Comparison of System Boards: STIROTERMAL BASIC vs DUO vs DRY vs SOLO

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Comparison of STIROTERMAL system boards: BASIC vs DUO vs DRY vs SOLO

If you've ever tried to get your bearings in the range of system insulation boards for underfloor heating, you've probably come across a whole range of similarly sounding names and numerical designations. STIROTERMAL BASIC, DUO, DRY, SOLO – what does this actually mean, how do these variants differ, and which one should you choose for a specific job? This is one of the most common questions we get from installers, builders, and investors, and it deserves a thorough answer without unnecessary simplification.

In this article, we will discuss each type of board in detail – construction, dimensions, thermal parameters, suitable applications, and common installation pitfalls. By the end, you should confidently know which board to reach for when facing a specific project.

What are system boards and why the choice matters

A system insulation board is not just a piece of polystyrene. It is a precisely shaped product with studs, grooves, or profiles that perform several functions at once: thermal insulation downward and sideways, mechanical guidance of the pipe in a precise grid, separation of different floor layers, and – in some types – also heat accumulation or distribution through diffusion layers.

Choosing the wrong type of board often only becomes apparent after years: the floor heats unevenly, stripes of warmer and colder zones appear on the surface (the so-called zebra effect), or condensation and moisture buildup occur in the layers. Investing in the right product from the start is therefore many times cheaper than later diagnostics and repair.

All types of STIROTERMAL are made of expanded polystyrene (EPS) with increased rigidity, suitable for loaded floor structures. The difference between the variants is therefore not in the basic material, but in the surface geometry, the presence of additional layers, and the way the pipe is installed.

STIROTERMAL BASIC (UHP55) – the base for wet installation

System insulation board UHP55 (STIROTERMAL BASIC) is the best-selling and most affordable variant in the entire family. It is a stud board with an even grid of studs, between which the pipe simply clips in without the need for any additional fastening elements. The stud height is usually 45 mm, and the total board thickness (including the base) reaches 55 mm – hence the designation UHP55.

Construction and dimensions

The studs are cylindrical in shape, with diameter and height optimized for pipes with a dimension of 16×2 mm or 17×2 mm – the most commonly used hose sizes in underfloor heating. The stud grid is 50×50 mm, which allows the pipe to be laid at spacings of 50, 100, 150, 200, 250, and 300 mm. This flexibility is extremely important in practice: in the perimeter zone of a room you need a denser grid (100–150 mm), while in the center of the room 200–250 mm is sufficient.

Standard boards are manufactured in dimensions of 1,400 × 800 mm, although other formats can be found with some distributors. The board weight is low, and handling is easy even for a single worker.

Thermal properties

The thermal conductivity coefficient (λ) of the EPS used in these boards is typically 0.031–0.035 W/(m·K), depending on the specific class. For a thickness of 55 mm, the thermal resistance is R = d/λ ≈ 55/0.033 = approx. 1.67 m²K/W. This is a sufficient base for most applications in new buildings; however, for floors above unheated spaces (garage, basement, exterior), it is necessary to either increase the thickness of the insulation layer under the board or choose a thicker variant. This topic is covered in more detail in the article What thickness of polystyrene is needed under underfloor heating in our Knowledge Center.

Suitable applications for BASIC

Wet installation – pouring cement screed or anhydrite (the most common case)
New buildings with standard insulation requirements (above a heated basement or above ground with added insulation)
Apartment buildings, family houses, office spaces
Projects with limited floor build-up height, where extra thickness is not needed

It is important to lay BASIC boards on a flat, stable surface. Underneath them, a separating foil 601001H is usually placed, which prevents cement laitance from the concrete from penetrating the insulation, reduces acoustic transmission, and serves as a vapor barrier in applications above ground.

STIROTERMAL DUO (UHP51) – a two-layer concept for better distribution

System insulation board UHP51 (STIROTERMAL DUO 11) represents a higher level of the product line. The DUO concept refers to a two-component system: the EPS board itself is supplemented with a top layer with precise grooves or slots that clearly determine the position of the pipe and prevent it from shifting during screed pouring.

What "11" means in the designation DUO 11

The number 11 refers to the height of the top profiled layer in millimeters. This layer is made of stiffer EPS or a combined material and is fused, or glued, onto the base insulation board. The total thickness of the system is therefore 51 mm (base) + 11 mm (profile) = 62 mm without the screed layer. The exact dimensions may vary slightly depending on the production batch, so always check the current technical data sheet.

Advantages of DUO over BASIC

The main benefit of the two-layer system is the precision of pipe placement. With BASIC boards, when laying the pipe more densely (e.g. 100 mm) or with stiffer tubing, the pipe may slightly slip out from between the studs, leading to uneven screed coverage and, in extreme cases, thermal bridges. DUO eliminates this problem, because the grooves physically secure the pipe from above and below.

Another benefit is a more even surface before laying the screed layer – the top profiled section forms a continuous plane, which also makes it easier for workers to move across the board during installation without the risk of deforming a loosely clipped pipe.

When to choose DUO

Larger areas with a high number of pipe circuits, where precise laying saves time and material
Projects with stricter requirements for uniform temperature fields (gyms, showrooms, open-space offices)
Situations where multiple installers work on the area at the same time and errors need to be minimized
Combination with anhydrite screeds, which are more sensitive to uneven coverage thickness above the pipe

STIROTERMAL DRY (UHPD) – system for dry installation

Polystyrene for dry underfloor heating UHPD (STIROTERMAL DRY) is a structurally different product from the previous two. The key difference is that it is not intended for wet installation (screed pouring), but for so-called dry mounting, where aluminum lamellas or dry gypsum boards perform the distribution function.

How dry installation works

The DRY board has milled U- or Ω-shaped grooves on the surface, into which the pipe is inserted. Above the pipe (and in the spaces between the grooves), aluminum distribution lamellas are then placed, which fill the gaps around the pipe and create a continuous thermally conductive surface. Dry screed boards or floor elements are placed directly on this aluminum surface (without any wet process).

For heat distribution in a dry system, the aluminum foil for dry underfloor heating is essential, ensuring lateral (sideways) heat spreading from the pipe across the entire lamella surface. Without this element, you would have a local hot spot above the pipe and cold zones between the pipes – the classic zebra effect.

When dry installation is the right choice

Dry installation is not just a fad – it has very specific technical justification in the following situations:

Renovations: When a wet screed cannot be added due to its weight (ceilings with limited load-bearing capacity) or when the investor doesn't want to wait several weeks for it to dry. Dry boards can be loaded almost immediately.
Wooden structures: Beam ceilings, wooden floors, where moisture from the screed could cause permanent damage to the supporting structure.
Attic apartments and penthouses: Where floor height is critical and every millimeter counts.
Phased renovations: When heating needs to be put into operation quickly, without waiting for the wet layer to cure.

However, you should know that the dry system is more expensive in terms of materials (aluminum, dry boards) and requires more precise work – any gap in the aluminum lamellas will show up in the floor's temperature field in winter. This topic is covered in detail in the article Dry vs wet underfloor heating: which polystyrene and foil are suitable in our Knowledge Center.

STIROTERMAL SOLO – a single-layer solution for special situations

The SOLO variant is less frequently offered, but irreplaceable for certain types of buildings. It is a simpler, thinner board without a special stud profile or milled grooves – the pipe is routed on the surface and fixed with clips or mounting rails. SOLO is essentially a "flat" EPS board optimized for possible combinations with other mounting systems or for situations where the investor has their own pipe fixing system.

Where SOLO makes sense

Combination with metal mounting rails (e.g. U-rail system), where the stud shape is not needed
Industrial halls with a special pipe grid that does not correspond to the standard 50 mm stud grid
Low-energy houses with a very dense pipe grid and thin covering, where a flat board with a smaller thickness is more advantageous
Projects where the customer combines different manufacturers and needs a "neutral" insulation layer

In practice, SOLO sells much less than BASIC or DUO – most customers prefer the convenience of an integrated pipe routing system. If you are considering SOLO, we recommend first consulting the project with the technical department, because proper pipe fixation is critical for a flat board in terms of thermal bridges. You can find more on this topic in the article How to properly lay pipes in a system board without thermal bridges.

Table comparison: BASIC vs DUO vs DRY vs SOLO

Parameter	BASIC (UHP55)	DUO (UHP51)	DRY (UHPD)	SOLO
Installation type	Wet	Wet	Dry	Wet / combined
Pipe routing	Between studs	In grooves of the DUO layer	In milled grooves	Clips / rails
Board thickness	55 mm	~62 mm	variable	variable
Pipe grid	50 mm and multiples	50 mm and multiples	according to milling	arbitrary
Need for separating foil	Yes (under the board)	Yes (under the board)	No (dry system)	Yes (under the board)
Need for aluminum foil	No	No	Yes (heat distribution)	Optional
Time to commissioning	Slow (screed drying)	Slow (screed drying)	Fast (immediate)	Medium
Material cost	Low	Medium	Higher	Low–medium

Practical scenarios from the field

Scenario 1: New family house construction, standard conditions

The customer is building a two-story family house, with a heated basement below the first floor. The floor will be ceramic tile in the entire lower zone (living room, kitchen, hallway) and laminate wood flooring in the bedroom. Recommendation: STIROTERMAL BASIC UHP55 is an absolutely adequate choice here. The 55 mm insulation is sufficient above a heated basement, and the stud system allows optimal pipe placement with a shorter grid in the perimeter zones and a standard grid in the central zones. A separating foil is placed under the boards. The total floor build-up height will be approx. 55 + 65 (screed) + 10 (adhesive and tile) = 130 mm – a standard value for a new building.

Scenario 2: Renovation of an apartment in a panel building, limited height

The customer is renovating an apartment on the 4th floor of a panel building. The floor height must be a maximum of 80 mm (due to doors and stairs). The existing screed must be removed and replaced with underfloor heating. Ceiling height is critical. Solution: STIROTERMAL DRY with a total build-up height of approx. 30–40 mm (board) + 4 mm (aluminum) + 22 mm (dry board) + 5 mm (vinyl) = approx. 61–71 mm. The customer thus stays within the required limit and, in addition, the system is commissioned in 2 days instead of 4–6 weeks with a wet process.

Scenario 3: Industrial hall with hypocaust-type heating

A production hall with an area of 2,000 m², requiring a uniform temperature field of 22 °C, with a pipe grid of 200 mm. The customer has their own metal mounting rail system. In this case, SOLO can be a suitable choice – no stud geometry is needed, the customer screws the rails into the board and fixes the pipe into them. This saves on the board price while providing a flexible grid.

Scenario 4: Showroom with strict heat homogeneity requirements

An auto club or fashion store with large-format glossy stone tiles. On such a surface, even minimal temperature unevenness is visible. STIROTERMAL DUO is ideal here – precise pipe fixation in the grooves guarantees even spacing across the entire area, and the anhydrite screed then creates a homogeneous accumulation layer, so the customer will not see any striping on the tiles.

What to watch out for with all types of boards

Regardless of the type of system board chosen, some common rules apply, the non-compliance of which leads to problems. We address floors that crack or warp in a separate article, Why the floor above the insulation board cracks or warps, but let's briefly summarize the basic causes:

Uneven substrate: The flatness tolerance of the substrate before laying the system board should be a maximum of 5 mm/2 m. Any unevenness will transfer to the final floor.
Missing expansion joints: For areas over 40 m² or sides longer than 8 m, expansion joints must be provided. System boards must not cross this expansion zone – the pipe passes through the joint in a protective sleeve.
Poor-quality board joints: Boards must be laid in a bonded pattern (like bricks), not with continuous straight joints. A continuous joint is a thermal bridge and a mechanically weak point.
Insufficient coverage above the pipe: With a wet screed, the minimum coverage above the top of the pipe must be at least 30–45 mm of cement screed or 35–50 mm of anhydrite. Less than this risks cracking of the screed above the pipe.

You will find a detailed guide to the entire installation process in the article Installing a system insulation board for underfloor heating step by step, and typical installation errors are summarized in the article Common mistakes when laying polystyrene and system boards for underfloor heating.

Economic evaluation of the choice

Comparing only the price per m² of the board is misleading. A proper economic analysis must include the entire system, including related materials and labor:

BASIC: The lowest board price, but requires a separating foil and wet screed (additional costs, drying time of 4–8 weeks depending on thickness and conditions).
DUO: Higher board price, but savings on the number of fixing elements and on installation time (fewer errors = fewer repairs).
DRY: The highest system price (board + aluminum + dry screed board), but zero cost for wet screed and zero waiting time. In renovations, the difference in total project cost is minimal or in favor of the dry system when you factor in the shortened project time.
SOLO: Low board price, but higher installation costs (clips, rails, labor-intensive laying) – advantageous only for specific projects.

For practical decision-making: unless you have a specific reason to go another way, choose BASIC for standard new buildings and DRY for renovations. DUO is the right choice when precision is required or for larger commercial projects. We also write about how to choose the right board in the separate article How to choose the right system insulation board for underfloor heating.

Frequently Asked Questions (FAQ)

Can I combine STIROTERMAL BASIC with an anhydrite screed, or is it suitable only for cement screed?

STIROTERMAL BASIC is compatible with both types of screed layers – cement and anhydrite. With anhydrite, it is important that the board surface has sufficient adhesion and that the board is fully embedded (anhydrite is poured and fills the gaps between the studs). We recommend using the DUO board for anhydrite applications – the closed groove profile prevents the anhydrite from seeping under the pipe, which could cause it to lift. A separating foil is always mandatory between the supporting structure and the board.

Is aluminum foil necessary with STIROTERMAL DRY, or is the board alone sufficient?

Aluminum foil, or distribution lamellas, are practically indispensable in a dry system. Without them, the thermal conductivity between the pipe and the dry screed board is minimal – the air in the grooves acts as a thermal barrier. The result is an extremely uneven temperature field. Aluminum foil for dry underfloor heating is therefore an integral part of every DRY installation and should always be included in the system calculation.

How to prevent BASIC boards from moving during screed pouring?

BASIC boards are usually not mechanically anchored to the substrate – underneath them is a smooth separating foil. The pipe laid in the studs and properly connected to the manifold creates a network that stabilizes the boards. The critical areas are the edges of the room and transitions between fields – there, edge strips should be applied and the edge boards glued to the edge strip with PU adhesive. During anhydrite pouring, the boards should not be walked on – anhydrite is pumped and applied using a hose, not a classic trowel.

What is the minimum thickness of cement screed above the pipe with STIROTERMAL DUO?

For cement screed (C20/25 or higher), a minimum coverage of 45 mm above the top of the pipe applies. For anhydrite (liquid poured screed, e.g. AE20), it is possible to go down to a minimum of 35 mm above the top of the pipe. These values are not just technical recommendations – they are the minimums needed for the screed layer to function as a thermal board and not just as a thin covering. Screed below the minimum cracks and, in extreme cases, lifts. You can find more on this topic in the article What thickness of polystyrene is needed under underfloor heating.

Can STIROTERMAL DRY be used in a bathroom?

Yes, but with conditions. The dry system in a bathroom must be supplemented with a waterproof coating (waterproofing) applied over the dry board before laying tiles. Tiles in the bathroom with a dry system are usually glued to Fermacell or a similarly moisture-resistant dry board. Note: standard OSB or gypsum board are not suitable for wet rooms without waterproofing. It also applies that the weight load and vibrations (e.g. sink, bathtub) must not exceed the system's load capacity – always consult the manufacturer.

Why are system boards sold in m² and not per piece – and how do I calculate how many I need?

System boards are usually packed in bundles with a precisely defined number of pieces and total area in m². To calculate the required quantity: measure the net area of the room excluding fixed furniture (bathtubs, shower stalls, floor-built-in cabinets). Add 5–10% to this area for waste (cutting edges, drain openings, and the like). Always order boards for one more package than the calculation indicates – ordering a small additional quantity later is more expensive due to shipping. You can find an overview of all available formats directly in the category Polystyrene, system boards on atria.sk.

Conclusion: which board to choose – a quick decision guide

To conclude, let's summarize the choice into a clear schematic decision tree for practice. If it's a new building with wet technology and standard insulation requirements, STIROTERMAL BASIC (UHP55) is your first choice – affordable, proven, easy to install. If you have a larger commercial project or strict requirements for temperature uniformity, look at STIROTERMAL DUO (UHP51). For renovations under time pressure, wooden structures, or height constraints, reach without hesitation for STIROTERMAL DRY (UHPD) – and don't forget to order the aluminum distribution foil along with it. Leave SOLO for specialized projects where you have a clear reason to deviate from the standard system.

No system board alone will guarantee excellent underfloor heating – equally important is proper hydraulic balancing of the system, the quality of the screed and its drying, proper expansion joints, and finally the choice of floor covering with suitable thermal resistance. We address all these topics in other articles in our Knowledge Center.

Do you have a question on this topic?

Can't decide, or are you dealing with a specific situation in your home? Write to us - we're happy to help.