How to choose an insulation board for floor heating - thickness, material and requirements

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How to choose insulation board for floor heating – thickness, material and requirements

The insulation board is one of those elements of floor heating that is often overlooked or not given enough attention during planning. Yet it is precisely this that determines whether the system will be energy efficient or whether expensive thermal energy will escape downwards instead of heating the room. From practice, I know that mistakes in this layer of the floor construction are very difficult to correct – in the best case, it will result in higher energy consumption, and in the worst case, you won't even achieve the planned floor temperature even at maximum pump performance.

In this article, we will look at how to correctly choose an insulation board for floor heating in terms of material, thickness, thermal resistance, acoustic properties and mechanical resistance. We will also look at what the standard says, what practice requires and where the most common pitfalls are.

Why insulation under floor heating is key

Floor heating works with a heat transfer medium in the range of 30–55 °C, while the floor surface should be at most 29 °C in living areas and up to 35 °C in the bathroom according to hygiene standards. The system therefore works with low temperatures and relatively large areas – which is its advantage in terms of comfort, but at the same time it means that the heat flow is evenly directed in all directions including downwards.

Without sufficient thermal insulation under the pipes, a significant part of the energy heats the floor structure, the ceiling below it and ultimately the adjacent apartment unit or the ground. In practice, I have seen cases where with insufficient insulation, up to 30–40 % of the thermal output is directed into the structure instead of the room. This means that the system not only increases operating costs, but in extreme cases it is not even able to meet the performance requirements of the project.

In addition to the thermal function, the insulation also performs an acoustic function – it reduces footfall noise – and a mechanical function – it ensures even placement of the pipes and rigidity of the base for anhydrite or cement screed floor. Therefore, when choosing an insulation board, it is important to assess several parameters at the same time, not just the thermal conductivity coefficient.

Types of insulation boards – overview of materials

Several basic materials are used in the market for insulation under floor heating. Each of them has its advantages and disadvantages, and therefore the choice depends on the specific use, the required thermal resistance, the available construction height and the financial budget.

EPS – expanded polystyrene

Expanded polystyrene (EPS) is by far the most commonly used material in residential construction. It is cheap, easily available and available in various densities. For floor heating, EPS with a minimum bulk density of 20 kg/m³ (EPS 70) is used for less loaded floors, but EPS 100 or EPS 150 with a thermal conductivity coefficient λ = 0.033–0.040 W/(m·K) is more commonly recommended.

A disadvantage of EPS is relatively lower thermal resistance per centimeter of thickness compared to PIR boards. To achieve a value of Rλ = 1.25 m²K/W (a typical requirement for a floor above a heated space), you need a minimum thickness of 48 mm with EPS with λ = 0.038 W/(m·K). For a floor on the ground or above an unheated basement space, the requirements are significantly higher.

Important: for floor heating, only EPS with increased rigidity (so-called floor EPS or "EPS-T" for ground) with a pressure strength at 10% deformation CS(10) of at least 70–100 kPa should be used. Standard facade EPS should only be used in exceptional cases in floors, as it compresses under mechanical load and causes cracking of the screed.

PIR – polyisocyanurate foam

PIR boards are a solution where you have limited construction heights. The thermal conductivity coefficient of PIR boards reaches λ = 0.022–0.026 W/(m·K), which means that to achieve the same thermal resistance, you need about half the thickness compared to EPS. PIR boards with a thickness of 30 mm can achieve a thermal resistance comparable to 50–60 mm EPS.

PIR is also suitable for areas with a higher risk of moisture, as its closed cells absorb minimal water. The price of PIR boards is significantly higher than EPS, but in many cases it is the only reasonable choice – for example, in the renovation of an apartment where you don't want to raise the floor level too much, or for floors above unheated garages.

PUR – polyurethane foam

Polyurethane boards have similar thermal properties as PIR (λ = 0.022–0.028 W/(m·K)) and also low wettability. Their mechanical rigidity can vary according to the formulation – for floor heating, look for types with CS(10) of at least 80–100 kPa. In practice, PUR boards appear less frequently than PIR, but in some ranges they are more affordable.

Mineral wool – rock and glass wool

Rock wool (Rockwool and similar) is used very rarely in floor constructions with wet screed, as it absorbs moisture and can lose rigidity under dynamic load (walking). However, there are rigid boards made of rock wool (e.g. "Steprock" type) with high pressure resistance, which are used in special applications – especially where there is a requirement for non-combustibility (A2 fire reaction class). For standard residential floor heating with anhydrite screed, rock wool is not a common choice.

System insulation boards with nubs or grooves

A special category are so-called system boards – EPS or PIR boards with shaped surfaces (nubs, channels or grooves), into which the pipes are laid directly without the need for further fixing. These boards combine the function of thermal insulation and fixing system in one. They are more expensive than plain EPS boards, but they reduce labor and save time during installation.

A detailed comparison of system boards and standard grids is discussed in the article System board vs. grid film – what is better for your base? in this Knowledge Center.

Required insulation thickness – standards and practice

According to STN EN 1264 and related thermal engineering standards, the thermal resistance of insulation under floor heating must be sufficient to direct the heat flow primarily upwards – into the heated space. The specific values vary depending on what is located under the floor.

Floor above the external environment or an unheated garage

This is the most demanding case. The EN 1264-4 standard recommends a minimum thermal resistance of the insulation for a floor above the external air of Rλ ≥ 2.0 m²K/W. In practice, this means:

EPS 100 (λ = 0.038): minimum 76 mm, in practice 80–100 mm is used
PIR (λ = 0.023): minimum 46 mm, in practice 50–60 mm is used

Floor on the ground in direct contact with the soil

For a floor on the ground (first above-ground floor without a basement), the requirement is similar: Rλ ≥ 1.25–2.0 m²K/W depending on the climatic zone. For Slovak conditions (design temperature –15 °C), the following is commonly designed:

EPS 100 on the ground: 80–120 mm (depends on a detailed thermal calculation)
PIR: 50–70 mm

I would like to point out that the used EPS must be resistant to moisture and ground moisture – it is a special type marked as EPS-T or EPS P (for ground). Regular roof or façade polystyrene is not suitable here.

Floor between two heated floors

This is a situation that is technically the easiest to solve, but where insulation is often underestimated. Since there is a heated space below you, the losses downwards are smaller – but they still exist and you still pay for them. For the situation between floors, EN 1264-4 defines a minimum Rλ = 0.75 m²K/W. This corresponds to, for example:

EPS 100: minimum 28 mm, in practice 30–50 mm is used
PIR: minimum 17 mm, in practice 20–30 mm is used

From the perspective of acoustics and comfort, it is recommended to use at least 30–50 mm EPS in the situation between floors, especially due to footstep sound insulation.

Practical example – a typical apartment building, middle floor

In most apartment buildings built after the year 2000, I deal with floor heating on the middle floor (above and below a heated apartment). Here, the minimum thermal requirement of Rλ = 0.75 m²K/W is met with 30 mm EPS 100. For acoustic reasons, however, I design 40 mm EPS with acoustic surface treatment or a combination of 30 mm EPS + acoustic mat. The total construction thickness of the assembly then comes to 110–130 mm: 40 mm insulation + 70–90 mm anhydrite screed.

Mechanical strength of insulation – what is often underestimated

The selection of insulation board cannot be based only on thermal resistance. Each board must be able to withstand the load acting on it during the pouring of the screed and during operation. An anhydrite screed with a density of around 2 000 kg/m³ and a thickness of 65 mm weighs about 130 kg/m². In addition, the live load on the floor is added (in apartments 150–200 kg/m², in office spaces 300–500 kg/m²).

For residential use (apartments, single-family houses), a compressive strength CS(10) ≥ 100 kPa is sufficient. For commercial spaces, warehouses or areas with heavy traffic, a minimum of CS(10) ≥ 200 kPa is recommended, or EPS 200 or PIR with a confirmed load-bearing capacity. If you use a softer EPS, the screed will gradually settle and cause cracking of the joints – with tiling, you will notice it by cracking joints and a hollow sound when stepping.

Another factor is the sensitivity of the insulation to temperature. EPS is sensitive to temperatures above 80 °C – but this does not happen in floor heating, as the temperatures in the system do not exceed 55–60 °C. PIR is more resistant, which you will appreciate especially during pressure testing of the system with hot water.

Separation film and its role in the floor structure

A separation film is always laid between the insulation board and the anhydrite screed. Its function is twofold: it separates the wet screed from the insulation (preventing the absorption of moisture into the EPS), and it prevents the leakage of cement milk or anhydrite mixture into the joints of the insulation. The film also forms a sliding bed, which allows the screed to freely thermally expand without pulling the insulation away.

For this function, an ideal choice is separation embossed film 0.1×1030 mm with AL surface – 50 m². Its embossed surface (grid with, for example, a 50 mm pitch) also serves as an orientation aid when laying the pipe – you can see exactly whether you are maintaining the planned spacing. The aluminum layer reflects heat upwards, which slightly improves the efficiency of the system. The film must be laid with overlaps of at least 10 cm and the overlaps are joined using metallic tape for joining films 55 mm wide – this prevents the formation of gaps through which anhydrite could penetrate.

More about when a waterproofing layer is also needed (not just a separation film) can be read in the article Waterproofing film under floor heating – when and how to use it?

Acoustic properties of insulation – footfall noise

Floor heating in apartment buildings must also meet acoustic requirements according to STN 73 0532. Footfall noise is one of the most common complaints in panel and masonry apartment buildings. An insulation board under floor heating helps to reduce footfall noise, but not all boards do so equally.

Softer EPS with lower density better absorbs dynamic load, but has worse mechanical strength. Harder EPS 200 is mechanically better, but less acoustic. The solution is special acoustic boards with an integrated soft layer (e.g. EPS T with a bottom mineral wool layer), or a combination of rigid thermal insulation and a thin acoustic mat on the surface.

In a standard apartment building with a 65 mm anhydrite screed and ceramic tiles on an intermediate floor, a reduction of footfall noise by 20–25 dB can be achieved with 40 mm EPS 100, which in combination with the original load-bearing structure usually suffices to meet the regulatory requirements (Lnw ≤ 58 dB for an apartment above another apartment).

Practical tips for laying insulation

From practice, I know that even a quality board can cause problems if it is not laid correctly. Here are the most important rules:

The base must be flat and dry. Deviations in the base greater than ±5 mm per 2-meter rule will cause the board to "float" in some places and break under load. Level the base with self-leveling compound or a sand bed.
Lay the boards in a staggered pattern. Just like bricks – each row must be shifted by half the length of the board, so that the joints are not continuous. Continuous joints are a weakness where the screed can shift or crack.
Edge strips are mandatory. Around the entire perimeter of the room and around each fixed element (columns, pass-throughs), it is necessary to use a dilatation edge strip (usually 8–10 mm EPS or foam), which allows thermal expansion of the screed. Without it, the screed will crack when heated.
Pass-throughs under doors must be solved correctly. Each space (room) should have its own dilatation field. Pass-throughs under doors are solved with a joint with a steel strip or with a gap filled with silicone.
Protect the insulation during construction. EPS is sensitive to organic solvents, acetylene (some contact adhesives) and UV radiation. Do not leave it uncovered on the site for long periods.

After laying the separation film, it is time to fix the pipe. For this, either fixing strips, clips or system boards are used. Pipe fixing strip 16–18 mm in length 1 m is a solution that allows for quick laying in a regular grid – the strips are attached to the film with staples and the pipe clicks into them. For turns and bends, fixing bends for PEX pipe with diameter 16–18 mm are used, which prevent the pipe from springing in bends. For straight sections, an alternative is pipe fixing clip 50 mm for straight runs.

Selection of insulation for special situations

Apartment renovation with low remaining space

This is the most common problem in the renovation of panel apartments. The original floor is milled or demolished, leaving only the bare ceiling slab, and it is necessary to fit insulation, pipes and a screed into a total height of 100–120 mm. In such cases, PIR boards with a thickness of 30–40 mm together with a dry anhydrite or a thinner wet anhydrite of 50–55 mm are the only solution that will meet the thermal resistance (Rλ ≈ 1.30–1.74 m²K/W at PIR λ = 0.023).

Single-family house on ground – new construction

Here you have the space to design the composition without compromises. A standard solution is 80–120 mm EPS 100 T on waterproofing, separation film, anhydrite 70 mm. Total thickness 160–200 mm. If you want to achieve the energy standard A0 or passive standard, consider 200 mm EPS or 100 mm PIR. In a passive house, the ground floor is one of the most important places of heat loss and insufficient insulation here will never be compensated by wall insulation.

Floor heating in the bathroom

The bathroom is specific due to its humidity and small area. It is always recommended to use a waterproof coating or film under the floor insulation in the bathroom to prevent moisture from rising through the insulation into the screed. EPS used in the bathroom should be closed-cell structure (EPS-T) with low water absorption. PIR is a better choice here in terms of moisture resistance. More on this topic can be found in the article Waterproof film under floor heating – when and how to use it?

Floor heating systems with cooling

If you plan to use a floor heating system for summer cooling as well, higher demands are placed on the insulation. During cooling, moisture can condense on the surface of the screed (dew point), so all elements of the composition must be water-resistant or protected. EPS-T is also suitable here, but it is necessary to seal the edge strips and films thoroughly. This topic is covered in detail in the article Floor heating and cooling – what must the floor composition meet?

Common mistakes in the selection and installation of insulation

Over the years of contract practice, I have seen several typical mistakes that repeat:

Using facade EPS in the floor. Facade EPS (e.g., EPS-F or "white box" from the hardware store) has too low compressive strength (CS(10) 10–30 kPa) and is compressed under the load of the screed and usage. The result is tile cracking after 1–2 seasons.
Insufficient insulation thickness on the ground. The most common cost-saving mistake – the investor wants to save on insulation and installs 30 mm instead of 100 mm. Result: the system cannot heat the rooms in freezing weather, and the consumption is 2× higher than planned.
Missing edge strips. Without edge strips, the screed has nowhere to expand and will crack after several seasons of thermal stress – usually in corners and at transitions.
Incorrect joining of the separation film. Films without gluing or without metallic tape tear during anhydrite pouring. Anhydrite penetrates under the film, into the insulation joints and causes uneven hardening of the screed.
Insulation without protective film before concreting. If you do not cover the insulation with a separation film before pouring the screed, free water from the anhydrite will saturate the surface of the EPS and slow down the hardening of the screed, leading to uneven strength.

A more detailed overview of errors during the entire laying process can be found in the article Common mistakes in laying the pipe of floor heating and how to avoid them.

Summary – how to decide

Selecting an insulation board is not a matter of feeling or price per m². It is a technical decision that will affect the energy efficiency and lifespan of the entire system for decades. Here is a brief decision guide:

You have enough height and it is a new construction or ground floor → EPS 100 or EPS 150, thickness 80–120 mm according to the thermal calculation.
Renovation, little space → PIR 30–50 mm, thermal resistance Rλ ≥ 0.75 (interfloor) or Rλ ≥ 1.25 (ground/garage).
Bathroom or room with humidity → EPS-T or PIR + waterproof layer.
Commercial spaces, heavy use → EPS 200 or PIR with confirmed CS(10) ≥ 200 kPa.
Passive house → 150–200 mm EPS or 80–100 mm PIR.

In any case, it applies: investment in quality and sufficiently thick insulation pays off in the first years of operation by saving on heating. And what should not be postponed: proper installation, edge strips and thorough joining of the separation film.

Most frequently asked questions (FAQ)

Can I use standard facade polystyrene as insulation under floor heating?

No. Facade EPS has too low compressive strength – typically CS(10) only 10–30 kPa. Under the load of an anhydrite screed and usage, it will compress, leading to tile cracking and an uneven floor. Use floor-specific EPS (EPS 100, EPS 150, EPS 200) with a minimum CS(10) of 70–100 kPa for apartments, or PIR boards for floor heating.

What is the minimum insulation thickness under floor heating in an apartment building?

For an interfloor situation (above and below heated apartment), the minimum thermal resistance is Rλ = 0.75 m²K/W. This corresponds to approximately 28 mm EPS 100 or 17 mm PIR. In practice, at least 30–50 mm is recommended due to acoustics (footstep noise). For ground floors, the minimum requirement is significantly higher: Rλ ≥ 1.25–2.0 m²K/W.

Is a separation film necessary if I have a system board with nops?

It depends on the specific product. Most system boards with nops have their own surface closure (laminated PE film) that serves as a separation film – but only on the top surface. Joints between boards and transitions to the edge strip must always be covered and sealed. If the board manufacturer recommends an additional separation film, follow their instructions.

Can the insulation under floor heating be wet when pouring anhydrite on it?

Surface moisture from anhydrite is not a problem for EPS if the insulation is properly covered with a separation film. Problems arise if the EPS is soaked (e.g., it has been stored for a long time without protection from rain or condensation). Wet EPS loses its thermal insulation properties and tends to long-term degradation. Always check the dryness of the insulation before pouring the screed.

Does the insulation for floor heating differ from that for floor cooling?

Thermal requirements are the same. However, during cooling, moisture condensation can occur on the floor surface and within the composition at the dew point – so the entire composition, including the insulation, must be moisture-resistant. PIR is more suitable than standard EPS for cooling applications. Details on the requirements for the composition can be found in the article Floor heating and cooling – what must the floor composition meet?

What is better – a system board with nops or standard EPS with a grid film?

It depends on the situation. A system board simplifies pipe laying and is ideal for smaller areas or DIY installation. Standard EPS with a grid film and fixing strips is cheaper for larger areas and offers greater flexibility for atypical pipe spacing. A detailed comparison of both approaches is in the article System board vs. grid film – what is better for your subfloor? If you plan a comprehensive process from insulation to pouring, also read Installation of floor heating step by step – from insulation to pouring.

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 are happy to help.