Step-by-step installation of a system insulation board under underfloor heating

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Step-by-Step Installation of a System Insulation Board for Underfloor Heating

The system insulation board is the cornerstone of every well-functioning underfloor heating system. Without it, heat simply doesn't go where it should – upward into the room – but escapes downward into the structure, needlessly heating your neighbor's ceiling or losing energy into the foundation. Proper installation of this board isn't rocket science, but it requires precision, a systematic approach, and knowledge of several key details that most installers underestimate. In this article, I'll guide you through the entire process – from substrate preparation, through laying the boards, to installing the pipes and the final check before pouring the screed.

If you're still deciding which board to choose, I recommend first reading the articles 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 assume you've already chosen your board and want to go straight to the installation technique.

What You Need Before Starting: Materials, Tools, Substrate

Successful installation of a system board begins long before you lay the first panel on the floor. From experience, I know that most problems – cracks in the anhydrite, uneven heating, bubbles under the foil – arise precisely because the installer skipped the preparation phase and jumped straight into laying the boards.

Materials and Quantities

Basic material list for standard wet installation (concrete or anhydrite):

System insulation board – calculate the room area + 5–8% for cutting waste. For ordinary rectangular rooms, 5% is sufficient; for L-shapes or angled walls, add up to 10%.
Edge expansion strip – around the entire perimeter of the room, including door openings. The standard height of 150 mm is sufficient for a build-up height of up to 80 mm.
Separation foil – specifically Separation Foil 601001H serves as a vapor barrier and protection against moisture from below, as well as against cement laitance seeping into the joints between boards. Foil overlaps should be at least 10 cm, and along walls it should be extended at least 15–20 cm above the finished floor.
Pipe – for system boards with a 150 mm grid, 16×2 mm or 17×2 mm PE-RT/PEX pipe is most commonly used.
Foil adhesive tape – PE sealing tape for overlaps and joints at the walls.

For dry installation (without a cement underlay), the list is somewhat different – instead of separation foil, you work with aluminum foil for dry underfloor heating, which ensures even heat distribution across the whole area. More about this in the article Dry vs. Wet Underfloor Heating: What Polystyrene and Foil Are Suitable.

Tools

Long spirit level (at least 2 m) and a rotary laser
Notched trowel or roller for adhesive (if bonding)
Sharp long-blade knife or a polystyrene saw (jigsaw with fine teeth)
Tape measure, pencil, 90° square
Foil stapler (staple gun with 10–12 mm staples)
Rubber mallet for fitting boards
Water hose level or digital laser level for checking flatness

Step 1: Preparing and Checking the Substrate

The substrate must meet three basic conditions: it must be level, dry, and solid. Neglecting any of these conditions will cause problems later.

Flatness of the Substrate

The maximum tolerance for unevenness of the substrate under the system board is ±5 mm per 2 m straightedge. In practice, it's usually worse – old concrete can be off by ±15–20 mm, which is a problem. The board tolerates some leveling, but depressions greater than 8–10 mm cause the board to "hang" in the air and deflect under load, which later leads to cracking of the screed or even cracking of the boards themselves.

Knock down bumps larger than 3 mm with a hammer and chisel, and fill depressions larger than 5 mm with a leveling compound. No compromises. In Slovakia, this is most often underestimated during renovations of old panel buildings, where the floor can really be very uneven.

Dryness of the Substrate

The moisture content of concrete must not exceed 2% (CM method) before laying the insulation. Fresh concrete needs at least 4–6 weeks to dry, depending on thickness and ventilation. Anhydrite dries faster in this respect, but is also more sensitive – above 0.5% moisture, wooden flooring must not be used on top of it. Measure moisture using the CM method, not a surface probe – the latter only gives misleading results.

Strength and Cleanliness of the Substrate

The substrate must have a compressive strength of at least C16/20 (concrete class). Grind away any weathered or crumbling surface, brush it, or apply a primer if needed. Sweep thoroughly. Oil, paint, loose particles – all of this must be removed, otherwise the insulation board has nothing to sit on and the foil won't bond.

Step 2: Laying the Separation Foil or Vapor Barrier

The separation foil serves several functions at once: it protects the polystyrene from moisture from below, prevents cement laitance from seeping into the joints between boards, and also acts as a sliding layer that helps with expansion of the concrete screed. We use it both before and after laying the boards – depending on the system.

In a standard wet system, Separation Foil 601001H is laid on a clean, leveled substrate before the system boards. The foil strips should overlap by at least 10 cm, with the overlaps taped using PE tape. The foil must also run up the walls – extend it 15–20 cm higher than the expected height of the final floor. The excess is trimmed off after the anhydrite or concrete has hardened.

Important detail: at foil strip overlaps, the upper strip must go over the lower one (not under it), so that water flowing from above – for example during construction work – runs off along the foil and out, not into the insulation. It's the same principle as roof tiles.

Step 3: Installing the Edge Expansion Strip

Before laying the boards themselves, install the edge expansion strip around the entire perimeter of the room. This step is always done before the boards, not after – the strip must be fixed to the wall and rest on the substrate or foil, not on the board.

The expansion strip must be:

Sufficiently high – extend it at least 20 mm above the planned height of the final floor (covering + screed + board). So if you have a 55 mm board, 65 mm anhydrite, and 10 mm tiling = 130 mm from the carrier, the strip must reach at least 150 mm.
Continuous – overlap it at the corners, don't stretch it tight. If you have an L-shaped room, the strip must run around the entire perimeter, including internal corners.
Tight against the wall – it must not bulge or fold under, otherwise the cement screed will flow behind the strip and form a rigid bond with the wall, eliminating the expansion function.

At door openings (where there is no wall, just lintels or frames), use an expansion profile embedded in the screed – the so-called "T-profile" made of aluminum or stainless steel, which divides the screed into separate fields.

Step 4: Laying the System Boards – Procedure and Techniques

This is the core of the whole job. System boards are laid using interlocking joints (tongue-and-groove or stud system), so they don't need to be glued – at least not with most standard systems. The exception is sloped surfaces or structures where the boards are bonded.

Laying Direction and Layout Logic

Always start laying from the corners – ideally from the corners near the entrance door, so that at the end you don't have to walk over already-laid boards to press in the last row. Choose the laying direction to minimize waste – usually parallel to the longer wall of the room.

Before you start laying, sketch out on paper (or, if experienced, in your head) how the boards will come out at the end. If the last row would come out narrower than 100 mm, shift the entire row at the start by half a board width – a symmetrical result is better than a narrow final row that won't hold well.

Laying the First Row

The first row is the most important – it determines the direction and plane of the entire area. Lay the boards tight against the expansion strip, not directly against the wall. There must be an expansion strip between the board and the wall, not air. Join the boards together – the tongue-and-groove system must fit without forcing. If you need to use a hammer, don't force the board – either the joint is uneven, or the board's lock is damaged.

Check the first row with a spirit level in two directions. If the substrate is properly leveled (see Step 1), the boards should sit evenly. If you feel a board rocking, look for a bump underneath it and remove it.

Laying Subsequent Rows

Each subsequent row must be laid with the joints offset by at least half a board's width from the previous row – the same principle as bricklaying. This is essential! If you were to lay boards with continuous joints (i.e., the joint from one row continues directly into the next row), you would create a line of weakness where the screed could easily flex or crack.

Cutting Boards to Size

For cutting system boards made of EPS (expanded polystyrene), a long, sharp-bladed knife works best – unheated, no hot wire. A fine-toothed wood saw also works, but leaves an uneven cut edge. A hot-wire cutter (polystyrene knife) is the fastest and cleanest, but most professionals don't have one on site.

Important: if you're cutting a board with a stud profile, make sure the stud grid on the cut piece lines up with the grid of the neighboring boards. On the UHP55 STIROTERMAL BASIC system board, the stud spacing is 50 mm, which corresponds to the common pipe grid of 100–150–200–250–300 mm. If the stud grid doesn't match, the pipe won't be able to run correctly at the joint between two pieces.

Step 5: Laying the Pipe into the System Board

Once the entire area is covered with boards, it's time to lay the pipe. A system board with studs makes this job considerably easier than the traditional method with clip rails – you simply press the pipe between the studs and it holds. Even so, a few rules must be followed.

Pipe Spacing and Heat Output

The most common pipe spacing for a standard living space is 150 mm. In bathrooms, hallways, or areas with high heat loss (glass walls, corner rooms), reduce the spacing to 100 mm. For peripheral zones along exterior walls, a "denser edge" is recommended – the first 2–3 rows of pipe at 100 mm spacing, the rest at 150 mm. This is common practice in Slovakia for new buildings with higher heat losses through the building envelope.

For more on how to design pipe layout without thermal bridges, see the article How to Correctly Lay Pipe in a System Board Without Thermal Bridges.

Technique for Pressing the Pipe In

Press the pipe into the studs evenly, without pulling or sharp bending. The minimum bending radius for 16×2 mm PE-RT pipe is 5× the outer diameter = 80 mm, and for 17×2 mm pipe it's 85 mm. A smaller bending radius leads to an oval cross-section (ellipsoidal pipe deformation) and reduced flow, and in extreme cases to permanent damage. In practice, when laying a meander loop (U-shape or serpentine), try to maintain a radius of at least 100 mm at every turn – that's a safe limit.

Pressure Testing the Pipe Before Pouring

Before any further step – before pouring the anhydrite and before any other work – fill the circuit with water and pressurize it to 4–6 bar. Leave it under pressure for at least 24 hours (some manufacturers require 48 hours). Monitor the pressure gauge. If the pressure drops, look for a joint or a damaged spot. Pouring screed over a leaking pipe is one of the worst situations an installer can allow – repairing it requires breaking up the screed.

Step 6: Final Preparation Before Pouring – Checklist

Before calling in the screed contractors (anhydrite screed installers), go through this checklist:

✓ All boards are properly joined, with no waves or bumps between neighboring boards
✓ The edge expansion strip is continuous and sufficiently high
✓ The separation foil has no major tears, and all overlaps are taped
✓ The pipe is pressed into the studs along its entire length, with no bulging
✓ The circuit is pressurized and the gauge has shown stability for more than 24 hours
✓ Pipe passages through expansion joints are protected with sleeves (e.g., PVC pipes)
✓ The manifold is installed and accessible (not covered)
✓ The board height from the carrier matches the project design

Specific Situations: Bathroom, Balcony, Staircase

Bathroom

In a bathroom, watertightness of the foil and joints is the most important factor. Use only certified PE foil with a thickness of at least 0.15 mm (preferably 0.2 mm). Tape all foil overlaps with waterproof PE tape, not ordinary construction tape. In bathrooms with underfloor heating, it's recommended to combine the system insulation board with a waterproofing coating on top of the screed – this provides double protection. Under a bathtub or shower tray, continue the insulation without interruption.

Balcony or Loggia

Underfloor heating on a balcony or loggia is less common in Slovak conditions, but not unheard of. Here, expansion joints are the most critical – the balcony experiences much greater temperature fluctuations than the interior. Separate the balcony screed from the interior screed with an expansion profile, and never connect both circuits into one without an expansion joint at the transition point.

Dry Underfloor Heating

For dry installation (e.g., a gypsum board floor or wooden carrier board instead of anhydrite), a different type of system board is used – for example STIROTERMAL DRY (UHPD), which has directly molded grooves for the pipe and aluminum sheets for heat transfer. On this board, you then lay aluminum foil for dry underfloor heating, which distributes heat from the pipe across the whole board surface. The substrate preparation procedure is the same, but laying the boards and pipe is somewhat simpler – no pressure test before pouring is needed, since there is no pouring at all.

Most Common Installation Mistakes and How to Avoid Them

Over the years of practice, I've seen many installations – both good and bad. Here are the mistakes that occur most often:

Unprepared substrate – the installer lays the boards directly on uneven concrete, the board bends, and the screed cracks. Solution: always measure, always fix before laying.
Missing or too low expansion strip – the screed is rigidly bonded to the wall, and when the temperature changes, it strains and cracks. I've seen rooms where the strip was completely omitted – the anhydrite ends up looking like a map of Africa.
Foil without overlaps or without taping – cement laitance flows into the board joints, the seams between boards harden, and the system's expansion capability disappears.
Boards laid without offset – straight joints running across the entire width of the room are a ticking time bomb for any poured screed.
Pipe laid without a pressure test before pouring – if a joint is faulty, you'll only find out once the screed is done. Breaking it up is several times more expensive than a 24-hour test.
Too small a bending radius – the pipe whitens at the bend, which can lead to a leak even after years of operation.

You'll find more details on these issues in the articles Common Mistakes When Laying Polystyrene and System Boards for Underfloor Heating and Why the Floor Above the Insulation Board Cracks or Buckles.

Choosing the Right Board for Your Project

For standard apartments and family houses with a wet system (anhydrite or cement screed), we recommend the UHP55 STIROTERMAL BASIC system insulation board – the 55 mm thickness provides sufficient thermal insulation for most construction situations. For floors above unheated spaces (garage, basement, unheated hall), a greater thickness is recommended – in that case, choose the UHP51 STIROTERMAL DUO 11 system insulation board, which combines the system layer with a greater insulation thickness. For more on choosing boards according to thermal requirements, see the article What Thickness of Polystyrene Is Needed Under Underfloor Heating.

Frequently Asked Questions (FAQ)

Do I need to glue the system insulation board to the substrate, or is loose laying sufficient?

For most standard systems with anhydrite or cement screed, gluing isn't necessary – the boards are held in place by their own weight, mutual interlocking, and later by the screed itself. Exceptions are sloped surfaces (ramps), thin boards prone to movement during work, and systems where the manufacturer explicitly requires bonding. In such cases, use an adhesive compatible with EPS (dispersion-based, not solvent-based – solvents dissolve EPS!).

What should I do if the stud grid doesn't line up at the joint when cutting a board?

This is the most common problem when starting the first row. If the stud grid doesn't line up correctly with the neighboring board, you won't be able to run the pipe smoothly through the joint. Solution: always check where the joint will be before cutting, and adjust the cutting position of the studs so that the studs on the cut edge line up with the studs of the neighboring board. In most systems (including STIROTERMAL), the stud spacing is 50 mm, so the cut piece must have a length that's a multiple of 50 mm.

Can I lay the system board directly on old tiles or vinyl?

Technically yes, if the old covering is solid, fully bonded (no hollow spots), free of bumps, and level within a tolerance of ±5 mm/2 m. In practice, however, old tiles often only partially adhere and can come loose under the weight of the new layer – creating a hollow spot, which is the worst-case scenario for a system board. Always check by tapping whether the covering is solid. If not, remove it. What you save "by not removing the old covering" you'll pay for many times over when repairing a cracked screed a year later.

How should pipe be handled when passing through an expansion joint in the screed?

At every point where the pipe passes through an expansion joint in the screed (e.g., a transition between two fields or a passage through a doorway), the pipe must be protected with a sleeve – either a short section of larger-diameter PVC pipe or a special expansion sleeve. The sleeve must extend at least 300 mm beyond the joint on each side (600 mm total). This rule applies without exception – without a sleeve, the screed will "shear" the pipe during expansion, leading to mechanical damage after a few years.

How long should I wait after laying the boards before pouring the anhydrite?

The boards themselves don't need any waiting time – once a successful 24-hour pressure test is complete, you can pour immediately. What's more important is that the room temperature is at least +5°C during both pouring and curing of the anhydrite (the optimum is +15 to +25°C). The anhydrite must also not be exposed to drafts during the first 24–48 hours of curing – rapid drying of the surface before the core has set leads to microcracks.

Can a system board be combined with electric underfloor heating?

Yes, some system boards have grooves designed for electric heating cables (usually 5–7 mm in diameter). If you're planning electric heating, check when choosing a board whether the grooves match the diameter of your cables. For dry electric heating (heating film), the procedure is different – the film is laid directly on the board or on a leveling layer, without pouring. In this case, aluminum foil as a heat-distributing layer is especially important.

Conclusion: Why It's Worth Doing It Right

Installing a system insulation board is a job you only see once – once the screed is poured, you can no longer access it. Every mistake made during substrate preparation, board laying, or foil installation can show up years or even decades after completion – cracks in the floor, inefficient heating, moisture problems. On the other hand, a correctly done installation requires almost no maintenance, and the system works reliably even after 30–40 years.

Invest time in preparation, uncompromisingly check the flatness and dryness of the substrate, never skip the pressure test, and consistently maintain the joint offset when laying the boards. These are the four rules that separate installations customers remain happy with for years from those an installer is afraid to revisit.

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.