Floor heating and cooling – what must the floor composition meet?

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Floor heating and cooling – what must the floor composition meet?

Floor heating is now a standard solution in new buildings and renovations. But when cooling is added to it – and more and more customers are considering this combination – the requirements for the entire floor composition become significantly stricter. It's not enough to just lay the pipe, pour anhydrite, and forget about it. A composition that works reliably for heating may fail for cooling if it is not properly designed from the ground up.

In this article, we will examine each layer of the floor composition as an expert would after decades of practical experience – from the load-bearing structure to the final covering. We will explain what each layer must meet, what specific values and dimensions are, where most implementations fail, and what needs to be addressed differently if the system is to cool.

Why the combination of heating + cooling is more demanding than heating alone

The basic physical problem is simple: during cooling, the floor surface is cold, and the air above it is warmer. If the air humidity reaches the dew point, water will start to condense directly on the floor surface or beneath it – in layers where you don't want it. This phenomenon is destructive to most building materials and significantly limits the choice of coverings and most adhesives.

The second complication is thermal expansion differences. A system that operates at surface temperatures of 26–29 °C in winter and cools to 18–22 °C in summer regularly subjects the entire floor composition to cyclic stress in the range of 8–11 °C. Not every covering and not every adhesive can withstand this long-term without damage.

The third factor is the regulation of indoor humidity. Without active control of the relative humidity in the room (via heat recovery or air conditioning), floor cooling is practically unusable in the typical European climate zone during the summer months. However, this is not a matter of floor composition – it is a systemic issue for the entire house.

Protection against condensation consists of three things: proper regulation of the cooling medium temperature (the minimum medium temperature must not drop below the dew point), a hygrostat that automatically stops cooling at excessively high air humidity, and the selection of a final covering that can withstand condensation without permanent damage.

Load-bearing structure – the foundation that must not be underestimated

The composition of floor heating and cooling begins with the load-bearing structure. Whether it is a reinforced concrete ceiling slab, Ytong prefabricates, a wooden beam ceiling, or a ground base, each structure imposes different requirements on the subsequent layers.

The most important property of the load-bearing structure from the perspective of floor heating is its stiffness and bending strength. The distribution layer with the pipe is a relatively rigid monolith that cracks on a flexible base. Therefore, for a wooden beam structure, large-area wet systems (anhydrite, concrete) are not recommended – or only with special structural measures (insertion of boards, reinforced screed). Dry systems (system boards with distribution boards) are much more suitable here.

With a ground base, moisture is critical. If the soil is not sufficiently drained and there is no quality waterproofing under the thermal insulation, moisture migrates upward and damages the entire composition. This is not just an aesthetic problem – during summer cooling, the combination of moisture from the base and cooling can cause condensation in the insulation layers or under the screed.

Thermal insulation – the most important layer of the entire composition

Thermal insulation is the key layer in terms of functionality. Its role is to prevent the loss of heat (or cold) downward and to direct the energy flow into the room. Without sufficient insulation, the entire system loses efficiency and operating costs increase.

For heating systems, minimum thicknesses of thermal insulation under the floor in Slovakia are regulated according to STN EN 1264 and according to the thermal technical standard STN 73 0540. In practice, the following rule applies:

Floor over an unheated space (garage, unheated basement): minimum 100–120 mm EPS with λ ≤ 0.033 W/(m·K), optimally 140–160 mm. For cooling, it is advisable to go beyond these values, as it reduces the risk of condensation from below.
Floor over ground (directly on the ground base): minimum 100 mm, commonly 120–140 mm according to the project calculation.
Intermediate ceiling slab (over heated space): minimum 30–50 mm insulation. Here, primarily sound insulation (e.g., EPS T 4000, footstep insulation), thermal function is secondary.
Ground floor over ground with the requirement for cooling: minimum 120–150 mm, recommended XPS (extruded polystyrene) instead of EPS due to higher compressive strength and lower water absorption.

We will discuss this topic in more detail in the article How to choose an insulation board for floor heating – thickness, material, and requirements in this Knowledge Center. Here we only emphasize aspects relevant to cooling: insulation must be vapor-tight or must be supplemented with a vapor barrier if the ground base or an unheated space with higher humidity is below it.

Compressive strength of insulation during cooling

During cooling, thermal insulation behaves the same as during heating in terms of mechanical load. But beware – if you choose XPS, its λ is indeed lower (higher insulation capacity for the same thickness) and the compressive strength is higher (from CS(10)150 kPa), but you must account for a higher thermal expansion coefficient compared to EPS. For wet systems (anhydrite), this is not a problem, but for dry systems, it must be considered in expansion joints.

Separation and Waterproofing Foil – When and Which One?

This layer is often underestimated or not specified at all in the project documentation. However, it plays two roles: it separates the insulation from the screed layer (preventing cement milk from seeping into the pores of the insulation, which would reduce its insulating properties), and at the same time it serves as a vapor barrier or waterproofing layer, if necessary.

In everyday practice, we encounter two types:

Textured separation foil: a polyethylene foil with protrusions or a grid, which also serves to hold the pipe in place without additional fasteners. For example, textured separation foil 0.1×1030 mm (AL) – the aluminum surface also functions as a reflective barrier, and the connection between the strips is made with an aluminum tape, for example, metallic tape 55 mm × 50 m, which ensures an airtight and strong bond between the foil strips without the risk of cracks in the joints during expansion.
Waterproofing foil (PE foil, min. 0.2 mm): it is mandatory for ground subfloors. It extends over the walls by at least 150 mm and the joints are welded or taped. This is not about separation, but about protecting the entire construction from capillary moisture.

For combined heating and cooling systems, we always recommend combining both functions – a textured aluminum-coated foil as separation, plus a full-fledged waterproofing layer underneath for floors above ground level or above unheated, damp areas. For more details on waterproofing foil, see the article Waterproofing Foil under Floor Heating – When and How to Use It?

Fixing the Pipe – Precision That Affects Performance and Lifespan

The pipe must lie at the correct depth in the screed layer and must not move during pouring. If the pipe floats or shifts, the coverage (thickness of anhydrite above the pipe) will change, which will affect the evenness of surface temperatures and may lead to localized overheating of the floor covering.

In practice, fixing is solved in three ways:

Textured foil with snap-in: the pipe is simply snapped into the foil grid without additional accessories. Fast, but suitable only for straight sections and regular spacing.
Fixing strips: pipe fixing strip 16–18 mm, supplied in meters is placed on the insulation and allows for precise setting of the spacing. Suitable for long straight sections and when different spacings are needed in different parts of the room.
Individual fasteners: pipe fixing clip 50 mm, straight is used either individually (nailing into the insulation), or in combination with strips at curves and direction changes. For curves at the ends of serpentine loops, there is a special solution – pipe fixing arc PEX 16–18 mm, which holds the pipe at the correct bending radius (minimum bending radius for PEX 16 is approx. 8× diameter = 128 mm) and prevents kinking (bending) at tighter curves.

More about pipe fixing, including a comparison of individual systems, can be found in the article Fixing of Floor Heating Pipes – Clips, Strips and Arcs.

Distribution layer – anhydrite or concrete? And how thick?

The distribution layer is the layer into which the pipe is embedded and which distributes heat (or cold) across the entire floor surface. It is the layer with the highest thermal capacity in the entire construction – and precisely for this reason, the system reacts slowly to changes and "holds" the temperature for a long time after the source is turned off.

Anhydrite (calcium sulfate – CA)

Anhydrite is currently the most commonly used material for the distribution layer in floor heating. It has excellent fluidity (self-leveling), minimal shrinkage, excellent thermal conductivity (λ ≈ 1.6–2.0 W/(m·K) after hardening and drying), and the possibility of a thickness above the pipe as low as 25–30 mm.

The minimum total thickness of the anhydrite layer for pipe DN 16 is 50–55 mm (25–30 mm coverage above the pipe + 20–25 mm below the pipe into the insulation is fixed material). For cooling, there is no technological reason to change the thickness, but it should be remembered that a thicker screed = higher thermal inertia = slower reaction to temperature changes. Conversely, a thinner screed = faster regulation, but a higher temperature gradient on the surface. For living areas, the optimum is around 60–70 mm total anhydrite thickness.

Important: anhydrite is sensitive to moisture during hardening and drying. Before laying the final floor covering, the residual moisture must be below 0.5 % CM (for heated floors) or below 0.3 % CM (for cooling, where condensation is a risk). Therefore, anhydrite must be properly dried before laying the floor covering – first naturally for at least 28 days, then active heating (gradually increasing the temperature by max. 5 °C/day) up to the maximum operating temperature and back. This process takes at least 14 additional days.

Cement screed (concrete C20/25 or C16/20)

Cement screed is less thermally conductive (λ ≈ 1.0–1.4 W/(m·K)) and shrinks significantly more – it requires expansion joints every 40 m² or at direction changes, doorways, and under. It is cheaper in material, but more expensive in labor and slower to dry. For combined heating and cooling systems, cement screed is not disadvantageous, but it requires more precise expansion planning, as temperature cycles between winter and summer will intensify expansion movements.

The minimum thickness of cement screed over DN 16 pipe is 30–40 mm (greater than with anhydrite), total screed thickness minimum 65–75 mm.

Dilation of the load-distributing layer – a critical point in a combined system

Dilation joints in the load-distributing layer are necessary. They are divided into:

Perimeter dilation strip: min. 8–10 mm foam (PE foam strip) along the entire perimeter of each room, including beams, columns and corners. Installed before pouring. Reduces transmission of footfall noise and absorbs dilation movements of the load-distributing layer.
Area dilation joints: every 40 m² with anhydrite, every 25–30 m² with concrete. Joints must pass through the entire thickness of the load-distributing layer (not just surface cuts). The pipe must be protected by a protective sleeve (flexible plastic tube, at least 300 mm on each side of the joint) – twice as important in a combined system due to cyclic dilation movements.
Joints at door frames and transitions: each opening = joint. With cooling, floor movements in the range of 6–8 °C annually are greater than with heating alone (where temperature only changes upwards from the base temperature).

Final floor covering – the most important choice in a combined system

Selection of the final floor covering is crucial in a combined heating and cooling system. Each covering must be explicitly certified by the manufacturer for use with underfloor heating. With cooling, the requirements are stricter – the covering must withstand condensation, temperature cycles and humidity changes.

Ceramic tiles and stone

The absolute best choice. Ceramic tiles have high thermal conductivity (λ = 1.0–1.5 W/(m·K)), resistance to moisture, temperature changes and condensation. They are mechanically stable, do not warp and do not deform. The only condition is the use of flexible adhesive class C2S1 or C2S2 (according to EN 12004), i.e. deformable adhesive with a minimum extensibility of 2.5–5 mm. The grout must also be flexible (class CG2WA). Large formats (60×60 and larger) require full coverage (100% bed coverage) and joints of at least 3–4 mm.

LVT and SPC vinyl coverings

Luxury vinyl tiles (LVT – Luxury Vinyl Tile, SPC – Stone Plastic Composite) are the second best choice. They must be certified for underfloor heating (maximum screed temperature 27 °C) and must have a thermal resistance lower than 0.15 m²·K/W (ideally below 0.10). SPC has a mineral composite core and is dimensionally more stable than pure LVT – it better withstands temperature cycles during cooling. Floating installation is suitable, glued installation is even better for optimal heat/cold transfer.

Wood and wooden coverings

Solid wood is problematic in a combined system. Wood is hygroscopic – during cooling it absorbs moisture from condensation and during heating it dries out. This cycle leads to cracking, warping and jointing. If the customer insists on wooden covering, the only acceptable option is:

Multi-layer wooden floors (engineered wood) with a decorative layer of max. 3.5 mm, glued installation (not floating), marked by the manufacturer for underfloor heating.
Solid wood only with strictly controlled interior humidity (relative humidity 45–55 % year-round) and under the condition that cooling is not used in cities with high summer humidity.
The thermal resistance of the wooden covering must not exceed 0.15 m²·K/W. For 22 mm solid oak (λ ≈ 0.19 W/(m·K)) R = 0.022/0.19 = 0.116 m²·K/W – still within the limit, but adhesive and penetration add further resistance.

Carpets

Carpets are generally unsuitable for underfloor heating and cooling – high thermal resistance (R = 0.10–0.40 m²·K/W depending on the type) significantly reduces system performance and with cooling, mold growth under the carpet is a risk in case of condensation.

Requirements for the thermal resistance of the entire construction

The total thermal resistance of all layers above the pipe (load-distributing layer + adhesive + covering) must not exceed the values set by STN EN 1264-4. In practice, the following applies:

Maximum thermal resistance above the pipe: 0.15 m²·K/W (ideally below 0.10 m²·K/W)
Ceramic 10 mm on anhydrite 50 mm: R ≈ 0.007 + 0.03 = 0.037 m²·K/W ✔
LVT 5 mm on anhydrite 50 mm: R ≈ 0.01 + 0.03 = 0.04 m²·K/W ✔
14 mm parquet board (multi-layer) on anhydrite: R ≈ 0.10 + 0.03 = 0.13 m²·K/W ✔ (just within the limit)
22 mm solid parquet board: R ≈ 0.116 + 0.03 = 0.146 m²·K/W ✔ (very tight, without reserve)
Medium type carpet: R ≈ 0.20–0.30 m²·K/W ✗ (not compliant)

Spacing and pipe diameter – influence on surface temperature uniformity

With heating, we accept a surface temperature gradient (difference between the area above the pipe and the area between the pipes) of up to 3–5 °C. With cooling, this gradient is much more critical – if the pipes are too far apart, the cold strip above the pipe may drop below the dew point, while the area between the pipes remains warm. Condensation will thus not appear evenly, but in strips – and that is deadly with wooden covering.

Therefore, for combined systems the following applies:

Maximum spacing for cooling: 150 mm (optimal 100 mm for cooling capacities above 30 W/m²)
With a spacing of 200 mm, the surface gradient during cooling is too large – not recommended for cooling applications.
Pipe diameter: PEX-A or PE-RT DN 16 or DN 20; with larger areas and lower pressure drop, DN 20 reduces hydraulic losses.

A detailed calculation of spacing and pipe diameter is covered in the article What pipe diameter and spacing do I need for underfloor heating? in this Knowledge Center.

Boundary conditions and special situations from practice

Cooling in the bathroom

The bathroom is one of the most risky spaces for floor cooling. Here you combine increased air humidity (with a shower, RH can temporarily reach 80–90 %), cold surfaces and waterproof covering (ceramic). In practice, most designers prohibit cooling in the bathroom or design a separate zone with regulation. If you still plan to cool the bathroom, the hygrostat must be set to a maximum of 50 % RH to activate the system.

Reconstruction – low construction space

Height of the construction is a limiting factor in reconstructions. If you have only 60–70 mm total height available, there is no room for proper thermal insulation. In such cases, the options are: dry systems (system board with aluminum distribution sheet, thickness from 25–30 mm), pipe DN 14 instead of DN 16, or accepting lower system performance and supplementing it with another heat source.

A comparison of system board and grid film can be found in the article System board vs. grid film – what is better for your subfloor?

Ground floor with high groundwater level

Standard waterproofing is not sufficient here. If groundwater rises seasonally above the floor level, the pressure stress on the waterproofing is significantly higher. In such cases, it is recommended to install a sub-drainage, asphalt waterproofing (e.g., SBS film with pressure-resistant properties), and solid sub-base concrete screeds. Cooling is unsuitable under such conditions or must be combined with active humidity control.

Most frequently asked questions (FAQ)

Can floor heating cool without modifying the floor construction?

Mostly not without changes. A standard construction designed only for heating may have unsuitable floor coverings (massive wood, carpet) or lack humidity control. Before activating the cooling mode, you need to verify: type of floor covering and its resistance to moisture, maximum dew point temperature at a given indoor humidity, setting of the control (hygrostat), expansion joints and their condition, and the correctness of the minimum temperature of the cooling medium (minimum 16–17 °C at normal summer humidity).

What is the minimum temperature of the cooling medium in the pipes?

The practical minimum temperature of the medium for floor cooling in living areas is 16–17 °C. A lower temperature would compress the floor surface below the dew point even at relatively low air humidity. The safe medium temperature depends on the current dew point in the room, which must be measured by a hygrostat. The dew point at 26 °C and 55 % relative humidity is approximately 16.3 °C – thus, under these conditions, the medium must have at least 17–18 °C with a safety margin.

Do I need to change the entire floor construction if I want to add cooling to an existing floor heating system?

Not always. If you have ceramic tiles or SPC vinyl, pipe DN 16 with a maximum spacing of 150 mm, and a functional control system, it is sufficient to add a hygrostat and set the minimum medium temperature to 16–17 °C. If you have solid wood or a larger pipe spacing (200 mm or more), cooling without replacing the floor covering or at least without strict indoor humidity control is practically impossible to operate safely.

How does the thickness of the load-bearing layer affect the cooling speed of the room?

Directly. A thicker anhydrite or concrete layer has a higher thermal capacity (storage) – it cools more slowly, but also stores heat at night and releases it during the day. For summer cooling, night operation (night cooling of the screed) is ideal, when the outside temperature is lower and the heat pump's cooling capacity is higher. A 60–70 mm anhydrite thickness is a good compromise between storage and regulation speed.

Do I need a different expansion joint for floor cooling than for heating?

The same rules apply for the placement of joints, but in a combined system, expansion movements are greater. The temperature difference of the screed in summer (18–20 °C surface) and in winter (28–30 °C surface) is 10–12 °C compared to a system with heating only, where the screed never drops below room temperature (20 °C). Therefore, protecting the pipes in the joints with protective sleeves is absolutely mandatory, and the perimeter expansion strip must be at least 10 mm, not just 8 mm.

Can I stick LVT vinyl directly on old ceramic tiles if I want to add floor cooling?

Yes, but under certain conditions: the old ceramic tiles must be firmly attached (no hollow spots), flat (tolerance max. 3 mm on a 2 m straightedge), and clean. The thermal resistance of LVT (up to 5 mm) + old ceramic (10 mm) is acceptable. However: the total construction thickness will increase, cooling and heating performance will decrease by 15–25 %, and the entire system's hydraulics must be recalculated. An advantage is that vinyl withstands condensation during cooling better than wood – it does not absorb water as dramatically.

Conclusion

The floor construction for combined floor heating and cooling is not just an engineering matter – it is a decision that will influence comfort, reliability, and the system's lifespan for decades. The most important points to take away from this article:

Thermal insulation must be sufficiently thick and vapor-tight (XPS for cooling, minimum 100–160 mm).
Separation and waterproofing layer protects the insulation and the entire construction from moisture.
Load-bearing layer (anhydrite CA-C16-F4 or concrete C20/25) must be of correct thickness, sufficiently dry, and have expansion joints.
Pipes must be firmly fixed – with grid film, fixing strips, or clamps with curves – and this applies even to the simplest implementations.
The final floor covering must be certified for floor heating and resistant to moisture during cooling – ceramic tiles are by far the most suitable.
The entire system must be supplemented with a hygrostat and minimum medium temperature control, without which cooling is risky at normal summer humidity.

A detailed step-by-step guide from the first layer to the pouring can be found in the article Installation of floor heating step by step – from insulation to pouring, and if you are selecting specific products for your project, check out the entire category floor heating and cooling on atria.sk.

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