What Thickness of Polystyrene Is Needed Under Underfloor Heating

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What thickness of polystyrene is needed under underfloor heating – a complete technical guide

This is a question we deal with practically every day with our customers. And although it might seem that the answer is simple – "the more, the better" – reality is much more nuanced. The thickness of the insulation layer under underfloor heating directly affects how much heat goes down into the concrete (where you don't need it), how much goes up into the room (where you want it), and the overall energy behavior of the system. At the same time, the thickness of the insulation board is a construction parameter – it takes up construction height, affects door thresholds, stair heights, and other architectural details.

In this article, we'll go through everything from the basics of thermal resistance physics, through normative requirements, to specific recommendations for different types of floors and real-life situations.

Why is the thickness of insulation under underfloor heating important at all

Underfloor heating works on the principle of radiant heat – the heated floor surface radiates heat into the space. The pipes (or electric cable) are laid in a construction layer, which is bounded from below by thermal insulation. If the insulation is weak, a significant part of the generated heat escapes downward – into the structure, into the ground, or into a neighboring housing unit. This has two unpleasant consequences:

Energy inefficiency: The boiler or heat pump must supply more energy to maintain the required room temperature.
Longer warm-up time: A poorly insulated floor heats up more slowly because a significant part of the energy first "saturates" the concrete structure beneath it.

On the other hand, insulation that is too thick unnecessarily increases the construction height and in practice brings only minimal additional thermal gain – the law of diminishing returns applies very consistently here. Finding the right thickness means finding the optimum between thermal resistance, construction height, and price.

Basic physical concepts: thermal conductivity coefficient and thermal resistance

To be able to talk sensibly about thicknesses, we need to clarify two concepts:

λ (lambda) – thermal conductivity coefficient [W/(m·K)]: Indicates how much heat passes through a 1 m thick material at a temperature difference of 1 K. The lower the value, the better the insulator. Standard expanded polystyrene (EPS) has λ = 0.031–0.045 W/(m·K) depending on density and quality.
R – thermal resistance [m²·K/W]: Calculated as d/λ, where d is the layer thickness in meters. The higher the R, the less heat passes through the structure. For insulation layers under underfloor heating, R values are decisive.

Example: Polystyrene 80 mm thick with λ = 0.035 W/(m·K) has a thermal resistance R = 0.080 / 0.035 = 2.29 m²·K/W. This is a real value that designers work with in calculations.

What does the standard EN 1264 and Slovak legislation say

Underfloor heating designers rely on the standard EN 1264 (Water based surface embedded heating and cooling systems), which, among other things, defines minimum requirements for the thermal resistance of insulation. The standard distinguishes situations according to what is located beneath the floor structure:

Heated room in the same residential building (e.g., apartment below an apartment): Minimum thermal resistance of the insulation layer R ≥ 0.75 m²·K/W
Unheated room below (e.g., cellar, garage): R ≥ 1.25 m²·K/W
External environment / ground (ground floor on terrain): R ≥ 1.25 m²·K/W, in practice designers recommend higher values

Slovak Decree No. 364/2012 Coll. (implementing the Act on Energy Performance of Buildings) sets minimum values for the thermal resistance of the entire floor structure for new buildings. For new family houses, the requirement is R ≥ 3.5–5.0 m²·K/W for a floor on the ground, which, combined with the other layers of the structure, again leads to the need for quality insulation under the pipes.

In practice, this means that for a ground floor on terrain you need at least 80–120 mm of EPS under the screed with piping, while for an intermediate floor above a heated space, 20–40 mm is sufficient. Let's break this down in detail.

Thickness according to floor location in the building – specific recommendations

1. Ground floor on terrain – the most critical case

This is the situation where the thickness of the insulation layer under underfloor heating matters most. Beneath the structure is the ground, which at a depth of about 1–2 m has a relatively stable temperature of around 8–12 °C. The temperature difference between the screed with piping (typically 30–45 °C) and the ground is enormous – heat losses downward are extreme without sufficient insulation.

A typical composition of a ground floor on terrain in a new family house looks like this: compacted soil → gravel bedding → base concrete screed → waterproofing → thermal insulation EPS (100–150 mm, sometimes more) → system insulation board with piping → screed → floor covering.

For a ground floor on terrain, we recommend a total thermal insulation thickness under the piping of at least 100 mm, and in new buildings with low energy consumption (low-energy houses, passive houses) 120–150 mm or more. In practice, this is achieved by combining:

Bottom layer: standard EPS 100 or EPS 150 S (more load-resistant) in a thickness of 80–100 mm
Top layer: system insulation board with studs for laying the pipe – for example the System Insulation Board UHP55 (STIROTERMAL BASIC), which has a total thickness of 55 mm and is designed exactly for situations where you need a combination of thermal resistance and convenient pipe installation

The resulting thermal resistance of the insulation itself with 100 mm EPS (λ=0.035) + 55 mm system board: R = 0.100/0.035 + 0.055/0.035 ≈ 2.86 + 1.57 = 4.43 m²·K/W. Without further construction layers, this fully meets even stricter energy performance requirements.

2. Intermediate floor above an unheated space (cellar, garage)

The situation is similar to a ground floor on terrain – below the floor is a cold environment. The standard requirement of R ≥ 1.25 m²·K/W for the insulation itself is a minimum. In practice, we recommend:

A system insulation board with a thickness of 55–80 mm as the sole insulation layer, if the construction height is limited
Alternatively, a combination of 50 mm EPS + system board – total thermal resistance around 2.5–3.0 m²·K/W

In a garage below a living room, we have repeatedly seen situations where the investor underestimated the insulation thickness and then wondered why the underfloor heating did not reach the required temperature, even though the boiler was correctly sized. When we measured surface temperatures, it turned out that more than 40% of the heat was going into the garage ceiling.

3. Intermediate floor above a heated room (typical apartment building)

Here the situation is completely different. Below the floor is a heated space – for example, a neighbor's apartment. The temperature difference is minimal (typically 5–10 °C between floors). Heat losses downward are small. Here, the insulation serves two functions:

Preventing cross heat flow: Your underfloor heating system should not "heat" the neighbors below you
Impact sound damping: Sound insulation (which is just as important as thermal insulation for intermediate floors)

For this situation, a system board with a thickness of 25–40 mm is sufficient. The standard requires R ≥ 0.75 m²·K/W, which at λ = 0.035 means at least 26 mm – this is adequately covered by most standard system boards. An excellent choice for limited heights is the System Insulation Board UHP51 (STIROTERMAL DUO 11), which combines a low overall thickness with sufficient thermal resistance for intermediate floors above heated spaces.

Impact of the building's energy class on the required insulation thickness

The building's energy class significantly affects the actual thickness of insulation needed under underfloor heating. The designer determines exact values based on heat loss calculations and balances, but here are practical reference values:

Energy class / building type	Ground floor on terrain	Above unheated	Above heated
Older renovation (class E–F)	80–100 mm	50–60 mm	25–30 mm
Standard new building (class C–D)	100–120 mm	60–80 mm	30–40 mm
Low-energy new building (class A–B)	120–150 mm	80–100 mm	40–55 mm
Passive house / near-zero energy	150–200 mm or more	100–120 mm	55–80 mm

Note: These values are indicative. For a specific project, always base your decision on the project documentation and calculation. If you don't have a designer, a good aid is a technical advisor from the system board manufacturer or the building's energy certificate, from which you can determine the current heat losses.

System board as part of the overall insulation layer – not a replacement for the bottom insulation

Many people make a fundamental mistake here: they buy a system insulation board (e.g., 55 mm thick) and think that's enough. The system board is designed so that pipes can be laid into it – it has studs or grooves with the correct spacing. However, its insulating thickness is dimensioned only for the "top" layer. For a ground floor on terrain, the system board alone is insufficient, and you must place an additional layer of standard EPS or XPS beneath it.

Such a combination is the standard recommendation for a ground floor:

Bottom layer: EPS 100 S or EPS 150 S, thickness 80–100 mm (standard insulation boards)
Top layer: system insulation board for laying pipes, e.g., STIROTERMAL BASIC (UHP55) with a thickness of 55 mm
Between the layers, don't forget proper overlapping and possible gluing, so that thermal bridges do not occur

For intermediate floors above a heated space, on the other hand, the system board alone is sufficient. Here, a thickness of 25–55 mm is a standard and adequate choice.

Dry vs. wet underfloor heating: does the insulation thickness requirement differ?

In terms of thermal insulation under the system, there is no fundamental difference between the dry and wet installation methods. The underlying physics is the same – heat must not escape downward. The difference lies in the type of system board used and what type of foil or heat distribution plate is added.

For dry underfloor heating (without concrete pouring, with wooden joists or plasterboard panels), a special type of system board is used – for example the Polystyrene for Dry Underfloor Heating UHPD (STIROTERMAL DRY), which is designed so that the distribution plates and pipes fit directly into the grooves without the need for pouring. Under it, the same logic of thickness applies as with the wet system.

An important detail with the dry system: since there is no screed to act as a thermal buffer, the surface of the board and the aluminum distribution plates are more sensitive to thermal bridges. Therefore, not only the total insulation thickness matters here, but also its quality and continuity. To improve heat distribution on the surface, an aluminum foil for dry underfloor heating is used, which reflects heat upward and significantly improves the uniformity of surface temperatures. You can read more about the combination of these elements in the topic Dry vs. wet underfloor heating: what polystyrene and foil are suitable.

Renovation vs. new construction – limitations and solutions

In a new building, you have the luxury of designing all the structure layers from scratch. In a renovation, you run into height limitations – every extra millimeter is a problem (door thresholds, stairs, frame height). This is exactly the situation where the trade-off between thickness and insulation quality is most apparent.

Practical approaches for renovations:

Using XPS instead of EPS: Extruded polystyrene has a lower λ (typically 0.030–0.033 W/(m·K)), so you achieve the same thermal resistance with a smaller thickness. Disadvantage: higher price.
Minimum system board thickness: For example, for an intermediate floor above a heated space, a board of 25–30 mm is sufficient, a height that almost every renovation can accommodate.
Separation foil: During renovations, don't forget about expansion and separation. The Separation Foil 601001H separates the new screed from the walls and ensures that the new structure does not crack in the wrong places. More on this topic in the separate section Separation and aluminum foil in underfloor heating: when and how to use them.
Grinding or milling the substrate: If the old screed is thicker than needed, partial grinding allows maintaining the height balance. However, this is additional construction work.

Impact of insulation thickness on heating water temperature and system response

This is a dependency that many investors don't expect at all: the thickness of the insulation under underfloor heating directly affects what water temperature you must maintain in the pipes to achieve the required floor surface temperature and room air temperature.

If the insulation is weak, the system must "fight" the losses downward – it must operate at a higher water temperature (e.g., 45–50 °C instead of the optimal 35–40 °C). This has far-reaching consequences:

The efficiency of the condensing boiler decreases (condensation occurs only up to an inlet temperature of about 55 °C, but the lower the temperature, the higher the condensation)
The COP (heating output per unit of electrical energy) of the heat pump drops dramatically – for every 1 °C increase in output temperature, COP drops by about 2–3%
Underfloor heating loses its comfort feature – the floor surface temperature may be too high (above 29 °C, which the standard does not allow for living rooms)

Properly dimensioned insulation allows the system to operate at lower water temperatures – and this is exactly the economic and comfort advantage of underfloor heating over radiators.

Special situations: balconies, bathrooms, entrance halls

A few special situations from practice where insulation thickness needs special consideration:

Bathroom: Bathrooms usually have limited areas and higher humidity. Use EPS 150 S or XPS (more moisture-resistant), thickness according to location (above heated space / on the ground floor). Emphasis on waterproofing above the insulation layer.

Entrance hall / vestibule: These spaces are critical – many thermal bridges through door thresholds, frequent opening, cold air from outside. A greater insulation thickness than standard is recommended, along with consistent perimeter expansion strips. In practice, we have seen how a poorly insulated entrance hall "eats up" the energy of the entire system.

Balcony with underfloor heating: Balconies are exposed to the external environment on three sides. Here, the insulation under the pipe must have R ≥ 3.0 m²·K/W, which for standard EPS (λ=0.035) means at least 105 mm. In addition, the insulation must be moisture-resistant – almost always XPS. Connecting the balcony's heating to the main house circuit is technically demanding and requires a separate control circuit.

Practical calculation examples from real projects

Example 1 – Family house, ground floor on terrain, low-energy standard: The customer had a construction height available from the base to the final floor of 250 mm. We designed: 10 mm tiles + 65 mm anhydrite screed with piping + 55 mm STIROTERMAL BASIC system board + 100 mm EPS 100 S = 230 mm. The remaining 20 mm for base concrete and leveling layer. Resulting thermal resistance of the insulation: (0.055 + 0.100) / 0.035 = 4.43 m²·K/W. The heat loss calculation through the floor showed a reduction from the originally estimated 850 W/m² to 380 W/m² – savings of almost 55%.

Example 2 – Apartment building, 3rd floor above a heated apartment unit, renovation: Available height only 85 mm. Solution: 8 mm vinyl flooring + 50 mm cement screed with piping + 27 mm UHP51 system board = exactly 85 mm. Thermal resistance of the insulation: 0.027/0.033 = 0.82 m²·K/W (XPS with λ=0.033). We met the minimum standard requirement of 0.75 m²·K/W, the floor works well, and the neighbors below do not pay for heating someone else's apartment.

Example 3 – Renovation of a garage into a living space: The garage was unheated, now being converted into a study. The customer wanted underfloor heating. Problem: the garage was recessed, height was not limited, but the customer was surprised by the amount of insulation needed. We designed: EPS 150 S with a thickness of 120 mm (load-resistant, moisture-resistant) + 55 mm system board + 65 mm screed + 10 mm tiles = a total increase of 250 mm. The customer was initially skeptical, but after the calculation we showed him that weaker insulation would mean 40% higher annual heating costs.

Most common mistakes in choosing insulation thickness – from practice

We've seen dozens of them and they keep repeating:

Customer buys only a system board without bottom insulation for a ground floor on terrain and wonders why consumption is high. A system board without a bottom layer is insufficient for a ground floor on terrain.
Replacing EPS with lower-quality foam polystyrene (e.g., packaging type): This has worse λ and worse mechanical resistance. Under the screed, you must use facade or floor EPS with a declared λ value.
Underestimating thickness in a renovation with a heat pump: A heat pump requires low water temperatures, which requires sufficient insulation. A compromise in thickness = the need to operate the pump at higher temperatures = lower COP = higher electricity bill.
Omitting expansion strips: This is not directly about insulation thickness but relates to installation. If the boards are laid without expansion from the walls, the screed may crack. Read more in the topic Why does the floor above the insulation board crack or wave.
Same insulation thickness for the whole house: In practice, the ground floor should have different parameters than the intermediate floor. One type of board for everything is a compromise, not an optimum.

This topic is covered in more detail in a separate article Common mistakes in laying polystyrene and system boards under underfloor heating.

How to calculate the thermal resistance of layers yourself – a simple procedure

The calculation is not complicated. Procedure:

Find the λ of each layer (from the product's technical data sheet or from the standard)
Calculate R for each layer: R = d [m] / λ [W/(m·K)]
Add up the R of all layers: R_total = R₁ + R₂ + R₃ + ...
Compare with the requirements of the standard and the project documentation

Calculation example for a ground floor in a low-energy house:

Tiles 10 mm, λ = 1.0 W/(m·K): R = 0.010/1.0 = 0.01 m²·K/W
Anhydrite screed 65 mm, λ = 1.2 W/(m·K): R = 0.065/1.2 = 0.054 m²·K/W
EPS system board 55 mm, λ = 0.035 W/(m·K): R = 0.055/0.035 = 1.57 m²·K/W
Bottom layer EPS 100 mm, λ = 0.035 W/(m·K): R = 0.100/0.035 = 2.86 m²·K/W
Total R = 4.49 m²·K/W – suitable even for low-energy construction

Frequently Asked Questions (FAQ)

Can I use just one 55 mm system board for a ground floor on terrain?

No, for a ground floor on terrain a single 55 mm system board is insufficient. Its thermal resistance is only about 1.57 m²·K/W, which does not even meet the minimum normative requirements for a floor on terrain (R ≥ 1.25 m²·K/W could technically still be met, but considering all layers and the building's energy performance, this is deeply below the recommended minimum). Always combine the system board with an additional bottom layer of EPS with a thickness of at least 80–100 mm.

Is XPS (extruded polystyrene) better than EPS (expanded polystyrene)?

XPS has slightly better thermal insulation properties (λ ≈ 0.030–0.033 vs. EPS ≈ 0.035–0.040 W/(m·K)) and significantly better resistance to moisture and mechanical load. However, it is more expensive. For standard intermediate floors, EPS is fully sufficient. XPS pays off where there is moisture (ground floor on terrain below waterproofing), limited height (you achieve the same R with a smaller thickness), or high load (floors in warehouses, garages).

Can I lay underfloor heating on old EPS without a system board?

Technically yes, but in practice this is not done well. Without a system board, you must fasten the pipes with clips into the EPS or using grid strips, which is slower and less precise. A system board with studs or grooves ensures precise pipe spacing, faster installation, and prevents pipe displacement when pouring the screed. If you have old EPS of sufficient thickness, place a system board on top of it as the upper layer.

How many millimeters of polystyrene will be added to the total floor height when installing underfloor heating?

The total increase depends on the composition, but typically for a wet system, count on: system board 25–55 mm + screed 50–70 mm + floor covering 8–15 mm = a total increase of 83–140 mm. For a dry system without screed, the increase is smaller: system board 30–55 mm + wooden panels/distribution plates 15–25 mm + floor covering = 55–95 mm. For a ground floor on terrain, add the bottom layer of EPS 80–120 mm as well.

Can the insulation thickness under underfloor heating be adjusted or fixed later?

Unfortunately not – the insulation is permanently built in after the screed is poured. If you underestimated the thickness during construction, the only remedy is demolishing the entire structure, which is extremely costly. That's why it pays off to invest in the correct design and sufficient insulation from the very beginning. For an extra few tens of euros per m² of better insulation, you will save annually on heating costs, and the investment will pay for itself in 3–7 years.

Do I need a separation foil under the system board as well?

Separation foil is typically laid above the system board (between the board and the screed) – it prevents cement slurry from getting into the board's studs and serves as an expansion layer. Between the bottom EPS layer and the system board it is not strictly necessary, but some installers use it here too. More on this topic in the article Separation and aluminum foil in underfloor heating: when and how to use them. On the market, for example, there is the Separation Foil 601001H, which is designed exactly for these applications.

Conclusion: correct insulation thickness is an investment, not a cost

The thickness of polystyrene (or overall thermal insulation) under underfloor heating is one of the most important decisions when designing the entire system. Poorly designed insulation cannot be fixed later without demolition, and its consequences – high energy consumption, low thermal comfort, low heat pump COP – will trouble the investor for decades.

Basic rules to conclude:

Ground floor on terrain: at least 100 mm total insulation, for low-energy houses 120–150 mm or more
Intermediate floor above unheated space: 60–80 mm
Intermediate floor above heated space: 25–40 mm according to the standard and energy class
Always combine the system board with a bottom EPS layer for a ground floor on terrain
For renovations, consider XPS for a lower thickness with the same thermal resistance
You can calculate the thermal resistance yourself, but leave the final dimensioning to a designer or technical advisor

If you are still undecided which type of system board is right for your specific

Do you have a question about this topic?

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