Effects of Decorative Veneer and Structure on the Thermal Conductivity of Engineered Wood Flooring

When it comes to flooring for a room with an under-floor heating system, it turns out that how well it works depends on not only the wood type but also the structure of the floorboards. Qingqing Chen, Xiaolei Guo, Jie Wang and Pingxiang Cao from the Faculty of Material Science and Engineering, Nanjing Forestry University show us the differences.

Engineered wood flooring (EWF) has gained popularity as an alternative to solid wood flooring. Commonly used for floors that need to be heated, engineered wood flooring needs to reach the required temperatures in the shortest time. Therefore, improving its thermal conductivity of is an important way to increase efficiency and save energy. This study examines four decorative veneer types and three different structures of engineered wood flooring. All samples were placed in a laboratory simulating an environment with a heating system and their temperature was measured three times every five minutes. Findings from this research will provide the know-how to improve thermal conductivity in EWF.


The samples tested had three types of structures as shown in Figure 1. The first structure (A) consisted of 4mm thick decorative face (upper) veneer, 9mm thick poplar core boards and 2mm thick poplar back (lower) veneer. The second type (B) has a 4mm thick face layer (made up of a 1.2mm thick decorative veneer (top) and three layers of plywood) and the rest of its structure is similar to structure A.  The third type (C) was made of 1.2mm thick decorative face veneer, a seven-layer plywood core and 2mm of poplar back veneer.

According to a study conducted by Seo et al, the thermal transfer performance of flooring is dependent on the thickness of the material used. The transversal thermal conductivity of wood increases with density, temperature and moisture content. All three types of EWF tested have the same dimensions: 910mm long, 125mm wide and 15mm thick. Four species of hardwoods with varying densities were selected as decorative veneer for this study. In order to avoid the influence of moisture content on the results, the original moisture content was controlled within a certain range for all samples. 


Testing environment and equipment

The experiment was conducted in a laboratory simulating an environment with a floor heating system (as shown in Figure 2). A heating system manufactured by O.S PANTO S.R.L (Italy), SEF DKC18, was used. In the lab, a copper pipe was installed with a narrow pitch in a cement mortar ground. Hot water from a boiler was supplied to the floor coil, an X-L pipe underneath the floor surface. A layer of polyethylene foam covers the cement mortar ground. The laboratory was also equipped with a humidifier, which regulated the overall humidity by spraying water vapour in the room. 

Thermal conductivity measurement

The thermal conductivity of the flooring samples were measured using a surface thermocouple thermometer DT-613 produced by CEM. The temperature of the top and bottom surfaces of the samples was simultaneously measured with a NR-81533B probe from CEM. 

Plan of the experiment

As discovered in studies by Kang et al, the perfect flooring surface temperature ranged from 22.0 to 38.8 °C. This experiment was carried out in late autumn when the outdoor temperature was typically 15°C during daytime. Hence , the laboratory temperature set for the experiment was 35 ± 2 °C. On account of the heat loss during the hot water transfer through the cement mortar to the engineered wood flooring, the temperature of the hot water was set to 40 ± 2 °C. 

At first, the primary temperature of the top and bottom surfaces of each engineered wood flooring sample was measured and recorded. Then, the temperature control system of the laboratory was activated and set to 40 ± 2 °C. Four or five hours later, the indoor temperature was raised to the needed temperature. The temperature of the floor was also measured and recorded at 35.5 °C. Afterwards, all the samples were placed on the ground in a flat orientation with the decorative veneer facing upwards. The top and bottom surface temperatures of each sample was measured three times every five minutes, and the average value is recorded. The experiment was stopped when the variation was too small to detect differences in the final measured data when compared to the previous data.

Results and Discussion

Influence of Different Types of Decorative Veneer

The values of the primary temperature (Tp), final temperature of the upper surface (Tf.up), final temperature of the lower surface (Tf.lower), average final temperature of the lower surface (Tf.lower.avg ) and the temperature variation between the final temperature of the upper surface and the average final temperature of the lower surface (Tv) of the respective samples have been shown.

As seen in table 3, all samples’ upper and lower surface temperatures rose with time. The lower surface temperatures for samples of different structures were also relatively close. The average of all the samples’ lower surface temperatures was 35.3 °C. The temperature variation for the samples with eastern black walnut was the smallest while temperature variation was the largest for samples that used cherry as decorative veneer. The heat loss ranking for the four types of wood used, from small to large, is as follows: eastern black walnut, birch, maple and cherry. The thermal conductivity of the four types of wood is ranked as follows: eastern black walnut > birch > maple > cherry. Thermal conductivity is inversely proportional to heat loss. Thermal conductivity is proportional to wood density.

Comparing the final upper surface temperature of the four different veneers, eastern black walnut has the highest temperature while cherry has the lowest temperature regardless of the structure. The greater the change, the faster the temperature rises. The higher the density of the wood used, the faster the temperature changes and the better the thermal conductivity. This is because the distance between the molecules becomes shorter as density increases, making heat transfer easier. Some explanation about air  Eastern black walnut transfers heat the fastest followed by birch, maple and cherry respectively. 

Influence of Different Structures

For eastern black walnut veneer, the temperature variation of structure A was the smallest while the variation for structure C was the largest. A similar trend in temperature variation was also observed for the three other types of wood veneers. Therefore, structure A loses the least heat, followed by structure B then structure C.

The change in upper surface temperature for the three flooring structures have been shown. For the period between 10 minutes and 20 minutes (during which there is an obvious upward trend), temperature variation for structure A was the largest, followed by structure B then structure C. A larger temperature variation indicates superior thermal conductivity. Thus, structure A demonstrated the highest thermal conductivity among the three structures tested. This may be because the adhesives for all samples with structure A are the same, so the gap between layers is small and heat loss during heat transfer is minimised. Structure A is the densest followed by  structure B and C. In line with the law that thermal conductivity is proportional to wood density, structure A has the highest thermal conductivity and structure C has the lowest thermal conductivity. Figure 5 also shows that from 20 minutes onwards, the temperature for structure A across all four wood types increased faster. At the same time, the temperature for structure C across all four wood types rose the slowest.

Key Findings

1.Comparing engineered wood floorings that have the same structure but different decorative veneers, it was found that the higher the density of the wood used (as the veneer), the better the thermal conductivity and the lesser the heat lost during transfer.

2.Comparing samples with the same type of wood veneer but different structure, it was found that the structure also influenced thermal conductivity. Structure A has the highest thermal conductivity while structure C has the worst among the three structures.

3.Based on the wood types and structures tested, engineered wood flooring with structure A and an eastern black walnut veneer is the best choice for someone looking to install an under-floor heating system. 

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