Underlay for Underfloor Heating

Underlays for underfloor heating – the importance of thermal resistance and stability

Thermal resistance (R) determines a material's ability to conduct or block heat flow. For underlays intended for underfloor heating, its value should not exceed 0.05 m²K/W to allow heat to freely penetrate the floor layer and effectively heat the room.

Underlays with excessive thermal resistance limit the effectiveness of the heating system. This leads to longer floor heating times, increased energy consumption, and uneven heat distribution. In some areas, the temperature may be significantly higher than in others, reducing user comfort and making heating control difficult.

Thermal conductivity is not the only important parameter. The underlay must also provide adequate mechanical stability. An overly soft material can cause micro-movements of the panels when walked on, leading to loose joints and surface deformation. Therefore, underlays used for underfloor heating should combine low thermal resistance with high compressive strength to ensure the floor's durability for years to come. They should also level minor unevenness, which facilitates installation as it does not require additional preparation.

What underlays to choose for underfloor heating?

The choice of underlay depends on the type of flooring panels, as different finishing materials vary in thickness, flexibility, and installation method. Vinyl flooring (LVT, SPC, with a mineral core) requires a different type of underlay than laminate flooring.
 

Underlays for vinyl flooring on underfloor heating

Underlays for vinyl flooring must be thin, hard, and highly thermally conductive. Since vinyl is thinner than laminate and more susceptible to point loads, it requires a high-density underlay to prevent deformation. PUM (polyurethane-mineral) underlays perform best, offering the lowest thermal resistance (R ≤ 0.006 m²K/W) and the highest compressive strength (≥ 400 kPa). Alternatively, PEHD underlays can be used, but their compressive strength must be at least 200 kPa to stabilize the panels, prevent loose joints, and level subfloor irregularities.
 

Underlays for laminate flooring on underfloor heating

Laminate flooring, mainly due to its lower price, remains a common choice for homes and apartments. Although its thermal resistance is higher than that of vinyl flooring, it can still be used with underfloor heating, provided the correct underlay is selected.

A well-chosen underlay improves heating efficiency, but for laminate flooring, acoustic insulation is equally important. Laminates are stiffer than vinyl, generating more noise, especially in multi-story buildings. An inadequate underlay can lead to a knocking and resonating sound effect, reducing user comfort.

Underlays should reduce both impact sound (transmitted through the floor to lower rooms) and reflected sound (which spreads within the interior). The best choice is mineral underlays, which not only conduct heat excellently but also reduce impact sound (ΔLw) and effectively absorb reflected sound (RWS).

Some natural underlays, such as cork, have good acoustic properties, but due to their high thermal resistance, they are not suitable for underfloor heating.

Choosing an underlay based on the type of underfloor heating

Underfloor heating can be hydronic or electric, and the choice of underlay for panels should consider the system's operation and the flooring's technical requirements.

Hydronic underfloor heating relies on hot water pipes embedded in the screed. Heating is uniform but slower than in electric systems. The underlay must have low thermal resistance (R ≤ 0.05 m²K/W) and provide adequate mechanical stability. Polyurethane-mineral and PEHD (high-density polyethylene) underlays are the best choice as they effectively conduct heat and stabilize the floor.

Electric underfloor heating uses mats, films, or heating cables placed just below the floor surface. This system heats up faster but requires a thinner underlay (≤ 1.5 mm) with the lowest possible thermal resistance (R ≤ 0.02 m²K/W). In this case, PUM underlays perform best, minimizing energy loss and allowing for rapid floor heating.
 

PEHD and PUM underlays can be used in both systems, but for electric underfloor heating, their minimal thickness and low thermal resistance are paramount to ensure heat quickly transfers to the floor. In hydronic heating, the underlay additionally stabilizes the floor, as larger and more uniform heating can cause the floor to expand over a greater area.

What are the consequences of choosing the wrong underlay?

How can an inappropriate underlay affect the system's performance? Underfloor heating can have its efficiency limited. A poorly chosen underlay also impacts the floor's durability.

The most common mistake is excessive thermal resistance, which blocks heat flow. The result is slower floor heating and higher operating costs. The heating system must work longer, and uneven temperatures may occur in the room – the floor will be warmer in some places and remain cool in others.

An underlay with insufficient compressive strength can cause floor deformation. Lack of adequate support leads to localized deformations and weakened joints, significantly affecting the durability of vinyl flooring. In laminate flooring, an overly soft underlay exposes the joints to excessive stress, leading to gaps and loose connections.

Another problem can be improper sound damping. An excessively thick or flexible underlay causes the floating floor to spring, resulting in a characteristic "clicking" sound when walked on and reducing panel stability.
 

Polystyrene (XPS) underlays are not suitable for underfloor heating because extruded polystyrene foams have high thermal resistance, which blocks effective heat flow and significantly reduces the heating system's efficiency.

The absence of an adequate vapor barrier in rooms with increased humidity can lead to moisture condensation under the floor, promoting panel swelling and mold growth. This particularly applies to hydronic systems, where a vapor barrier should be integrated with the underlay or used as a separate protective layer.

Vinyl flooring and underlays for heating – an effective duo

Underfloor heating requires the use of appropriate finishing materials. Mineral core panels are among the best options, offering low thermal resistance and good heat conductivity. Due to their thin wear layer, they heat up quickly, and their dimensional stability prevents deformation under temperature fluctuations. Unlike laminates and parquet, they do not expand or contract, eliminating the risk of gaps. To maximize heating efficiency, it is essential to use a suitable underlay that ensures optimal heat transfer and protects the floor from damage.

PUM underlay – the best choice for underfloor heating

Polyurethane-mineral (PUM) underlays are the best choice for vinyl flooring installed over underfloor heating. Arbiton Multiprotec 1000, as an example of this type of underlay, boasts exceptionally low thermal resistance (0.006 m²K/W) and high compressive strength (≥ 400 kPa), ensuring maximum heating system efficiency. Its stable structure evenly distributes loads, protecting panel joints from micro-damage and preventing localized floor deformation.

An additional advantage of PUM underlays is their moisture resistance, allowing them to be used on various subfloors, including concrete screeds and old ceramic tiles. This makes them suitable for both new constructions and renovated interiors where removing previous floor layers is not feasible.

How does the underlay affect daily comfort and savings?

The right underlay is not just about technical parameters but primarily about tangible benefits you will experience daily. Thanks to proper heat conductivity, the floor heats up faster, meaning lower heating bills – the system doesn't need to run at full power for extended periods to achieve a comfortable temperature. This also translates to greater thermal comfort underfoot, which you and other household members will appreciate. A warm floor means more pleasant mornings and the ability to walk barefoot even on colder days.

A level, stable floor means lasting durability. A well-chosen underlay prevents panel joints from loosening, keeping the floor looking great for longer and avoiding costly repairs. You won't have to worry about removing the entire floor, as stable support extends its lifespan. It's an investment in long-term comfort, savings, and hassle-free floor use every day.