Dry vs wet underfloor heating: which polystyrene and foil are suitable

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Dry vs. wet underfloor heating: what it actually means and why it matters

When deciding on an underfloor heating system, one of the first and most important technical questions is: will you go the wet way, or the dry way? This choice directly determines what type of polystyrene, system board and foil you will need underfoot – and therefore what you need to order, how it is installed and what results you can expect. In practice, I often see people confusing these terms, combining the wrong components, or getting confused on site. So here I'll break down both systems from the ground up, show where they differ in physics as well as in implementation, and specifically say which boards and foils belong to which system.

Wet underfloor system: pipes cast in anhydrite or concrete

The wet system is the classic approach. The pipe – whether PEX, PEX-Al-PEX or PE-RT – is laid into the grooves of a system board or fixed with clips onto a smooth insulation board, and the whole thing is cast with a layer of anhydrite or cement screed, typically 45–65 mm thick above the pipe. Anhydrite is dominant in underfloor heating today because it conducts heat better, can be laid in a thinner layer (minimum 35–45 mm above the pipe) and cracks less. Cement screed requires greater thickness and attention must be paid to expansion joints.

Physically, it works like this: the screed layer forms a heat storage mass that heats up slowly and cools down slowly. The water temperature in a wet underfloor heating circuit typically ranges between 35 and 55 °C, while the floor surface temperature should not exceed 29 °C in living areas and 35 °C in bathrooms. The system is robust, durable and has excellent thermal inertia – if you want fast control (e.g. heat recovery ventilation with solar gain in transitional seasons), a wet underfloor system is not the ideal choice, because the screed responds to thermostat adjustments with a delay of several hours.

Which boards belong to the wet system

For the wet system you need a system insulation board that:

has sufficient compressive strength to bear the weight of the screed during and after pouring (minimum CS(10)150 kPa, i.e. 150 kPa at 10% deformation, ideally CS(10)200 or more for anhydrite)
contains fixing studs or channels for laying the pipe at the required spacing (100, 150, 200, 250 mm)
has an edge tongue and groove or another connecting method without thermal bridges
is covered with a foil or laminate that prevents anhydrite from seeping under the board

A typical example of a product for the wet system is the System insulation board UHP55 (STIROTERMAL BASIC). This is a classic studded board made of expanded polystyrene EPS with a compressive load rating suitable for both anhydrite and cement screeds. The studs are arranged so that a pipe of 16 × 2 mm or 17 × 2 mm can simply be pressed into the grid without any additional fixing accessories – the studs hold the pipe in the chosen position. Edge joints are solved with tongue and groove, which eliminates thermal bridges in the seams. This board is laid directly onto a cured subfloor concrete or an existing floor (in renovations), while ensuring the substrate is level (tolerance max. ±3 mm over a 2-metre straightedge).

For situations where you need slightly wider stud spacing or a combination with an edge strip matching the board thickness, the System insulation board UHP51 (STIROTERMAL DUO 11) is suitable. It is designed with a double layer – a studded working surface on top and a smooth insulation layer below – which gives a better overall thermal resistance at the same construction height. It's suitable for projects where you want to save on floor build-up thickness but don't want to go below the recommended thermal resistance values (typically R ≥ 0.75 m²K/W for floors above heated spaces and R ≥ 1.25 m²K/W for floors above unheated spaces according to standards).

Dry underfloor system: pipe without concrete, heat via aluminium heat-distribution plates

The dry system is a completely different principle. The pipe here is not laid in wet screed, but inserted into pre-milled channels in special polystyrene, with aluminium heat-distribution lamellas (profiles) lying above the pipe. These lamellas carry heat from the pipe laterally across the surface and heat the dry working layer (most often two layers of OSB boards, MDF, plasterboard, special cement-fibre boards, or cast floors with a minimal thickness). The result: the system has almost no thermal storage mass, responds to control much faster (in the order of tens of minutes, not hours), the construction height can be significantly lower and there's no need to wait for the screed to dry after installation.

The dry system is particularly suitable:

for renovations where it's not possible to increase the floor height by an extra 80–100 mm
for wooden beam ceilings, where a wet screed is not structurally suitable
for spaces with intermittent heating (cottages, offices with weekend breaks), where fast response pays off
for projects requiring very quick installation without a technological pause

A specific product for this system is Polystyrene for dry underfloor heating UHPD (STIROTERMAL DRY). Unlike studded boards, it has no studs, but instead has a precise network of milled channels of various widths (according to pipe diameter – typically 16 mm or 20 mm). The pipe is laid into these channels without forced bending, which is an advantage especially with larger diameter pipes. An aluminium heat-distribution plate then lies on top of the channels, creating a thermal bridge between the pipe and the wear layer.

Key difference: compressive strength and board surface type

Compressive strength is precisely the technical parameter that divides these two worlds. In the wet system, the board is pressed on by a column of anhydrite or concrete, which creates hydrostatic pressure during pouring, and after curing there is an additional mass of roughly 100–130 kg/m² above the polystyrene. In the dry system, instead there are OSB boards 18–22 mm thick and the wear layer – the total load is several times lower, but the flatness of the channels and precision of spacing are all the more important, because the aluminium lamellas must fit perfectly.

The conclusion: never insert a board designed for the dry system (UHPD / STIROTERMAL DRY) into a wet system. Its channels are not designed to hold anhydrite in place, and above all – it has no studs, so the pipe is held in a different way. Likewise, the reverse doesn't work either: you cannot use a studded board for the wet system as a base for the dry system, because the studs would create air gaps under the OSB and the OSB would not be flat or stable.

Separation foil for the wet system: what it does and why it's mandatory

With the wet system, there's one component that's very easy to forget during installation, but without which the whole system becomes problematic: the separation foil. This is a layer of polyethylene foil laid either directly on the system board (if the board doesn't have a pre-laminated surface) or as separation between the polystyrene and the anhydrite.

The Separation foil 601001H serves this purpose. Its function is threefold:

Preventing infiltration of the pour: Anhydrite and cement slurry are very fluid, and without a barrier they would flow through the seams of the boards under the polystyrene, where they would create cold bridges and degrade the thermal insulation.
Shrinkage separation: The screed changes volume as it cures. The foil allows it to slide on the substrate without transferring stresses to the polystyrene or the structure. Without the foil, cracks could appear in the screed concentrated exactly above the board seams.
Vapour tightness: The foil prevents moisture transfer from the substrate (e.g. if it's a slab on grade without waterproofing) into the thermal insulation.

The foil is laid with an overlap of at least 100–150 mm between individual strips and must also be carried up behind the edge strips. Joints are taped with adhesive tape – this is a detail that in practice is often forgotten during faster-paced installation, and the result is that anhydrite penetrates under the system board and creates cold bridges. The problem then shows up on the thermostat's temperature reading – the floor doesn't reach the required surface temperature even at maximum system parameters.

Aluminium foil for the dry system: different function, different placement

The foil for the dry system has a completely different role. Aluminium foil for dry underfloor heating is not a separation layer from the pour – because there's no pour here at all. Its function is thermal reflection and heat distribution from the space between the pipes and the OSB working layer.

Here's how it works: the aluminium heat-distribution lamellas carry heat from the pipe upward, but the spaces between the lamellas are air pockets. Air conducts heat poorly, and could also radiate heat downward (into the polystyrene, where it would be wasted). Aluminium foil placed on the surface of the EPS UHPD (or under the OSB layer) reflects infrared radiation back upward and significantly improves the evenness of the temperature distribution across the floor surface. In practice, this means the floor is warmer and more even, and the energy consumed is used more efficiently.

The aluminium foil is laid over the entire EPS surface after the pipe has been installed, but before laying the OSB boards. Overlaps between strips should be at least 50 mm, and joints are again taped with aluminium adhesive tape. The foil must be smooth without folds that would prevent the OSB from sitting flat. At the edges, the foil avoids the edge strip – which remains free.

Comparison: which system where and why

For an overview, here are specific scenarios from common installations:

New-build family house, concrete slab on grade, all rooms: The wet system with anhydrite is the clear choice here. You would use the STIROTERMAL BASIC UHP55 or STIROTERMAL DUO UHP51 system board (depending on the thickness requirement), separation foil, edge strips and anhydrite screed min. 45 mm above the pipe. The total build-up height (EPS + screed) ranges from 80 mm (thinner board, 45 mm anhydrite) to 120 mm or more. Advantage: robustness, long lifespan, excellent heat output.

Apartment renovation, old cement screed, living room: This depends on the available height. If you can add 60–80 mm, the wet route with a thin anhydrite screed is feasible. If you only have 35–50 mm of space, the dry system STIROTERMAL DRY with OSB and vinyl flooring is more realistic. Keep in mind that the dry system has lower output per m² (roughly 15–25% less than wet) due to poorer thermal contact.

Wooden beam ceiling above a basement: Here the wet system is unsuitable for structural reasons – wet anhydrite weighs 100–130 kg/m², which in many cases is not permissible for historic beam ceilings. The dry system with EPS UHPD, aluminium foil, OSB and wear layer has a weight that is 70–80% lower. You just need to correctly assess the beam load capacity and calculate the rest with a designer.

Bathroom renovation: Here many people make a mistake – when there's enough height, they go straight for tiles on the wet system with a thin anhydrite layer. The dry system is problematic in a bathroom, because OSB is not suitable for humid environments without special treatment and detailed waterproofing solutions. Tiles above anhydrite are ideal for a bathroom – they have excellent thermal conductivity and tile adhesive creates a sufficient bond.

Thermal resistance of boards: numbers that determine consumption

For proper functioning of underfloor heating, the thermal insulation under the pipes must have sufficient thermal resistance Rd to direct heat upward (into the room), not downward (through the ceiling to neighbours or into the ground). The values used in projects come from the STN EN 1264 standard and from the thermal engineering requirements applicable in Slovakia.

Approximate minimum thermal resistance Rd values for the insulation layer:

Floor above a heated space (e.g. above a living room in the apartment below): Rd ≥ 0.75 m²K/W
Floor above an unheated space (garage, basement): Rd ≥ 1.25 m²K/W
Floor on grade in low-energy standard: Rd ≥ 1.50 m²K/W or more

Standard EPS with a declared value of λ = 0.035 W/(mK) at a thickness of 30 mm achieves Rd = 0.030 / 0.035 = 0.857 m²K/W, which covers the first scenario. For floors above unheated spaces, a thickness of at least 45–50 mm of this EPS is needed. Studded system boards have a slightly higher λ (around 0.036–0.038 W/mK) due to the studs and possible laminate surface, so actual thermal resistance for specific products is always stated in the manufacturer's technical documentation – always check it when designing. More on this topic can be found in the article What thickness of polystyrene is needed under underfloor heating and in the Comparison of system boards: STIROTERMAL BASIC vs DUO vs DRY vs SOLO.

Edge strip: a component not to be underestimated

In both systems – wet and dry – an edge expansion strip is essential around the entire perimeter of the room and around all penetrating structures (columns, pipework, door frames). The edge strip has two functions:

Expansion of the screed or OSB: Anhydrite screed changes volume during curing and with temperature changes. Without an edge strip, the screed would flow into the edge polystyrene, and as it hardened would crack or lift tiles near the wall. The dry OSB layer also expands with changes in humidity and without a gap would press against the wall.
Breaking the thermal bridge at the perimeter wall: The edge strip made of a softer material (PE foam, low-density EPS) reduces heat flow from the screed into the wall, which would otherwise show up as a cold zone near the walls.

The edge strip is installed first, before laying the insulation boards. It is glued directly to the wall and must be taller than the final finished floor surface will be (the excess is trimmed after the screed has hardened, before laying the wear layer). In practice, I often see workers underestimating the strips or not installing them at interior walls between rooms – this is a mistake, because thermal expansion doesn't distinguish exterior from interior based on physics, only based on where the installer put the strip.

Expansion joints in anhydrite screed: where and when

Another practical detail of the wet system: anhydrite screed in large rooms or at transitions between rooms must have expansion joints. The rules are approximately as follows:

maximum field area without a joint: 40 m² at a side ratio of max. 1:2
maximum side length without a joint: 8 m
mandatory joint at every door threshold
joints run through the entire thickness of the screed (not just to a depth of 1/3)
pipes passing through a joint must be protected by a protective sleeve extending 300–500 mm on each side of the joint

This is a technical detail that is sometimes forgotten in project documentation, and screed cracking is then dealt with years later. More on cracking floors can be found in the topical article Why does a floor above an insulation board crack or warp in the Knowledge Centre.

How to choose the right foil for wet vs. dry systems – summary

For clarity, here's a summary of foil selection:

Wet system: use the Separation foil 601001H. It is laid over the entire area of the system board before pouring the screed. Overlaps of at least 150 mm, taped joints. The foil must also be carried up behind the edge strips.
Dry system: use the Aluminium foil for dry underfloor heating. It is laid on the UHPD polystyrene after installing the pipe and aluminium heat-distribution lamellas, before laying the OSB. Overlaps of at least 50 mm, joints taped with aluminium tape.
Never combine the foils: A PE separation foil under OSB in a dry system won't cause a direct problem, but it won't provide thermal reflection and will therefore potentially reduce the output of the underfloor heating. Conversely, aluminium foil in a wet system may react with alkaline anhydrite (pH 9–11) and degrade.

Most common mistakes in selection and installation

From experience, I can list several recurring mistakes that unnecessarily increase repair costs or reduce performance:

A customer buys a studded board for the wet system because it was cheaper, and uses it in a dry system. Result: uneven floor, OSB wobbling on the studs, cracks in the wear layer.
The installer forgets the separation foil or doesn't tape it. Anhydrite flows under the board. A month later the customer reports that the floor doesn't heat well near the walls.
A thin board (20 mm) is used above an unheated basement. Thermal resistance is too low, energy flows mostly downward, consumption rises, and the top floor surface is noticeably colder at the edge (near the basement).
The system board is laid on an uneven substrate (deviation of 5–8 mm). The studs don't create perfect contact with the pipe, heat transfer is uneven, and the floor has warm and cold zones.
The edge strip isn't installed in time before pouring, and the installer just throws it around the perimeter without fixing it – the screed then pushes it out of place and a cold bridge forms at the edges, caused by direct thermal contact between the screed and the masonry.

These mistakes are covered in more detail in a separate article, Common mistakes when laying polystyrene and system boards under underfloor heating.

Choosing according to the wear layer: the material on top matters too

The choice between wet and dry systems is also related to what will be on top. For the wet system, the following are ideal:

Ceramic tiles – highest thermal conductivity (λ ≈ 1.0–1.5 W/mK), best system performance, recommended thickness with adhesive max. 12–15 mm
Stone tiles – similar to tiles, excellent conductivity
Vinyl flooring (LVT) – thermal resistance max. 0.15 m²K/W (according to ČSN EN 1264-2), which most products meet
Laminate – given with a thermal resistance up to 0.10–0.15 m²K/W; important to check this in the product's technical data sheet

For the dry system, the choice is slightly different. The OSB working layer restricts some types of flooring:

Laminate, vinyl (floating installation) – excellent fit, no restrictions
Glued wooden parquet – possible, but requires special flexible-type adhesives and following the parquet manufacturer's recommendations
Tiles on OSB – problematic without a special underlayment board (cement-fibre board on OSB), because OSB moves with moisture and this would cause grout cracking under tiles

Combining wet and dry systems within a single building

In practice, it's not unusual to have a wet system in a single family house on the ground floor (concrete slab, bathrooms, living areas) and a dry system upstairs (wooden structure, need to minimise weight). This is a completely legitimate and technically correct approach – it involves two parallel hydraulic circuits, or possibly two zones on one manifold, each with its own parameters (flow, water temperature). It's important that in such a building you don't mix materials: for the ground floor you buy studded boards and separation foil, for the upper floor STIROTERMAL DRY and aluminium foil.

Frequently Asked Questions (FAQ)

Can I use regular EPS without studs instead of a system board in the wet system?

Technically yes, but with reservations. Regular EPS without studs won't fix the pipe in place – you'll need to use fixing clips and a mesh underlay, which increases installation costs. In addition, you must ensure the entire EPS surface is covered with separation foil, since without lamination there's a risk of anhydrite seepage. A studded system board is ultimately more efficient – it saves installation time and reduces the risk of errors. A detailed overview can be found in the article How to choose the right system insulation board for underfloor heating.

How do I calculate how much foil to order for a given area?

For PE separation foil in the wet system, calculate room area × 1.15 (15% allowance for overlaps and waste). For aluminium foil in the dry system, × 1.10 is sufficient. Don't forget that the foil must also run up behind the edge strips at the wall – this adds an extra allowance for smaller rooms with a large perimeter. For example, a 5 m² bathroom with a 9 m perimeter, using 1 m wide foil and a 100 mm overlap up the wall = approx. 0.9 m² extra just for the edges.

Can I use STIROTERMAL DRY (UHPD) for a wet system if I block the channels with separation foil?

No, that would be incorrect use. The UHPD board has milled channels but no studs – anhydrite would flow into these channels, the polystyrene wouldn't hold the pipe in position during pouring, and the whole geometry would be disrupted. In addition, the compressive strength of UHPD is optimised for the dry system, not for wet screed. Always use products according to their intended purpose.

What is the minimum thickness of polystyrene for a dry system on a wooden ceiling?

The minimum thickness of EPS UHPD is primarily determined by the depth of the milled channels – a 16 mm pipe requires a channel at least 18–20 mm deep, so the board must be at least 20–25 mm thick. For thermal insulation requirements (above unheated space), a thickness of 30–50 mm may be needed. Always consult a project calculation – remember that the EPS thickness on a wooden structure is also limited by the available construction height between or above the beams.

Why is a layer of two OSB boards important instead of just one in the dry system?

A single 18 mm thick OSB board is rigid, but with the point-load heat transfer from the pipes through the aluminium lamellas, it may show slight deflections between the lamellas, which shows up on the wear layer (fine unevenness, cracking of laminate joints). Two layers of OSB (e.g. 2 × 9 mm or 2 × 12 mm) with staggered seams form a more rigid composite structure with better load distribution. This is the standard recommended build-up for a smooth and long-lasting floor function in a dry system.

Is separation foil needed even if the system board has a pre-laminated surface?

This depends on the specific product. Some premium system boards have a PE laminate on the surface that fulfils the separation function – in that case, there's no need to add additional foil over the boards' surface. However, it's always necessary to tape the seams between the boards with adhesive tape. If in doubt, always prefer to add separation foil – the cost of the foil is negligible compared to the overall investment in underfloor heating, but its absence can have serious consequences for the system's performance and lifespan.

Conclusion: the right choice from the start saves both nerves and money

Dry and wet underfloor heating are two different physical systems, each with its own justification – and each requiring different polystyrene, different foil and a different installation procedure. The wet system with studded boards (such as STIROTERMAL BASIC UHP55 or DUO UHP51), separation foil and anhydrite screed is robust, high-performing and maintenance-free in the long run. The dry system with UHPD polystyrene (STIROTERMAL DRY), aluminium foil and OSB working layer is fast, light and ideal for renovations or lightweight ceiling structures.

When choosing, always keep in mind: the type of substrate, the available build-up height, the required thermal resistance, the planned wear layer, and the speed of installation. And if you're not sure, start with a proper project including a hydraulic calculation – that's an investment that pays off many times over compared to any savings on materials. For more details on installation, see the article Installing a system insulation board under underfloor heating step by step and Separation and aluminium foil for underfloor heating: when and how to use them.

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