Step-by-step installation of floor heating - from insulation to pouring

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Installation of underfloor heating step by step – from insulation to pouring

Underfloor heating is now one of the most popular heating methods for single-family homes, apartments and commercial spaces. There are several reasons for this: even heat distribution, low temperature requirements for the heating medium (ideal for heat pumps), quiet operation and, not least, comfort – a warm floor under your feet. Despite its apparent simplicity, underfloor heating is a system where each step of the installation determines the final performance, lifespan and reliability of the entire installation.

In this article, we will go through the entire installation from the first board to the last liter of anhydrite – step by step, with specific numbers, practical examples and explanations of why things are done this way and not otherwise. If you are wondering how to choose the right insulation board, system board or grid foil, or what pipe dimensions are optimal for your project, you will find detailed information in other articles from our Knowledge Center – specifically in the topics How to choose an insulation board for underfloor heating – thickness, material and requirements and What pipe diameter and spacing do I need for underfloor heating?

Before the actual installation: what you need to know and prepare

The installation of underfloor heating may not seem like rocket science, but practice shows that most problems arise even before the first board is laid. At the same time, it is important to note: errors in this phase are difficult to correct later – either not at all, or at the cost of breaking up the floor.

Subfloor and its condition. Before anything else, check the concrete subfloor. It must be flat (deviation max. 5 mm per 2 m rule), dry (concrete moisture content under 4 % CM method), without sharp protrusions, cracks and loose areas. Any unevenness will be transferred to the insulation layer and later may cause cracking of the surface floor.

Hydro insulation. On the ground floor, above unheated basements, above garages and in bathrooms, you must treat the hydro insulation before insulation. We will deal with this topic in more detail in the article Hydro insulation foil under underfloor heating – when and how to use it?

Edge expansion strip. Before the insulation is applied, an edge expansion strip (usually PE foam, thickness 8–10 mm, height min. 150 mm) is fixed around the entire perimeter of the room. This strip separates the cement screed from the wall and compensates for thermal expansion. Without it, the screed will crack when heated.

Project and calculations. Professional installation always starts with a hydraulic calculation – number of circuits, pipe lengths, spacing, flows. In practice, we encounter installations "by eye" that work, but are inefficient. If you don't have a project, at least use the following rule of thumb: pipe spacing 15 cm for bathrooms and corridors, 20 cm for living rooms, 25 cm for well-insulated houses with low heat loss.

Step 1 – Laying thermal insulation

Thermal insulation is the heart of the entire system. Its task is to prevent heat loss downward and ensure that energy from the pipe spreads upward – to the walking layer and into the living space. At the same time, it serves as a mechanical base for fixing the pipe.

The most common materials are EPS (expanded polystyrene) in various types – standard, graphite (gray, better λ), or special boards with raised grid for pipe attachment. For wet environments and ground floors above the ground, XPS (extruded polystyrene) is chosen, which does not absorb moisture. In exceptional cases, when the structural height is very limited, boards made of PUR/PIR foam with an extremely low thickness and high thermal resistance are available.

The thickness of the insulation depends on the location of the floor in the building. For ground floors above unheated subfloors, standards (STN EN 1264, ÖNORM H 5151) recommend a minimum of 80–100 mm EPS 100. Between heated floors, 30–50 mm is sufficient, with the main function being the limitation of lateral heat loss and the elimination of acoustic transmission. A more detailed analysis of this issue can be found in the article How to choose an insulation board for underfloor heating – thickness, material and requirements.

We lay the boards dry, tightly together, in a staggered bond (like bricks) – never with crossed joints. In the joints, it is sufficient to use transparent adhesive tape, so that during the pouring of the screed, the joint is not filled and no thermal bridge is created. On the edges of the room, we fix the boards – the work is accelerated by a sharp knife and a straightedge.

Step 2 – Separation foil and its correct laying

A separation foil is laid on the thermal insulation. Its functions are threefold: it separates the insulation layer from the wet screed, prevents the escape of cement milk into the insulation (which would impair the thermal resistance) and at the same time facilitates the movement of the screed during thermal expansion. When using a grid (mesh) foil, it also serves as an orientation grid when laying the pipe at the exact spacing.

In practice, we recommend Separation Foil with Grid 0.1×1030 mm; AL – 50 m² from manufacturer IVAR. This foil has a printed millimeter grid, which allows you to precisely maintain the pipe spacing and direction – without having to constantly measure. The aluminum reflective effect also helps to reflect heat upward. Foils are laid with an overlap of at least 100 mm; overlaps are joined with Metallic tape for joining foils (55 mm × 50 m), which holds even during vibratory compaction of the screed and does not lose adhesion in the presence of moisture.

A common mistake we see in customer projects: the films are overlapped only slightly, or the overlap is neglected entirely. As a result, during the anhydrite pouring, the films float, shrink, and the pipes shift. The solution is simple: adhesive tape every 30–50 cm along the entire joint.

In rooms with significant insulation irregularities or when laying on a staircase, it is recommended to stretch the film upward along the edge expansion strip in the corners and fix it with a tape – this ensures full separation of the wet mix from the wall.

Step 3 – Pipe Fixation: Strips, Clamps, and Bends

This is the phase where a skillfully precise job differs from a hastily done one. The pipes must lie evenly, at the correct spacing, securely fastened so that they do not move or float during the pouring of concrete or anhydrite. The choice of fixation system depends on the subfloor and whether you are using system boards or a film with a grid.

Fixation Strips

For laying on a flat film with a grid (without system boards), the fastest solution is pipe fixation strip for 16–18 mm pipe, 1 m/100 pieces. These plastic strips are laid directly on the film in the direction of pipe laying and are fastened down through the film into the EPS subfloor. The mutual distance between the strips is usually 500 mm. The pipe is simply clicked into the strips – the entire row of strips forms a solid route for the pipe loop, with the pipe spacing determined by the choice of strips with the appropriate grid or by the mutual position of the strips.

In room corners and when crossing expansion joints, the pipe should never be bent with a sharp curve – pipe fixation bends for PEX 16–18 mm are used. These plastic bends ensure a minimum bending radius, which manufacturers of PEX pipes for DN 16 usually specify as 8–10 times the outer diameter (i.e., minimum 128–160 mm). A smaller radius may cause waviness and flow restriction, or possibly cracking of the outer layer during repeated thermal stress.

Clamps

For quick fixation on system boards with protrusions (knobbel boards, boss boards), strips are not necessary – the pipe is simply pressed between the protrusions. For alternative situations (passing by the manifold, fixing the supply/return pipe towards the manifold), pipe fixation clamps – 50 mm, straight are used. These clamps are nailed or clipped into the EPS board and hold the pipe in a stable position even when not clicked into a strip.

A detailed analysis of various fixation systems – where to use a strip, a clamp, or a bend and why – can be found in the article Fixation of Floor Heating Pipes – Clamps, Strips, and Bends.

Pattern of Pipe Laying

There are three basic patterns: spiral (snail, bifilar), meander (snake), and a combination. For most rooms, the most suitable pattern is the spiral – the supply pipe spirals from the outside inwards, the return pipe in the opposite direction, so that hot and cooler media alternate and the floor is heated evenly. The meander is suitable for long, narrow spaces (hallways, foyers), where the laying geometry is simpler. In practice, living rooms are divided into an edge zone (1.0–1.5 m from the outer wall) with a denser spacing of 10–15 cm and an inner zone with 20–25 cm – this compensates for the higher heat loss at the edges.

Step 4 – Manifold and Connection of Loops

The manifold is the central point of the hydraulic circuit for floor heating. Each loop (usually one room) has its own pipe branch connected to the manifold. The manifold is mounted before pouring into the manifold box, usually embedded in the wall or on the surface in a technical room.

Before pouring the screed, the manifold must be installed, the pipes in the loops connected to the manifold, and the entire system pressurized for a pressure test. The standard STN EN 1264-4 prescribes a pressure test at double the operating pressure, but at least 6 bar, for a minimum of 24 hours (ideally overnight before pouring). The pressure must not drop by more than 0.2 bar during this time. Only if the pressure test is successful can you call the concrete or anhydrite layer installer.

The length of one loop is limited by hydraulic resistance – for pipe DN 16 (outer diameter 16 mm, wall thickness 2 mm) the maximum loop length is usually 80–100 m, for DN 17×2 up to 120 m. Longer loops are hydraulically unbalanced and some parts of the floor are not heated evenly. In real practice, we most often encounter loops of 60–80 m with a spacing of 15 cm, i.e., with a loop area of approximately 9–12 m².

Step 5 – Pressure test before pouring

A pressure test is an indispensable prerequisite for any quality installation. It is carried out with air or water. A water test is more reliable – air is compressible and a small leak is hard to detect. Procedure:

Fill the system with water, vent each loop at the manifold.
Use a pump to increase the pressure to the specified value (e.g., 6 bar).
Close the loops and monitor the manometer for at least 24 hours.
Check the connections at the manifold, the ends and each connection on the pipe.
Record the results in the installation report with photographs – serves as proof for any warranty claims.

If the pressure drops, systematically search for the leak: first check all threaded connections at the manifold. In practice, 90 % of leaks occur precisely here, not in the pipe itself. PEX or PE-RT pipe without joints (a key advantage of the floor heating system – the pipe is always inserted in one piece without joints in the pour) is extremely reliable and with proper installation provides a lifespan of 50+ years.

Step 6 – Pouring the screed: anhydrite or cement wet screed

There are two main types of screed materials used for floor heating: anhydrite (flow screed) and cement wet screed. Each has its advantages and limitations.

Anhydrite flow screed (CAF)

Anhydrite (calcium sulfate) is today the dominant material for floor heating in new buildings. The reasons are clear:

Transfers heat very well – thermal conductivity coefficient λ ≈ 1.8–2.1 W/(m·K), which is significantly higher than cement screed (approx. 1.2–1.5 W/(m·K)).
Poured by pumping, not manually – quick application even for large areas.
Self-levelling – does not require vibration or manual smoothing; it spreads out on its own into the plane.
Minimum coverage over the pipe DN 16 is 25–30 mm (total screed thickness over the insulation is approx. 60–65 mm).
Low shrinkage – minimal risk of cracking, expansion joints are not required in the flat (only edge and door transition joints).

Disadvantage: anhydrite is not suitable for permanently damp areas (bathrooms, terrace extensions) without special surface treatment, as it is sensitive to moisture. Therefore, cement screed is usually chosen for bathrooms.

Cement wet screed (CT)

Classic cement screed (cement : sand ratio approx. 1:3 to 1:4, water-cement ratio w/c 0.4–0.5) is still used in bathrooms and in areas with higher humidity, as well as where investors or designers insist on traditional solutions. It is more robust and resistant to moisture and, with proper treatment, achieves excellent strength. Its disadvantage is higher shrinkage – expansion joints are required every 25–40 m² (for floor heating every 15–20 m² or max. 5 m width). Another disadvantage is lower thermal performance compared to anhydrite at the same thickness.

Thickness of the covering layer

Parameter	Anhydrite (CAF)	Cement screed (CT)
Min. coverage over pipe DN 16	25–30 mm	30–45 mm
Thermal conductivity λ	1.8–2.1 W/(m·K)	1.2–1.6 W/(m·K)
Expansion joints in the flat	Only edge joints	Every 15–20 m²
Suitability for bathrooms	Limited	Yes
Drying time (25 mm)	4–6 weeks	6–8 weeks

Step 7 – Procedure after pouring: drying and first heating

The poured screed requires time and proper conditions for sufficient drying. During the first 24–48 hours, no one should walk on the screed or place materials on it (exception: anhydrite usually allows walking after 24–48 hours under normal conditions). The room must be protected from drafts and the temperature must be above +5 °C.

First heating (also called "Belegreifheizung" or commissioning heating) is a crucial step that many investors underestimate. The standard EN 1264-4 prescribes: after at least 21 days from pouring, start heating with 25 °C supply water temperature for 3 days, then increase by 5 °C per day until reaching the maximum operating temperature, which is maintained for 4 days. The entire cycle takes about 2 weeks. The aim is to gradually remove residual moisture from the screed – rapid heating would cause cracking.

For anhydrite, heating is even more important, as the anhydrite slab must be sufficiently dry before laying the floor covering. The moisture content of anhydrite measured by the CM method must be less than 0.3 % (for non-heated rooms) or 0.5 % (for heated rooms). For wooden floors (solid, glued parquet), the limits are even stricter – usually max. 0.3 %.

Step 8 – Walking surface and final works

Once the screed is dry and tempered, you can proceed with the installation of the walking surface. Each material has different thermal properties and installation requirements:

Ceramic tiles and stone materials – ideal for floor heating. Low thermal resistance (practically zero with a thin adhesive layer), high thermal mass. Adhere using flexible adhesive suitable for heated floors (designation S1 or S2 according to EN 12004).
Vinyl floors (LVT, SPC) – an increasingly popular material, low thermal resistance, quick response to temperature changes. Manufacturers usually specify a maximum floor surface temperature of 27–28 °C. Most modern LVT/SPC floors are approved for floor heating.
Laminate floors – suitable if conditions are met (label "suitable for floor heating" on the packaging). The thermal resistance of the floor including underlayment must not exceed 0.15 m²·K/W. Therefore, classic soft foam is not laid under laminate, but a special thin underlayment with low Rλ.
Solid wood floors and parquet – the most sensitive material. Wooden floors are hygroscopic and react to changes in humidity. The maximum surface temperature is limited to 27 °C. The wood must be acclimatized before installation, and the screed moisture must be below 0.3 %. We recommend consulting the parquet supplier.

The topic of how the floor composition must be designed for a combination of heating and cooling is covered in the article Floor heating and cooling – what must the floor composition meet?

Most common mistakes in practice and how to avoid them

Over the years of practice, we have seen a wide range of errors in the installation of floor heating – from minor technical flaws to fatal structural failures. Here are the most common ones:

Omission of edge expansion tape – the screed has nowhere to expand and cracks during the first heating, sometimes even lifting the walking surface.
Incorrect insulation thickness – too thin insulation on the ground floor increases heat loss towards the foundation by 20–40 %, which directly increases operating costs for the entire lifetime of the house.
Too long circuits – a circuit longer than 100 m with DN 16 causes high pressure drop, uneven heat distribution and noise in the pipes.
Connections and bends in the screed – any connection in the screed is a potential risk. The pipe to the manifold must always go in one piece, without connections.
Omission of pressure test – investors want to save time and pour the screed without testing. After pouring, a minor leak takes several times longer to locate and the repair is several times more expensive.
Too quick start-up – starting heating immediately after pouring without observing the waiting period and temperature protocol leads to screed cracking.

A detailed analysis of this topic can be found in the article Common mistakes in the installation of floor heating pipes and how to avoid them.

System board vs. grid foil – short comparison in the context of installation

The question of whether to use a system board (bossed board with pins) or just a grid foil is one of the most practical questions when planning the installation. A system board simplifies pipe fixing – the pipe is simply pressed between the pins and holds by itself without additional clips. However, the price of a system board is higher. Grid foil and fixing strips are cheaper, but the installation takes longer and requires higher precision. A comprehensive comparison of both approaches including recommendations for specific situations can be found in the article System board vs. grid foil – what is better for your subfloor?

Checklist before pouring – what must be fulfilled

Before calling the anhydrite or concrete screed layer, go through this list and confirm each item:

The subfloor is flat, dry, without loose areas.
Hydro insulation is applied on the ground floor and in bathrooms (if prescribed).
Edge expansion tape is glued around the entire perimeter of each room.
Thermal insulation is laid without gaps, boards are staggered.
Separation foil covers the entire area with at least 100 mm overlaps, joints are taped with metallic tape.
The pipe is laid at the prescribed spacing, secured with strips, clips or a system board.
Circuits are connected to the manifold, the manifold is mounted in a box.
The pressure test (min. 6 bar, min. 24 hours) was successfully carried out and is documented.
The system remains pressurized during the entire pouring process.
The room is well ventilated during pouring and setting, temperature does not drop below +5 °C.

Frequently asked questions (FAQ)

What is the minimum thickness of the screed over the floor heating pipe?

For anhydrite (flowable) screed with DN 16 pipe, the minimum cover thickness over the pipe is 25–30 mm. For cement wet screed, a minimum of 35–45 mm coverage is recommended, because cement screed has lower thermal conductivity and a greater thickness ensures more even heat distribution. These values are minimums – coverage is increased under mechanical loading (industrial floors, warehouses).

Can I install floor heating in a bathroom?

Yes, bathrooms are very suitable for floor heating – tiles conduct heat very well and a warm floor is especially appreciated here. However, you must use cement screed instead of anhydrite (due to moisture), apply waterproofing under the tiles (sticker or system waterproofing foil), and install the edge expansion tape in the bathroom before waterproofing, not after it. Pipe spacing in bathrooms is chosen to be denser – usually 100–150 mm.

How long do I have to wait after pouring before I can lay the floor covering?

The minimum waiting time depends on the type of screed and its thickness. For anhydrite screed with a thickness of 60–65 mm, the rule of thumb is: 1 week for every 10 mm of thickness under normal conditions (20 °C, relative humidity 65 %). For a thickness of 65 mm, it is therefore at least 6–7 weeks. For cement screed, the times are similar or longer. Always measure the screed moisture with the CM method before laying tiles, vinyl or wood.

How many circuits do I need for a typical family house?

Approximately: one circuit per room, or two circuits for larger rooms (over 15–20 m²). A typical family house with 5 rooms + corridor + bathroom needs 7–9 circuits. The exact number depends on the heat loss of each room, maximum circuit length and hydraulic calculation. The manifold is selected according to the number of circuits – commonly 8-way to 12-way versions.

Do I need to make expansion joints in the screed for floor heating?

For anhydrite screed, edge expansion joints (tape around the perimeter) and joints at door transitions or changes in the main pipe direction are sufficient. Expansion joints in the slab are not mandatory for anhydrite up to an area of about 40 m². For cement screed, expansion joints are required every 15–20 m² or max. 5 m in one direction. Joints are filled with elastic sealant (e.g. PU sealant) and are also realized in the walking surface.

Can I use cooling in summer with floor heating?

Yes – technically, floor heating systems are capable of operating in cooling mode (circulation of cold water at about 16–18 °C). However, you must be careful: cooling poses a risk of condensation of moisture on the floor surface (dew point), which is unacceptable. Therefore, cooling is conditional on humidity control using dew point regulation and sometimes also forced air recovery. All details can be found in the article Floor heating and cooling – what must the floor composition meet?

Conclusion: quality hides in the details

The installation of floor heating is not complicated in its basic logic – layered composition, precise fixing of the pipe, pressure test, controlled pouring and tempering. The complexity lies in dozens of details, which, if neglected, will either result in significantly higher operating costs (poor insulation) or real structural damage (cracking, leaks, condensation). A properly dimensioned, professionally installed and well-controlled floor heating system will reliably serve you for 30–50 years without major maintenance. If you want to learn more about maintenance, air venting and operating parameters, we recommend the article Maintenance and air venting of the floor heating system and also the comprehensive section Frequently asked questions about area heating and cooling.

All key materials – separation foils, metallic tapes, fixing strips, arches and fasteners – can be found in the floor heating and cooling category on atria.sk. If you are unsure about your choice, describe your project to us – we will gladly recommend a specific solution based on room dimensions, type of subfloor and required performance.

Do you have a question about this topic?

Having trouble deciding or dealing with a specific situation in your home? Write to us – we are happy to help.