# Experimental Characterization of Thermal Conductivity with a New Compact Hot-Box Prototype
## 1 Introduction
(2021) at the University of Bologna (Italy) to evaluate the thermal conductivity of materials using an innovative, low-cost, and low-tech movable hot box (cube) with an approximate volume of 1 m\({}^{3}\)[11]. The cube was used as a hot box, and the laboratory environment was used to simulate the cold chamber. The structure of the cube was made of five 18 mm oriented strand board (OSB) panels and insulated with 80 mm thick expanded polystyrene (EPS) panels. The experimental apparatus was validated using a sample of OSB and EPS with dimensions of 0.5 m \(\times\) 0.5 m. It was found that the error was minimized throughout the test. The development of an affordable hot-box calorimeter was carried out by [PERSON] (2023) to determine the U-value of various 3D-printed building bricks [12]. The wall of the apparatus was built with one layer of expanded polystyrene (EPS) with a thickness of 100 mm and an exterior layer with a thickness of 6 mm thick of plywood. A mask wall made of EPS with a central hole was used to thermally separate the hot chamber and cold chamber. The specimen was supported by the mask with dimensions of 0.2 m \(\times\) 0.2 m. The cold chamber was controlled by a refrigerated circulator to vary the temperature. It was found that the apparatus estimated the sample performance with very good accuracy, with the maximum error reported as 6%. A low-tech hot box, labeled as the \"approachable hot box (AHB)\", was developed by [PERSON] et al. (2023) to evaluate the thermal conductivity of homogeneous materials [13]. The main innovation of the apparatus was that it focused on temperature and humidity instead of thermal load input as the standard hot box. The apparatus consisted of a hot chamber and a receiving chamber. The receiving chamber was open to the environment and was used to monitor temperature and humidity. The hot chamber was equipped with a wire electrical heater (25 W) to supply the thermal load, a fan, an Arduino temperature sensor, and a humidifier. The AHB was structured with both chambers having an exterior layer of 51 mm polystyrene insulation foam and a single layer of 16 mm plywood. The thermal transmittance adopted in the calculations was set at 33.40 W/m\({}^{2}\).K (R-0.17). Specimens of plywood and concrete with dimensions of 0.305 m \(\times\) 0.305 m were used to validate the experiment. It was concluded that the apparatus slightly overpredicted the expected values, particularly for the plywood specimen, with the error decreasing as the temperature gradient of the specimen surfaces increased. A novel experimental cell prototype for characterizing thermal conductivity based on the hot-box methodology is proposed in this study. The accurate measurement of thermal conductivity is essential for evaluating the suitability of materials in energy-efficient construction, particularly in the development of sustainable insulating materials. Traditional measurement methods, such as the guarded hot plate and the heat-flow meter, often require expensive equipment and controlled laboratory conditions, which can limit accessibility and flexibility in experimental setups. The proposed system is aimed to provide a more adaptable and cost-effective alternative while maintaining reliable measurement accuracy. Three key stages were involved in the development of the experimental cell. First, a prototype was designed to create a controlled environment for thermal characterization. Next, a control system was implemented to regulate the thermal load, ensuring a stable temperature gradient between the hot and cold chambers. Finally, the prototype was instrumented with high-precision sensors to monitor temperature variations and compute the thermal conductivity of any tested specimen. The new experimental cell was built with external dimensions of 81 cm (height) \(\times\) 61 cm (width) \(\times\) 122 cm (length), resulting in a total volume of 0.602 m\({}^{3}\). The testing of specimens with a fixed surface area of 25 cm \(\times\) 25 cm and a variable thickness of up to 10 cm is allowed.