How to Choose the Right System Insulation Board for Underfloor Heating
How to choose the right system insulation board for underfloor heating
Choosing a system insulation board is a decision that most investors underestimate – and then you wonder why the floor reacts slowly to temperature changes, why energy bills are higher than expected, or why strange cracks appear in anhydrite screeds two years after installation. The right board is not just "some polystyrene under the pipes". It is the thermal, acoustic, and mechanical foundation of the entire underfloor heating system, and it largely determines how efficiently, how quickly, and how long your system will work.
In this article, we'll explain in detail what to consider when choosing a board, what the differences are between the various types, what the standard says, what physics says, and what years of practical experience from real projects say. If you want a comparison of specific products from our range, also check out our article Comparison of system boards: STIROTERMAL BASIC vs DUO vs DRY vs SOLO, where the individual types are broken down side by side with specific values.
What a system insulation board actually does – and what people don't understand
A system board performs several functions at once, and this is where the first misconception arises: people think its primary role is thermal insulation downward. That is important, but not the only thing. The board also:
- Mechanically holds the pipe in place – studs or grooves fix the pipe at a precise spacing without additional fastening, which is a huge time saving during installation.
- Directs heat flow upward – thermal resistance downward prevents unnecessary losses into the concrete slab or above the basement.
- Dampens impact noise – depends on the specific product and its flexibility, but a proper board can reduce impact noise by 10–25 dB depending on the layer composition.
- Acts as a separation layer – separates the anhydrite or cement screed from the load-bearing structure and allows it to expand.
- Withstands mechanical loads from screeds and flooring – must withstand the pressure of wet screed (up to 2,000 kg/m³) as well as long-term loads from furniture.
Once you understand this, it becomes clear why "any old EPS" or construction polystyrene from facades cannot be used. These are not designed for these complex requirements.
Key parameters used to choose a board
1. Thermal resistance and insulation thickness (R-value)
The thermal resistance of a board is given in m²K/W. For underfloor heating, the Slovak standard STN EN 1264 applies, which sets minimum thermal resistance values for insulation under the system depending on what is beneath the floor. Simplified, this means:
- Above a heated space (e.g. floor between apartments) – minimum R = 0.75 m²K/W, in practice usually a board thickness of 30–40 mm EPS 200.
- Above an unheated space (cellar, garage, basement) – minimum R = 1.25 m²K/W, in practice 60–100 mm EPS or a combined structure.
- Above ground or exterior environment – minimum R = 2.0 m²K/W, which in practice means 100–150 mm of insulation, with the system board being only the top layer above additional thermal insulation.
This leads to a key point: a system board of 55 mm thickness alone is rarely sufficient as the only insulation above ground. This is a common mistake I see repeatedly on projects – the investor buys a 5 cm board, places it directly on the ground, and then wonders why the heat losses are enormous. The system board is the top of the insulation "cake", not the whole cake. You can read more about proper thicknesses in our article What polystyrene thickness is needed under underfloor heating.
2. Compressive strength (CS value)
This is a parameter that is most often ignored when choosing boards, and then we wonder why the studs break or why the board "bulges" under heavy furniture. For underfloor heating, these approximate values apply:
- CS(10) 100 kPa – minimum for residential rooms without special loading (apartment, family house)
- CS(10) 150–200 kPa – recommended for commercial spaces, garages, areas with heavy foot traffic
- CS(10) 200+ kPa – industrial applications, warehouses, shops
System boards for wet underfloor heating must withstand the pressure from fresh screed (anhydrite has a density of ~2,000 kg/m³) and from the flooring layer. When you lay 65 mm of anhydrite, that's a load of about 130 kg/m², to which you need to add furniture, people, and dynamic impacts.
3. Stud spacing and diameter (pipe fixation)
The stud system has a direct impact on the pipe spacing you can achieve and the pipe diameter the system works with. Standard system boards are designed for Ø16, Ø17, or Ø20 mm pipe. The studs are arranged so the pipe is laid at 50 mm, 75 mm, or 100 mm spacing (or multiples thereof). If you plan a spacing of 150 mm or 200 mm (for example in rooms with lower heat losses), you need to check whether the specific board allows this spacing.
4. Type of system: wet vs. dry underfloor heating
This is a key factor when choosing a board. If you are doing classic wet underfloor heating (pipes embedded in anhydrite or cement screed), you need a stud board designed for wet systems. If you are doing dry underfloor heating (without screed, with aluminum distribution plates and dry materials), you need a completely different type of board with a grooved or milled geometry.
The difference is not just in geometry – these are systemically different approaches with different physical properties. In dry systems, there is no thermal mass from the screed, which means a faster response, but also a greater impact from heating interruptions. You can learn more about this difference in the article Dry vs wet underfloor heating: what polystyrene and foil are suitable.
Overview of system board types from practice
Stud boards for wet systems (STIROTERMAL BASIC and DUO)
Classic stud boards are undoubtedly the most widespread type. System insulation board UHP55 (STIROTERMAL BASIC) is a typical representative of this category – EPS with a strength of CS(10)100 kPa, 55 mm thickness, stud surface for Ø16–20 mm pipe at 50 mm spacing. It is a proven choice for common residential applications: family house, apartment, holiday cottage.
If you need more in one package – e.g. if you want a board that has a laminated foil on the top side (which acts simultaneously as separation and prevents cement slurry from penetrating the board), reach for the System insulation board UHP51 (STIROTERMAL DUO 11). DUO 11 has a thickness of 55 mm, but the combination of basic EPS with a smooth surface layer and laminated foil simplifies installation – there is no need to lay a separate separation foil.
In practice, I most often encounter BASIC where there is an experienced installation crew that can lay the separation foil quickly and correctly. The DUO version is somewhat more expensive but speeds up the work and reduces the risk of errors – it's appreciated by smaller companies with less experienced workers or when working in tight spaces.
Boards for dry systems (STIROTERMAL DRY)
For dry underfloor heating, the Polystyrene for dry underfloor heating UHPD (STIROTERMAL DRY) is designed. This type has milled grooves for placing aluminum distribution plates and the pipe. It is used together with aluminum foil for dry underfloor heating, which ensures even distribution of heat into the flooring layer.
Dry systems with STIROTERMAL DRY are ideal for:
- Renovations in old apartments, where the floor level cannot be raised by 70–80 mm (wet system), but only by 30–40 mm
- Wooden ceilings (beam structures), where a wet system is not an option due to weight reasons
- Spaces where an immediate response to temperature changes is desired
- Summer houses and cottages, where heating is not continuous
Separation foil: when is it mandatory and when is it optional
The question of separation foil is common. Short answer: with a cement screed, you always need it. With anhydrite, it depends on whether the board has a laminated layer or not.
The reason is simple: cement screed has a pH of around 12–13 (a strongly alkaline environment) and chemically attacks the surface of EPS if it is not protected. Anhydrite is more neutral, but it still penetrates the pores of EPS and reduces its insulating properties. Separation foil 601001H is laid on the board before pouring the screed and simultaneously acts as a chemical barrier and a vapor barrier – preventing moisture from the screed from escaping downward, which is important with cement mixtures.
With STIROTERMAL DUO 11, the foil is integrated directly into the product – you just need to tape the joints. With STIROTERMAL BASIC and other stud boards without lamination, the separation foil needs to be laid separately, with overlaps of at least 10 cm and extended onto the walls behind the expansion strip. We cover this topic in more detail in the article Separation and aluminum foil for underfloor heating: when and how to use them.
Specific project scenarios – what I've seen in practice
Scenario 1: New family house build on a concrete slab above ground
This is probably the most common case. The concrete foundation slab is finished, electrical wiring and sewage are already laid on it, now underfloor heating needs to be installed. The heat loss of a ground-floor room is typically 30–50 W/m². What is done here:
Under the system board, 80–100 mm of XPS or EPS goes in (depending on the height level) as the primary insulation. On top of that comes the system stud board with a thickness of 30–55 mm. The total insulation thus reaches 110–155 mm, giving an R value of 2.8–4.0 m²K/W – fully in line with the standard and with realistically achievable heat losses. Without the bottom insulation, you would lose tens of percent of the thermal output toward the ground, and the system would have to work with higher water temperature, which reduces the efficiency of the heat pump.
Scenario 2: Apartment renovation in a panel building – ceiling above a cellar
Here the situation is different: the maximum permissible floor rise is 60–70 mm (limitations of doors, thresholds, connection to the corridor). The system board must be as thin as possible while maintaining sufficient insulation. Typical solution: a system board with a stud height of about 11 mm + EPS base of 30 mm (total 41 mm), or a 30 mm board without studs with 20–30 mm of bottom insulation. Above a cellar (unheated), the thermal resistance is usually borderline and must be carefully calculated.
Scenario 3: Wooden beam ceiling structure
Here a wet system with anhydrite is not possible – the weight is too high (65 mm of anhydrite = 130 kg/m², which is much more than the load-bearing capacity of old wooden ceilings). The solution is dry underfloor heating with STIROTERMAL DRY, where the system is much lighter. The total weight of the dry system (board + aluminum plates + OSB/fiber-gypsum board) is typically 25–40 kg/m², which is acceptable for a wooden ceiling.
Scenario 4: Bathroom with tiling and floor covering
A bathroom is specific due to increased humidity. Here, the quality of the separation foil and its correct connection to the waterproofing coating are critically important. The EPS must have a closed cell structure or be surface-protected to prevent moisture absorption. In bathrooms, the pipes should also be laid with a higher density (spacing 100–150 mm), because heat losses are higher due to forced ventilation, and tiles have higher thermal resistance than vinyl or wood.
What to watch out for when ordering and installing
On projects, I've seen unnecessary mistakes that resulted from carelessness when ordering or from underestimating details. Here are the most common ones:
- Mixing boards from different manufacturers – studs from different manufacturers may have different heights, causing steps between boards and uncontrolled thermal bridges at the transition from one board to another.
- Incorrect board orientation – some boards are asymmetric and must be laid with a specific side up. If you turn them around, the stud pattern doesn't match, and the pipe cannot be laid properly.
- Omitting the expansion strip – the expansion strip must run around the entire perimeter of the room and at door transitions. Without it, the screed cracks. Most system boards include it in the package, but some don't – check this.
- Insufficient sealing of joints – with a cement screed, if the joints between boards are not taped or covered with foil, cement slurry flows into the gaps and mechanically bonds the screed to the board, creating local thermal bridges and a risk of cracking.
- Laying pipe without adequately calculating heat losses – laying the pipe at the same spacing across the entire room is a mistake. Edge zones (near windows, exterior walls) need a higher pipe density than the center of the room.
You can find more about these mistakes and how to prevent them in the article Common mistakes when installing polystyrene and system boards for underfloor heating.
Table comparison of key parameters of system boards
| Parameter | BASIC (UHP55) | DUO 11 (UHP51) | DRY (UHPD) |
|---|---|---|---|
| Board thickness | 55 mm | 55 mm | 30–50 mm (depending on variant) |
| Heating system | Wet | Wet | Dry |
| Compressive strength | CS(10) 100 kPa | CS(10) 100 kPa | CS(10) 100 kPa |
| Integrated foil | No | Yes (laminated) | Separate aluminum foil |
| Pipe fixation | Studs | Studs | Grooves / milling |
| Separation foil needed? | Yes (separate) | No (integrated) | Aluminum foil |
| Suitability: wooden structure | No | No | Yes |
| Typical price (approximate) | Lower | Medium | Medium–higher |
Thermal bridges with system boards – where they occur and how to prevent them
A thermal bridge in a system board is not something you would see with the naked eye, but it shows up unpleasantly in the energy balance. Where do they most often occur?
At board joints: If the boards do not have a tongue-and-groove joint (step edge) and the joints between them are not taped, a direct thermal path forms through the joint. Solution: system boards with a tongue-and-groove joint or thorough taping of joints with aluminum tape.
At risers and pipes passing through the board: If a pipe (e.g. a riser) passes through the board and there is an air gap around it, heat escapes. Solution: wrap the riser with insulation (mineral wool or tubular insulation) and seal the gap.
At transitions between rooms with different temperatures: If one room is heated by underfloor heating and another is not, or they have different temperatures, there must be an expansion joint and insulation at the transition. Otherwise a direct bridge forms.
You can find more on this topic in the article How to correctly lay pipe in a system board without thermal bridges.
Economic perspective: is it worth paying more for a better board?
This is a question I get from every other customer. The answer is more nuanced than a simple yes or no.
The price difference between BASIC and DUO 11 is usually 15–25% in favor of the board with integrated foil. If we're talking about a typical 80 m² apartment, that's a difference of a few tens of euros in materials. The labor savings (one worker saves 2–4 hours by not laying the foil separately) usually compensate for or even exceed this difference. From this perspective, DUO 11 usually comes out economically better for larger areas.
Where is it worth investing in a thicker board or better EPS? Precisely at the upper threshold of energy certification. If you're on the border between energy class A0 and A1, better insulation under the underfloor heating can be decisive – and in the long run, every 10% of heat savings shows up over a decade of operation.
Frequently Asked Questions (FAQ)
Can I use ordinary facade polystyrene instead of a system board?
No, for several reasons. Facade EPS does not have studs for fixing the pipe – you would have to fasten the pipes with clips or plaster, which is time-consuming and less reliable. Facade EPS also has different mechanical parameters (usually lower compressive strength) and is not certified for use in contact with anhydrite or screed. You'll save on material purchase, but lose several times more on labor and risk problems with the system's functionality.
What is the difference between EPS and XPS for system boards?
EPS (expanded polystyrene) has lower thermal conductivity than XPS (extruded polystyrene) at the same density, but XPS has better moisture resistance and higher compressive strength. In practice, system boards are made almost exclusively from EPS, because the stud pattern is easier to form. XPS is used as the bottom insulation under the system board or in applications where the risk of moisture is higher (e.g. ceramics in contact with the ground). Stud system boards made of XPS exist, but they are considerably more expensive.
Do I need to add a separation foil with the dry system (STIROTERMAL DRY)?
With a dry system, aluminum foil for dry underfloor heating is used instead of a separation foil. This serves a different function than the separation foil in a wet system: it primarily distributes and reflects heat from the pipe upward to the flooring layer. Separation foil 601001H is not needed in a dry system, because there is no anhydrite or cement screed that would need to be separated from the EPS.
What minimum system board thickness is required for a typical apartment?
For an apartment on an intermediate slab (above a heated space), the minimum thermal resistance according to STN EN 1264 is R = 0.75 m²K/W. This corresponds to a thickness of about 28 mm at λ = 0.036 W/mK, which in practice means a board thickness of 30–40 mm. For a ground floor above an unheated basement, the minimum is R = 1.25 m²K/W, i.e. at least 45 mm of EPS, with 60–80 mm recommended in practice. If the exterior is below the floor (ceiling above a passage), the standard requires R = 2.0 m²K/W or more, which requires a combined structure. This topic is covered in detail in the article What polystyrene thickness is needed under underfloor heating.
How do I know that the system board is laid correctly and I can pour the anhydrite?
Before pouring, check: (1) all boards are firmly connected and don't lift when stepped on, (2) the joints are taped or the boards have a tongue-and-groove joint, (3) the separation foil is laid with overlaps of at least 10 cm and extended onto the walls above the screed level, (4) the expansion strip is installed around the entire perimeter of each room, (5) all pipes have been pressure-tested and are under pressure (min. 6 bar), (6) the pressure test lasts throughout the entire pouring period and at least 24 hours afterward.
What to do if an air pocket has formed under the boards (the board moves)?
An air pocket under a system board most often occurs due to poor substrate preparation – an uneven, crumbly, or non-cohesive surface layer. The board should be laid on a firm, level, and dry substrate. If a board "springs", it should be lifted, the cause removed (fill in, pour a thin layer of filling compound or leveling compound), and the board laid again. Under anhydrite, the board must not lose contact with the substrate – as it dries, the anhydrite would sag and crack.
Conclusion: choosing the right board pays off
A system insulation board is not a place to save money. It is the foundation of the entire system that you will use for 20–30 years. A correctly chosen board – with an appropriate insulation thickness, correct compressive strength, compatible geometry for your pipe diameter, and with an integrated or separately added foil – is the difference between a system that works efficiently for decades and a system that starts showing problems just a few years after installation.
If you're not sure which board is best suited for your specific situation, take a look at the entire range in the category Polystyrene, system boards, where you'll find an up-to-date overview of available products with technical data sheets. And if you come across a specific technical question regarding installation, also check out our detailed guide Installing a system insulation board for underfloor heating step by step.
Have a question on this topic?
Not sure how to decide, or dealing with a specific situation in your home? Write to us - we'll be happy to help.