 mm thick) must be fitted before laying the boards along the entire perimeter of the walls, around columns, load-bearing pillars – in short, against every vertical structure that the floor board touches.
Why is this critical? Anhydrite or cement screed shrinks and expands as it sets and later due to temperature changes (cyclic heating). If it has nowhere to "breathe," the pressure is transferred to the corners and cracks appear either in the screed or – worse – in the wearing layer (tiles crack, parquet buckles). The height of the expansion strip must exceed the total height of the floor structure including the wearing layer. After the installation and completion of the wearing layer, the excess is trimmed off and covered with a skirting board.
A typical mistake in practice: the expansion strip is fitted only to the thickness of the insulation, not to the full height. The screed thus has a gap at the wall in the lower part, but in the upper part (in the area of the screed itself and the wearing layer) it is firmly attached to the wall. This is not enough – cracks will still appear during thermal expansion.
Another version of the mistake: the expansion strip is omitted at door frames and staircase steps. It is precisely at transitions that screed movements are most critical. Every transition between two screed fields must have an expansion joint – in practice this is solved by placing a vertical joint in the doorway, which is then covered with a cover strip or filled with permanently flexible sealant.
5. Poor handling of the foil – folds, holes, and incorrect overlaps
Separating foil or aluminum foil is part of the floor structure that has a specific function, and therefore deserves the same attention as the insulation itself. Separating foil 601001H serves as a vapor barrier as well as protection of the insulation from moisture from the screed. Aluminum foil for dry underfloor heating also has a reflective function – it reflects heat upward and improves heat distribution in the dry system.
The most common mistakes when working with foil:
- Insufficient overlap of strips: Individual strips of foil must overlap by at least 100–150 mm, and the joint must be taped with self-adhesive tape. In practice, strips are sometimes just placed next to each other or have an overlap of only 30–40 mm without taping. When pouring anhydrite, the foil floats, the joints open up, and moisture from the screed penetrates directly into the insulation.
- Mechanical damage during installation: When workers walk on the foil with sharp tools or spiked footwear, the foil gets punctured. Every hole is a potential entry point for moisture. The foil is laid last, right before the screed – not hours or days in advance, when it is exposed to movement on the construction site.
- Folds and wrinkles: When the foil is not stretched evenly, folds form, which then "stand" in the screed and create places of reduced rigidity. In a dry system, folds under the distribution plates change the height level and cause instability of the wearing layer.
- Mixing up foils: Aluminum foil is not a substitute for separating foil – they are different products with different functions. More about this in the article Separating and aluminum foil in underfloor heating: when and how to use them.
6. The pipe is not properly fixed – it "floats" or shifts
System boards with studs (e.g., System insulation board UHP55 STIROTERMAL BASIC or System insulation board UHP51 STIROTERMAL DUO 11) allow the pipe to be laid directly into the grooves between the studs without additional anchoring. This is a great advantage – but also a trap for the inattentive.
Mistake no. 1: The pipe is not pressed sufficiently between the studs. If the pipe only "sits" partially, it can lift when the anhydrite is poured – the anhydrite floats it upward under pressure during pouring. Result: the pipe is too close to the surface of the screed, the thermal layer above it is insufficient (it should be at least 30 mm above the pipe), and the floor surface heats unevenly, causing stress in the tiles.
Mistake no. 2: The bend radius of the pipe in the corners is too small. Each type of pipe has a minimum bend radius (e.g., for PE-Xa 16 mm it is typically 5–8 times the diameter, i.e., a minimum of 80–128 mm). If the pipe is bent too sharply, the wall is damaged, the flow is restricted, and in extreme cases it can burst. The studs of the system board define the spacing, but you must monitor the direction of the bend yourself.
Mistake no. 3: Too large a spacing of loops in the room. The usual spacing is 100–200 mm. With a larger spacing (e.g., 250–300 mm), a "zebra" effect occurs – stripes of warmth and cold on the floor surface, which in extreme cases can even be felt with bare feet. For more details on correct pipe installation, see the article How to correctly lay pipes into a system board without thermal bridges.
7. Ignoring the manufacturer's prescribed installation conditions
Every manufacturer of system boards states the installation conditions in the technical documentation. These include things like: minimum air and substrate temperature during installation, maximum substrate moisture, method of joining boards, maximum load before pouring. This information is not "marketing recommendations" – it is the result of material testing.
A common problem: installation in winter at temperatures below +5°C. Polystyrene itself tolerates low temperatures well, but anhydrite screed must not be poured at temperatures below +5°C (and the room must be kept at a minimum of +5°C for at least 48 hours after pouring). If this is violated, the anhydrite does not crystallize properly, loses strength, and becomes brittle after drying. The insulation itself is not a problem in this case – but the whole structure is.
Another case: soaked insulation. Polystyrene is generally fairly resistant to moisture, but if a system board with a special surface (e.g., with a textile layer or laminated surface) is wet for a long time before pouring, delamination of the surface or biological contamination (mold) can occur. Therefore, protect the boards with tarps if the building is not roofed.
8. Incorrect screed thickness above the pipe and its consequences
The height of anhydrite or cement screed above the pipe axis is not a matter of chance. The STN EN 1264-4 standard specifies a minimum cover of 30 mm above the top of the pipe. In practice this means: if the pipe has a diameter of 16 mm and its axis is at a height of 8 mm above the board, the surface of the screed must be at a minimum height of 8 + 8 + 30 = 46 mm above the board. For a 20 mm pipe it is 10 + 10 + 30 = 50 mm.
Too thin a layer of screed above the pipe: uneven thermal loading, cracks, problems with the wearing layer, visible "stripes" on the floor. Too thick a layer: slowed thermal response of the system, higher thermal mass, slower response to changes in thermostat temperature. Both are undesirable – the goal is an optimum.
9. Skipping the leak test before pouring the screed
Before any pouring of the system with screed, it is absolutely essential to perform a pressure test on the entire pipe system. This applies equally to wet and dry systems (in a dry system, before closing with distribution plates). The test is performed at a minimum pressure of 6 bar (or 1.5 times the working pressure, whichever is higher) for at least 24 hours. During this time, the pressure must not drop by more than 0.1 bar.
In practice, the test is sometimes performed too quickly (only a few hours) or not performed at all – the installer "knows it's tight because the pipes are intact." The problem arises later: a failure under the screed = breaking up the floor, disposal of the wearing layer, remediation. The costs are ten times higher than an hour of a technician's work with a pressure gauge before pouring.
Equally important: the system should remain pressurized during the actual pouring of the screed. This way, any pipe damage that may have occurred due to movement on the construction site or during the pouring itself can be detected immediately.
10. Mistakes specific to the dry installation system
The dry underfloor heating system has its own specifics, and mistakes that are marginal for the wet system take on greater significance here. STIROTERMAL DRY polystyrene is designed for screed-free systems, where aluminum or steel distribution plates and the wearing layer (e.g., OSB boards, laminate, vinyl flooring) are laid above the pipe.
The most common mistakes in the dry system:
- Gaps between distribution plates: Every gap between aluminum distribution plates is a place where heat is not distributed evenly. The plates must be laid tightly next to each other. More about distribution foils in the article Separating and aluminum foil in underfloor heating: when and how to use them.
- Wrong thickness and type of wearing layer: In a dry system, the wearing layer has a direct effect on thermal resistance. A thick carpet or multi-layer laminate with underlay can increase thermal resistance so much that the system cannot deliver the required output.
- Missing aluminum foil under the wearing layer: In a dry system without foil, heat rises directly from the pipe locally – there is nothing to distribute it sideways. Thermal stripes are visible and noticeable.
- Incorrect fixing of OSB boards: OSB boards (or other rigid wearing underlay) must be joined together and must have expansion gaps from the walls. If they are glued or fastened to the board beneath them without expansion, they crack during temperature changes.
11. Thermal bridges at penetrations and corners
Thermal bridges are places where heat escapes outside the defined path. In the context of underfloor heating, the riskiest points are: pipe penetration through an expansion joint (without a protective sleeve), connection to the manifold (where the pipe leaves the insulation upward), room corners, threshold transitions, and places where the insulation is interrupted (e.g., under a staircase step).
At every pipe penetration through an expansion joint, the pipe must be protected by a protective sleeve at least 200 mm long on each side of the joint. This is not a recommendation – for a wet system it is a standard requirement. Without protection, movements of the screed during expansion are transferred directly to the pipe and can damage it over time or tear it out of the insulation.
The issue of thermal bridges is discussed in more detail in the article How to correctly lay pipes into a system board without thermal bridges.
12. Premature loading and starting the heating after installation
Anhydrite screed needs drying and curing time. A common thickness of 50–65 mm requires at least 4 weeks before starting the heating (under favorable conditions – temperature at least 15°C, ventilation). Cement screed is slower – at least 6 weeks. After this period, the heating is started gradually: it begins at an inlet temperature of 25°C and is increased by 5°C each day up to the design temperature. The entire process usually takes 10–14 days.
If the heating is started prematurely or too quickly, the result is diffuse cracking of the anhydrite – small cracks throughout the entire surface. Most of these are cosmetic in nature, but they can also be deeper and affect the strength of the screed. More about the causes of floor cracking can be found in the article Why does the floor above the insulation board crack or warp.
Equally dangerous is driving heavy machinery (forklift, pallet truck with a heavy load) onto fresh screed that has not yet sufficiently cured. System boards have a defined load capacity (e.g., EPS 150 has a compressive load of 150 kPa at 10% compression), but the screed before curing is much more susceptible to deformation.
Frequently Asked Questions (FAQ)
Can I use regular facade polystyrene (EPS 70) instead of a system board under underfloor heating?
Technically yes, if it meets the thickness and compressive strength requirements. However, EPS 70 has lower load capacity (70 kPa at 10% compression) and is not designed for floor loading. For underfloor heating, a minimum of EPS 100 or EPS 150 is recommended, and in some cases (intensive use, garages) even EPS 200. System boards with studs also have predefined grooves for the pipe, which greatly simplifies installation and ensures correct spacing. Using facade EPS without studs means manually anchoring the pipe with clips – which is more labor-intensive and error-prone.
What are the consequences if I forget the edge expansion strip?
If the expansion strip is missing, the screed has no space for thermal expansion. With cyclic heating (daily switching of thermostats on and off), the screed tries to expand – and if the wall does not allow movement, the tension is released in the form of cracks. Cracks typically appear in room corners and at openings (doors, windows). In the worst case, skirting tiles can be pushed off and tiles near the walls can crack. There is no solution without dismantling the floor – therefore it is better to fit the expansion strip before installation.
What happens if the separating foil is not properly taped at the joints?
With a wet system using anhydrite: anhydrite slurry (liquid phase) flows through untaped joints under the foil and into the insulation. This is not necessarily catastrophic in itself – the anhydrite will dry. But: moisture can remain longer in the insulation than on the surface, and when the heating is switched on, this moisture tries to escape upward – through the screed and the wearing layer. Consequence: laminate flooring buckling, tiles loosening at the joints, or wooden parquet turning brown. Taping the joints is a 2-minute job that can save you weeks of remediation.
How can I tell that the boards were installed correctly if the screed is already finished?
After the screed has dried, there are several checkpoints: surface flatness (2 m straightedge, max. 3 mm deviation), tapping the screed – a hollow sound indicates air pockets under the board. An infrared thermal camera with the heating switched on will show uneven thermal stripes (indicating pipe displacement or thermal bridges). A pressure test before pouring verifies the tightness of the system – this check must be performed before the pouring itself. Once the screed has dried, repair is not possible without partially dismantling the floor.
Is it necessary to let system boards acclimatize indoors before installation?
For polystyrene, acclimatization is not technologically necessary in the same sense as for wood. Polystyrene does not change dimensions due to moisture. It is more important that the boards are not exposed to sunlight for a long time before installation (UV degradation of the EPS surface) and extreme temperatures (above 80°C, EPS starts to deform, but this is rare in practice). If the boards came from a cold warehouse into a warm room, we recommend letting them sit for at least 2–3 hours before installation – not because of dimensions, but for easier handling and better contact with the substrate.
Can I glue system boards to the substrate with dispersion adhesive or PU foam?
Gluing boards to the substrate is generally permissible and in some cases even recommended (e.g., in a dry system, where boards must not slide during installation). However, the correct adhesive must be chosen: PU foam must be low-pressure and must not lift or deform the boards. Dispersion adhesives must be compatible with EPS (some solvents dissolve EPS). Always verify compatibility on a small sample before applying it over a large area. In a wet system, gluing is supplementary – the primary fixation is provided by the weight of the screed itself.
Conclusion: Installing the insulation is the foundation, not a formality
Insulation under underfloor heating is the kind of work people only talk about when it fails. When done correctly, nobody thinks about it – the system works, the floor is warm, energy consumption matches the design, and the floor lasts for decades. When done poorly, problems appear gradually: first small cracks, then a rocking floor, then moisture, and finally remediation that costs several times more than a correct installation from the start.
Most of the mistakes described in this article are not technically complex. They are things that require attention, patience, and adherence to the technological process – not special tools or special skills. If you are unsure about a specific step, see the article Installing a system insulation board under underfloor heating step by step or consult us directly.
Choosing the right materials is the first step – we recommend browsing the entire category of polystyrene and system boards, where you will find products for both wet and dry systems, including the necessary foils and accessories.
Do you have a question on this topic?
Can't decide or are you dealing with a specific situation in your household? Write to us - we'll be happy to advise.