## 1 Introduction  
The underfloor heating system is one of the oldest technologies for providing heating in buildings. The Ondol system was probably the first developed (Korea, 1000 BC) [1], and the Kang (China, 500 BC) [2] and the hypocaust (Greece/Rome 300 BC) [3] are other ancient technologies developed for this purpose. After the fall of the Roman Empire, underfloor heating disappeared from Europe during the Middle Ages, re-emerging in France and Britain in the 18 th and 19 th centuries [4]. In North America, the first radiant heat floor systems date from the end of the 19 th century, but the popularization of these systems worldwide probably only began after the Second World War [5]. Radiant floor heating is currently one of the fastest growing heating systems, thanks in part to the introduction of plastic piping [6], systems based on heat pumps [7], and renewable energies. Radiant floors have a number of advantages over other heating systems, including: high comfort levels [8], resulting from their ideal vertical temperature gradient [8,9]; high energy efficiency, due to the lower air temperature required to meet comfort conditions [9]; silent operation [8]; and their use of low water temperatures that are compatible with renewable energy [10,11,12,13]. For these reasons, radiant floor heating is gaining in popularity compared with more conventional systems [14,15]. Numerous investigations have been carried out on the thermal performance of radiant floors from different perspectives, such as control [13,16,17,18], thermal inertia [19,20], the use of PCMs [21,22,23,24,25,26,27], the arrangement of tubes [23,28], and the most suitable radiant surfaces [6], but fewer publications has been found on the influence of floor coverings properties, although, as [PERSON] and [PERSON] [29] showed in a parametric study, the thickness and thermal properties of the finish or covering have a major bearing on the performance of radiant floor heating. The coverings most commonly applied in radiant flooring are porcelain or ceramic tiles, natural stone, or wood. Low-thermal-resistance natural stone and ceramic materials may initially be deemed to feature higher thermal performance than wood [30], which is nonetheless used as a covering in radiant floors for subjective reasons: its aesthetics tend to be more highly esteemed and it is deemed more pleasing to the touch. Another advantage, according to [PERSON] et al. [31], is that with wood 'the surface temperature is more moderate and uniform'. In an empirical study of four types of laminate wood flooring with conductivities ranging from 0.091 W/(m\(\cdot\)K) to 0.12 W/(m\(\cdot\)K), [PERSON] et al. [32] found that when secured to the system with adhesives, wood delivered better results than when not secured. They contended that although more energy is needed for first-use start-up, such floors maintain a higher temperature for longer after the heating is turned off. That beneficial effect has also been studied in connection with renewable energies. [PERSON] and [PERSON] [33], for instance, studied the effect of the solar heat accumulating in wood floors. European standard EN 1264-2, 'Water based surface embedded heating and cooling systems--Part 2: Floor heating: Prove methods for the determination of the thermal output using calculation and test methods', specifies that thermal resistance values in floor coverings (R\({}_{\lambda,\text{B}}\)) exceeding 0.15 m\({}^{2}\)K/W lie outside its scope, a contention reinforced in Part 3 (on dimensioning), according to which values of R\({}_{\lambda,\text{B}}>\) 0.15 m\({}^{2}\)K/W should be avoided. This has been interpreted to constitute a hard-and-fast maximum and to mean that coverings with thermal resistance higher than this value are unsuitable for radiant floors. Design engineers have consequently challenged the suitability of wood, with its fairly high thermal resistance, as the covering of choice for radiant flooring, particularly in the coldest climates.