What thickness of insulation board is suitable for an apartment and a house
Why the thickness of the insulation board is the key to cost-effective and safe floor construction
In heating floor projects in apartments and family homes, we often encounter the same mistake that we see in every tenth implementation: investors and sometimes even installation companies try to save on materials where it hurts the most. I am talking about the thickness of the insulation boards. Many people think that if they lay a thin layer of insulation, the heat "will still get up there," just a few percent less efficiently. In practice, however, it is different. Choosing the wrong thickness not only leads to higher monthly energy bills but can also cause permanent damage to the building structure, the formation of mold, or even a violation of hygiene standards in the apartment.
The insulation board in a floor heating system is not just a passive material that separates the pipe from the concrete. It is an active component of the building's thermal envelope. Its main task is to direct the heat flow in one direction – upwards into the room. If you choose the wrong thickness, heat escapes downwards, heating the ceiling below (in the case of an apartment) or the ground (in the case of a house), which means your boiler works unnecessarily, consumes more gas or electricity, and you pay for heat that no one feels. In this article, we will look at technical details, real examples from our practice, and precise dimensions that you should use for different types of buildings.
Difference between insulation for an apartment and insulation for a house: Where are the limits?
The most common question we hear in our online shop is: "Should I use 4 cm or 6 cm?" The answer depends on whether it is an apartment space above an unheated attic, above a basement, or above ground level, and whether it is a standalone family house on the ground. The difference is not only in centimeters but in the physics of the process.
In the case of an apartment, where you have floor heating on the second floor and there is a neighbor's apartment below, the situation is specific. Here, it is not primarily about saving heat for you but about impact sound insulation. Your priority is to prevent the transmission of impact sound (footsteps) and at the same time minimize heat loss to the lower apartment. If you use a thick polystyrene board (for example, 10 cm) without special treatment, you may have a problem with increased floor level and thus with doors, but the main risk is that polystyrene by itself does not provide sufficient damping capacity for impact sound. Therefore, in apartments, we use special impact insulation with a different surface structure.
On the other hand, in a family house on the ground, the goal is to maximize thermal resistance. Heat from the floor tries to penetrate into the cold ground. If the insulation layer is too thin, the ground cools down and a so-called "thermal bridge" forms around the perimeter walls. In this situation, a simple rule applies: the thicker the insulation, the less heat loss. For a house on the ground, a minimum thickness of 10 cm to 15 cm is recommended today, and for low-energy houses (passive houses), this value approaches 20 cm.
Impact insulation: Why 6 mm is enough and 2 cm is unnecessary
Many customers ask why sellers recommend only 6 mm thick boards for impact insulation, when common polystyrene boards are 4 to 10 cm thick. The reason is technical and relates to acoustics, not thermal insulation. In floor heating in an apartment, we need a board that has two properties: it must be strong enough to carry the load of the concrete screed and floor covering without deformation, and it must have high acoustic resistance to impact sound.
Specialized impact insulation (for example, brand Ivar) is made from extruded polystyrene (XPS) or polypropylene, which has an open structure or a special surface profile. This structure works as a vibration damper. If we used a 10 cm thick board made of standard EPS, which is soft and flexible, we would feel a "softness" when walking on the floor and the sound of footsteps would be transmitted very intensively. On the contrary, a 6 mm thick board with a hard core and special surface (often labeled as TDZ - Designed Insulation Technology) ensures that the floor will be firm and the sound will be damped.
In practice, we have seen a case where a company saved on quality and used a thick insulation board on the balcony of an apartment instead of special anti-impact insulation. The result was that after laying the tiles and after a few months, micro-cracks appeared in the ceramic due to an unstable base. At the same time, the neighbor below complained about loud footstep sounds. The correct solution we then provided included Ivar TDZ anti-impact insulation boards in dimensions of 1.5 m or variant 1.0 m. These packages contain a precisely defined thickness of 6 mm, which is optimal for apartment conditions. Variants are also available in packaging of 12.5 m² or 50 m², which allows you to adjust your purchase precisely according to the area of your apartment.
Ground floor house: Where does the savings start and end?
In the situation where you are building a ground floor house, the logic completely changes. Here, the insulation board is the only barrier between the heat of your floor and the cold ground. The physical law is clear: thermal resistance (R) is directly proportional to the thickness of the material (d) and inversely proportional to the thermal conductivity (λ). The higher the R value, the better the thermal properties.
The Building Energy Efficiency Act (amendment to Act No. 555/2005 Coll.) sets minimum values of thermal resistance for ground floors. For Slovakia, these values range around 2.5 to 3.5 m²K/W depending on the region and type of building. Standard expanded polystyrene (EPS) has a thermal conductivity of approximately 0.038 W/mK. Extruded polystyrene (XPS) has a better value of around 0.035 W/mK. This means that to achieve the required thermal resistance, you need a thickness of:
- For a standard house (new build): At least 10 cm XPS or EPS with higher density (min. 30 kg/m³).
- For a low-energy/passive house: It is recommended to use 15 cm to 20 cm, or a combination of two layers of different materials (e.g. 10 cm XPS + 5 cm EPS).
Why is this so important? Imagine a situation where you built a house with 5 cm insulation. The heat from the floor quickly passes through this thin layer and heats the ground under the house. In winter, the ground under the house cools down, condensation of moisture forms, and in the worst case, mold appears in the basement or in the cellar. In addition, your floor heating will have to work at full capacity because the heat will constantly escape downwards. The difference in energy consumption can be up to 20% annually. This means that over 10 years, you will pay more for gas or electricity than the cost of quality insulation.
Practical examples from real estate projects: What happens if you save money?
As a technician who monitors thousands of implementations, I know that theory sometimes meets reality in practice. A case that impressed me was in a new building in Bratislava. The client wanted to save on insulation boards and gave the installer an order to use leftover old boards from other projects, which were only 4 cm thick. The installer approved it, because "the floor will still be warm".
What happened? After the first heating season, the client found out that it was cold in the corners of the rooms and the floor was difficult to heat. The boiler was working at maximum capacity, but the temperature in the apartment did not reach the desired 21 °C. After inspection, it was found that the sub-base layer was cold and the heat flow was going directly into the ground. In addition, moisture appeared in the basement. The only solution was to dismantle the entire floor, which cost the client thousands of euros. If the correct insulation boards had been used (for example, 10 cm XPS thickness), the system would have worked immediately and efficiently.
Another case concerned an apartment building. There, a thick insulation board without impact damping was used. The result was that the neighbors complained about loud footsteps. Since the floor was not sufficiently rigid, it slightly bent when walking and resonance was created. The solution was to rework the floor, which again meant high costs. The correct approach would have been to use anti-impact insulation 1.0 m in packaging 50 m², which is specifically designed for this purpose – a combination of thermal insulation and acoustic damping.
Another common problem is incorrect joining of boards. If you use boards with a small thickness, you must pay attention to perfect joints. If there are gaps, heat escapes through them. Therefore, we recommend using boards with a rebate (so-called "tongue and groove" joints), which ensure a tight connection. In our range, you will also find junction boxes for electrical conductors, which are essential for the installation of floor heating with regulation, so that you can safely connect the conductors without damaging the insulation.
Technical details: Material density and compressive strength
Thickness is not the only parameter to consider. The second critical parameter is the material density and compressive strength. For floor heating in ground floor houses, we recommend polystyrene with a density of at least 30 kg/m³ (EPS) or 35 kg/m³ (XPS). For apartments, a lower density is sufficient if it is impact insulation specifically designed for this purpose.
Compressive strength is important because the insulation board should not collapse under the weight of the concrete screed and floor covering. If you use a soft board with low density, the concrete may bend and cracks may form. This is why we have different types of boards for different applications in our portfolio. When choosing, always check the technical data sheet and verify that the product meets the standards for floor heating.
Another aspect is the installation of pump modules. If you have a large area, you need a properly dimensioned pump module for distribution units. This module must be compatible with your insulation and piping. If the insulation is too thin, it can lead to pipe overheating and subsequent damage to the system. Therefore, it is important that the entire system is designed as a whole.
Installation and common mistakes when laying insulation boards
Regardless of the board thickness you choose, incorrect installation can negate all savings. The most common mistakes include:
- Loose joints: If the boards are not firmly connected, gaps form through which heat escapes.
- Missing vapor barrier: In some cases, it is necessary to use a vapor barrier to prevent moisture from penetrating the insulation.
- Incorrect pipe placement: If the pipe is too close to the edge of the board, it can be damaged during concrete pouring.
- Ignoring expansion joints: For large areas, it is necessary to use expansion joints to prevent concrete cracking.
For a detailed guide on proper installation, we recommend reading the article on common mistakes when laying insulation boards on the subfloor, where you will find specific tips and tricks that will help you avoid these problems.
Lifespan and safety of system boards
One of the most common questions is the lifespan of insulation boards. Polystyrene and extruded polystyrene are materials that are stable and have a long lifespan. With proper installation and protection from UV radiation (which does not occur in the floor), they can last for decades. It is important, however, to ensure that the material is protected from chemicals that could damage it.
Safety is also a key factor. Insulation boards must meet requirements for fire resistance. Most modern boards are classified as difficult to ignite (B2) or non-combustible (A2), which means they are safe to use in living spaces.
Thermal bridges: Why insulation under the edge is key
A common mistake when calculating and laying insulation boards is ignoring the perimeter walls. Heat always tries to penetrate where it finds the least resistance, which means thermal bridges form in room corners and along perimeter walls. If you use only standard boards without additional edge insulation, these areas will be cold, which can lead to condensation and mold growth on the floor surface.
Why some installers use thinner boards? Difference in technology
In practice, you may encounter a recommendation to use thinner insulation boards (e.g., 4 cm or 5 cm) with the claim that "the system can handle it." The reasons are often lower material costs or an attempt to reduce the overall floor height in apartments with low ceilings. Technically, however, it is a compromise that is not worthwhile in the long run. A thinner board has a lower thermal resistance (R), which means that for the same temperature difference, more energy will escape into the subfloor.
While standard polystyrene with a thickness of 5 cm has a thermal resistance of approximately 1.3 m²K/W, a board with a thickness of 10 cm already reaches a value of 2.6 m²K/W. This means that with 5 cm insulation, twice as much heat escapes into the ground as with a 10 cm layer. In the case of apartments with a floor above the basement, the problem may be less obvious, but it still leads to higher boiler energy consumption. Therefore, we recommend always adhering to the minimum normative values set for the given climatic zone and type of building.
Influence of thermal conductivity of the material on the required thickness
Very often, customers compare only centimeters, but forget about the key parameter – thermal conductivity (λ). Extruded polystyrene (XPS) has better insulating properties than standard EPS. This means that 8 cm XPS can have the same thermal resistance as 10 cm of standard polystyrene.
The Critical Role of Edge Strips in Thermal Expansion
The thickness of the board itself is not the only factor affecting floor stability. When heating a concrete screed, it expands. If you do not have a properly dimensioned and installed edge strip, the pressure will be transferred to the side walls, leading to cracks in the floor or upward buckling of insulation boards.
FAQ: Most Frequently Asked Questions About the Thickness of Insulation Boards
Is it possible to use a thicker insulation board in an apartment?
Yes, it is possible, but it may not be economically advantageous. In an apartment, footstep insulation is most important. If you use a thicker board, you may have a problem with an increased floor level and thus with doors. Moreover, a thicker board may not provide better acoustic protection unless it is specifically designed for this purpose.
What is the minimum thickness for a ground floor in a house?
The minimum thickness for a ground floor in a house should be 10 cm for standard houses and 15 cm to 20 cm for low-energy or passive houses. This thickness ensures sufficient thermal resistance and prevents heat loss into the ground.
Can I use regular polystyrene instead of special footstep insulation?
We do not recommend using regular polystyrene instead of special footstep insulation. Regular polystyrene does not have sufficient acoustic resistance and may cause problems with impact sound. Special footstep insulation is designed to combine thermal insulation with acoustic damping.
What should I do if I have already installed the floor and find out that the insulation is too thin?
If you have already installed the floor and find out that the insulation is too thin, the repair will be laborious and expensive. You will have to remove the floor and replace the insulation board. Therefore, it is important to choose the correct thickness before starting the work. If you have any doubts, consult a specialist.
How does the thickness of insulation affect the speed of floor heating?
The thickness of insulation affects the speed of floor heating. Thinner insulation may cause heat to escape more quickly downward, meaning the floor will heat up more slowly. Thicker insulation ensures that heat is directed upward and the floor will heat up more quickly.
Cost of thick insulation vs. energy savings
Although thicker insulation is more expensive, energy savings over time more than compensate for it. In a standard house, savings can reach up to 20% annually, which means the investment can be recovered within 5 to 10 years.
Conclusion: Investment in the right insulation always pays off
Selecting the correct thickness of insulation board is one of the most important decisions when building a floor heating system. It is not only about the cost of the material, but about long-term energy savings, living comfort, and structural safety. For apartments, footstep insulation with a thickness of 6 mm is key, ensuring acoustic comfort and thermal insulation. For ground floors in family homes, a thickness of 10 cm to 20 cm is essential to minimize heat loss into the ground.
Remember that in construction, it is often said: "Frugal at the beginning, rich at the end." But when it comes to insulation boards, it is the opposite. Being frugal at the beginning (on materials) means being rich at the end (on energy bills and repairs). Therefore, it is worth investing in high-quality and properly dimensioned insulation boards. If you have any doubts, do not hesitate to contact our experts, who will be happy to help you choose the right product for your construction.
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.