Frequently Asked Questions About Polystyrene and System Boards for Underfloor Heating

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Frequently Asked Questions about Polystyrene and System Boards for Underfloor Heating

Underfloor heating is now standard in new buildings and increasingly common in renovations as well. Although it is a relatively simple principle, in practice many questions arise precisely regarding the selection and installation of the insulation layer – that is, the polystyrene and system boards. We have seen projects where the polystyrene was too thin, the boards were the wrong type, the foil was omitted, or the pipes were laid without any system at all. The result? A long system warm-up time, higher energy consumption, cracking screed, or a warped floor. This article answers the questions we receive most often – from the basic function of polystyrene to the details that determine whether the whole structure works properly.

What does polystyrene actually do under underfloor heating?

The first thing to understand: polystyrene under underfloor heating does not primarily function as thermal insulation for the room in the classic sense. Its main task is to direct heat upward – into the floor and into the room – and prevent energy from escaping downward, into the ceiling slab, foundations, or intermediate floor structure. We are therefore talking about a reflective and insulating function in one direction.

When a pipe with hot water (or an electric cable) lies on an uninsulated base, heat spreads evenly in all directions. The concrete slab beneath it absorbs just as much heat as the floor above it. Polystyrene with sufficient thickness and correct thermal resistance significantly limits this loss – in practice, without insulation, sometimes 30–40% of the energy goes "into the ground" instead of into the room.

In addition to this primary function, polystyrene also provides impact sound insulation – especially valuable in intermediate floor structures – and mechanically supports the screed to distribute the load evenly.

What is the difference between plain polystyrene and a system board?

This is question number one we get from people tackling this for the first time. The answer is fairly simple, but the consequences are significant.

Plain EPS polystyrene (expanded polystyrene panels) is a flat board without any relief. It provides thermal and sound insulation, but the pipe cannot be anchored into it in any way. The installer must fix the pipe using clips, fasteners, or spike rails, which extends the work and increases the risk that the pipe will shift when the screed is poured.

A system board has protrusions on its surface (studs, projections, profiles, or grooves) into which the pipe is inserted and held automatically at the required spacing. No additional fixation is needed, the spacing is automatically maintained, and the entire installation is faster and more precise. In addition, most system boards have a reflective (aluminum) layer on the surface, which improves heat distribution.

In practice, plain polystyrene without a profiled surface is now used more as a bottom insulation layer under a system board (when a greater overall insulation thickness is needed), or for dry systems, where the pipe is laid into special grooves between the panels.

Specific examples: System insulation board UHP55 (STIROTERMAL BASIC) is a classic stud board for wet systems – the pipe is inserted between the studs, and the screed is poured directly onto the board. Conversely, Polystyrene for dry underfloor heating UHPD (STIROTERMAL DRY) is a solution for dry systems, where there is no screed or concrete at all – the pipe lies in grooves and heat is distributed via aluminum plates.

What thickness of polystyrene is actually needed?

This topic is discussed in detail in a separate article "What thickness of polystyrene is needed under underfloor heating"; here we will look at practical shortcuts for common situations.

The STN EN 1264 standard (and related standards for floor thermal insulation) sets minimum values for the thermal resistance of the insulation layer under underfloor heating. For a floor above an unheated space (e.g., above a garage, above ground, above a basement), the minimum thermal resistance of the insulation is 0.75 m²K/W; for a floor between heated floors (where it's only about acoustics and redirecting heat upward), 0.50 m²K/W is sufficient.

What does this mean in practice, in millimeters of polystyrene?

Above an unheated basement or ground: minimum 80–100 mm EPS (with lambda 0.036–0.040 W/mK)
Above a garage or exterior: 100–140 mm, ideally more – commonly 200 mm in passive houses
Intermediate floor (heated space above and below): 30–50 mm is sufficient to meet the standard, but 50 mm is a reasonable minimum also from an acoustic standpoint
Renovation with limited floor height: system boards with a thickness of 20–30 mm exist, which are technically at the limit, but combined with a bottom layer of rigid insulation they are acceptable

Note: the system board alone (e.g., a 35 mm stud board) usually does not meet the thermal resistance requirements for a floor above the exterior – lower layers of flat EPS must be added. The system board addresses pipe fixation, not the entire thermal insulation.

Why are there different types of system boards on offer, and which one to choose?

If you have looked into the category Polystyrene, system boards, you have noticed there are several types of boards. This is not a marketing trick – each has its own justification for a specific type of building and system. You can find a detailed comparison in the article "Comparison of system boards: STIROTERMAL BASIC vs DUO vs DRY vs SOLO"; here are practical shortcuts:

STIROTERMAL BASIC (UHP55) – a classic stud board for wet systems with anhydrite or cement screed. The height of the studs allows a pipe of ∅16–20 mm to be easily snapped in, and the screed covers both the studs and the pipe. The board thickness including studs is 55 mm. Suitable for new buildings with sufficient structural height.

STIROTERMAL DUO 11 (UHP51) – this system insulation board UHP51 has a double profiled layer, which improves heat distribution and allows multiple pipe spacings without additional aids. Suitable where more precise control of pipe density is required (e.g., perimeter zones with wider spacing, zones near walls with narrower spacing).

STIROTERMAL DRY (UHPD) – for dry underfloor heating without screed. The pipe is laid into grooves milled into the polystyrene, aluminum plates are placed on top, and then the dry flooring (e.g., OSB, gypsum fiber boards). The advantage is much faster installation, no "wet phase," and quick commissioning. Suitable for renovations or wooden beam ceilings.

A practical example: a customer is renovating a cottage with wooden beam ceilings. Pouring a screed is unthinkable – weight, moisture, time. Here, STIROTERMAL DRY is a clear choice. Conversely, a developer building an apartment block with industrial concrete ceilings – there, STIROTERMAL BASIC is a fast and economical choice.

What is separation foil used for, and when should it be used?

Separation foil is a thin polyethylene film that is laid on the system board before pouring the screed. It has three tasks:

Prevent cement slurry from leaking into the insulation layer – cement slurry would fill the air gaps in the polystyrene, increase its thermal conductivity, and weaken the acoustic function.
Allow sliding movement of the screed relative to the insulation – the screed must be able to move freely during thermal expansion and contraction, and the foil forms a sliding layer (hence the term "separation").
Prevent moisture from wicking up from the substrate in case the system board itself is not vapor-tight.

Separation foil 601001H is designed exactly for this purpose. The foil is laid with overlaps of at least 100–150 mm, and the joints are taped. Note – the foil goes under the screed, above the system board. Not under the board (that is a vapor barrier, which has a different function).

The foil is not needed for a dry system (STIROTERMAL DRY), where there is no screed. However, it is always required for any wet system – whether anhydrite or cement screed.

What is aluminum foil and when do I need it?

Aluminum foil has a completely different function than separation foil. It is a reflective layer whose task is to distribute heat and reflect it upward.

In wet systems with stud boards, the aluminum layer is usually integrated directly into the surface of the system board (laminated). For dry systems without screed, it's different: without aluminum, heat from the pipe would rise only in spots (hot directly above the pipe, cold between the pipes). Aluminum foil for dry underfloor heating is laid above the pipes and ensures even heat distribution across the entire floor area.

A typical real-world situation: a customer installed dry underfloor heating without aluminum plates, with only an OSB board placed directly on the pipes. The result? You could feel exactly where the pipes were – walking barefoot, you could feel warm stripes alternating with cold ones. Thermal comfort was poor. After replacing and adding aluminum, the problem disappeared.

How is a system board installed – a practical procedure in brief

A detailed procedure is available in a separate article "Installing a system insulation board for underfloor heating step by step." Here we summarize the key points that are most often omitted during poor-quality installation:

The substrate must be level and clean. Unevenness greater than 5 mm under the boards causes the board to "work" under load, which can lead to screed cracking. Level the substrate before laying the boards.
Edge expansion tape. Before laying the boards, tape the entire perimeter of the room with expansion tape (also around columns, protrusions, pipes). A tape thickness of 8–10 mm is the minimum. Without it, the screed has nowhere to expand and it cracks.
Boards are laid with staggered joints. Just like bricks – joints of adjacent rows must not align on a single line. System boards have a tongue and groove on the side edges, or straight edges with a lock – always follow the manufacturer's instructions.
The pipe is laid into the studs, not on top of them. The pipe must be snapped in between the studs and sit down low – not hang on top of the studs. Incorrect pipe placement leads to thermal bridges and lower efficiency.
Pressure test before pouring the screed. Always pressurize the pipe to operating pressure (min. 6 bar, in practice 4–6 bar) before any wet process, and keep it under pressure throughout the entire pouring and curing process of the screed.

Wet vs. dry system: what does it mean for choosing polystyrene?

The wet/dry distinction is crucial for material selection. This topic is covered in a separate article "Dry vs. wet underfloor heating: which polystyrene and foil are suitable"; here we summarize the practical implications:

Wet system (anhydrite or cement screed) requires:

A system stud board (e.g., STIROTERMAL BASIC or DUO)
Separation foil above the board
Edge expansion tape
Sufficient screed thickness (min. 45 mm above the top of the pipe for anhydrite, 55–65 mm for cement)

Dry system (without screed, e.g., OSB + aluminum) requires:

Polystyrene with milled grooves for the pipe (e.g., STIROTERMAL DRY)
Aluminum plates / aluminum foil above the pipe
A dry floor board (OSB, gypsum fiber boards, min. 18–22 mm)
Separation foil is not needed here (there is no wet process)

Common installation mistakes that really do happen

Over the years of practice, we have seen the following recurring mistakes. The article "Common mistakes when installing polystyrene and system boards under underfloor heating" discusses them in detail; here we summarize the most dangerous ones:

1. Omitted or insufficient edge expansion tape. This is cause #1 of screed cracking. Without the tape, the screed has no room for thermal expansion and cracks – first around the perimeter, then in the middle. Sometimes the cracks are so large that you can see light through the floor down to the pipes. Repair is costly and usually means demolishing the entire floor.

2. Board joints on the same line. If the board joints are not staggered (not laid with an offset), a structural weakness occurs at those points. The screed above the joint cracks, sometimes appearing on the wearing layer as a visible crack.

3. Insufficient polystyrene thickness for the building's position. A customer in a flat area with a garage under the apartment saved on polystyrene and used only a 30 mm stud board without bottom EPS. Result after two years: cold floor on the ground floor, the system runs at maximum output and still isn't enough – energy is escaping down into the unheated garage. Fixing this situation is practically impossible without demolition.

4. Screed poured without pressurized pipes. If the pipe is empty and loaded with screed, it can deform – oval instead of round cross-section. This increases hydraulic resistance, distributes the load unevenly, and in extreme cases can crack the pipe during the first pressurization. Always pour the screed with the pipe under pressure of at least 4–6 bar.

5. Missing or damaged separation foil. Cement slurry leaks into the insulation board, degrades its thermal properties, and creates rigid bonds that prevent expansion. Result: screed cracking and reduced thermal efficiency. Why does this happen? The foil is laid in the morning, pouring happens in the afternoon, and in the meantime the foil gets torn from workers walking on it. Solution: tape the foil immediately after any damage.

How to correctly lay the pipe into the system board

The pipe is laid into the system board in what is called a serpentine or bifilar pattern. The serpentine pattern (spiral) is more suitable for larger rooms – the warm and cold pipe alternate next to each other, so the surface temperature difference is minimal. The bifilar pattern (parallel) is easier to install, but for long circuits a visible thermal gradient occurs (the inlet is much warmer than the outlet).

Pipe spacing depends on calculation, but typical values are:

100 mm – perimeter zones near exterior walls, bathrooms, very cold spaces
150 mm – standard living room with good insulation
200 mm – very well insulated new buildings with low heat loss
300 mm – sometimes used in passive houses, where underfloor heating serves more as tempering

Stud boards usually have studs on a 50 mm grid, so a spacing of 100, 150, 200, or 250 mm is easily achieved. This topic is covered in more detail in the article "How to correctly lay pipes into a system board without thermal bridges."

Why does the floor crack or warp over time?

This is the topic of a separate article "Why does the floor above the insulation board crack or warp." In brief: there are several causes, and polystyrene is not always to blame. The most common reasons are:

Increasing the temperature too quickly during the first system "warm-up" – the screed must not be heated abruptly; the first warm-up should be gradual: 2–5 °C / day, max 5 °C per day, for at least 4 days, then to the maximum operating condition
Insufficient expansion joints in rooms larger than 40 m² or in long corridors
Wearing layer without suitable adhesive / no expansion gap left for wooden floors
Mechanical damage to the insulation board before pouring the screed (walking on the boards with pointed tools, e.g., a stapler)

Frequently Asked Questions (FAQ)

Can I use ordinary facade EPS 70 or EPS 100 polystyrene under underfloor heating instead of a system board?

Yes, you can – but only as a bottom insulation layer. EPS 70 and EPS 100 are suitable base insulations (they have sufficient compressive strength of 70 or 100 kPa at 10% deformation), but they have no studs or grooves for fixing the pipe. Without a system board on top, you will have to fix the pipe with staples into the polystyrene, and the risk of the pipe shifting when the screed is poured is higher. Always use a system stud board for the layer directly under the pipe. A combination of bottom EPS + top system board is a common and correct solution for greater insulation thicknesses.

What is the difference between anhydrite and cement screed in terms of requirements on the polystyrene?

Both types of screed place the same requirements on the polystyrene in terms of compressive strength (min. EPS with CS(10)100 kPa). The difference is that anhydrite screed is more fluid and leaks into the studs and under the foil more easily, so the separation foil is absolutely critical with anhydrite and must not have even the slightest damage. Cement screed is denser, less "flowing," but places a greater load on the polystyrene. Both require the pipe to be pressurized during pouring.

What happens if I use polystyrene that is too thin?

There are two direct consequences: increased heat loss downward (lower energy efficiency, higher consumption) and a slower system warm-up and response. Polystyrene forms a "reservoir" of thermal energy – the thinner it is, the lower the thermal capacity, which can be an advantage or disadvantage depending on the control system. However, from an energy standpoint, insufficient thickness is always disadvantageous. For a floor above the exterior or an unheated space, the customer will feel the thin insulation immediately in their heating bills, while remediation is extremely costly (all layers would have to be demolished).

Are system boards also suitable for electric underfloor heating?

Yes, stud system boards can also be used for electric underfloor heating with heating mats or loose cables. The electric cable is inserted between the studs just like a water pipe. The choice of system board depends on the cable diameter – for thinner cables (∅5–7 mm), boards with smaller studs are suitable. For dry electric heating (foil heating), a flat insulation board with a reflective layer is recommended instead. The same principle applies: heat must be directed upward, not into the substrate.

Can I lay system boards in wet areas (bathroom, pool)?

EPS polystyrene is inherently resistant to moisture – it does not absorb water, does not mold, and does not rot. System boards made of EPS are therefore suitable for wet areas. However, note that for pools and wet zones, you must ensure waterproofing of the screed and covering, not the polystyrene. The system board itself can handle moisture, but if water penetrates to the pipes or joints, it can cause damage elsewhere. For bathrooms and wet zones, address waterproofing above the screed (sealing compounds, waterproofing strips) – this is a layer independent of the system board.

How long does it take before I can walk on the laid system board before pouring the screed?

The system board itself is practically walkable immediately – EPS with a load capacity of CS(10)100 kPa can bear the load of normal worker movement. However, watch out for damage to the studs from sharp objects and for deformation of the foil. Wait with any heavy loads (pallets of material, heavy tools) until the screed has been poured and cured. Before pouring the screed, always check the entire area: whether all pipes are correctly snapped in, the foil is undamaged, the edge tape is attached everywhere, and the pipe is under pressure.

Conclusion: Why it pays to do it right from the start

Polystyrene and system boards are layers that become invisible and inaccessible once covered with screed. This is exactly why their correct selection and installation deserve proper attention – any mistakes can only be corrected by demolishing the entire floor, which is many times more expensive than the cost of the entire insulation layer. Customers who saved a few tens of euros on the thickness of polystyrene or the type of system board out of the total investment in a renovation or new build usually regret it before they've spent their first winter in the new house.

The right choice starts with a specific product: for a classic wet system it might be STIROTERMAL BASIC (UHP55), for renovations and dry systems STIROTERMAL DRY (UHPD), for more demanding applications with variable pipe spacing STIROTERMAL DUO 11 (UHP51). Add to that the right foil, edge tape, and correct procedure – and the result will last for decades.

If you are not sure which product is suitable for your specific situation, check out other articles in the Knowledge Center – especially "How to choose the right system insulation board for underfloor heating" and "Separation and aluminum foil in underfloor heating: when and how to use them." These articles, together with this FAQ, form a complete guide from selection to installation.

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