# Experimental Characterization of Thermal Conductivity with a New Compact Hot-Box Prototype
## 1 Introduction
In both cases, for large specimens with a minimum dimension of 1 m \(\times\) 1 m, the specimen is placed between the hot and cold chambers. In the guarded hot box, heat flux is measured directly with a heat-flux meter on the surface of the specimen. It employs a guarded unit to ensure heat is moved only through the test specimen to minimize losses, and the thermal load in a hot chamber is provided using a control system. In the calibrated hot box, a guard unit is not used; heat flux is indirectly determined by comparing it with a calibrated reference. The guarded hot box is applied for small specimens compared with the calibrated hot box. In addition, the guarded apparatus has been used more frequently in the literature as they are more accurate. The main constraints of the hot-box methods that are based on \"Industrial Standard\" ISO or ASTM are identified as follows: the high cost of construction, significant laboratory space, high maintenance costs, structural complexity of the hot chamber, long testing durations, and the high energy consumption required [11]. According to [PERSON] et al., a single test conducted in an ASTM-certified laboratory could cost in the range of USD 6000 to USD 20,000 per specimen, and construction of a validated hot box under standards could cost up to USD 1,000,000 [12; 13]. Additionally, the use of a hot box by certified standards requires a technician with a solid background to understand and operate the technology. In the specialized literature, extensive efforts have been made to develop alternative methodologies for determining thermal conductivity using the approach of simple hot-box apparatus, a novel innovation that contributes to the preliminary calculation of thermal conductivity, as demonstrated by the following studies. The thermal transmittance of a wall was evaluated by [PERSON] et al. (2015) using a simple hot box-heat-flow meter method [14]. The specimen was a wall with a structure of one 240 mm insulation brick layer and two 20 mm plaster layers. The hot box had an exterior volume of 0.243 m\({}^{3}\) (900 mm \(\times\) 900 mm \(\times\) 300 mm). The apparatus wall was built with a 15 mm wood composite board exterior and 25 mm rubber sponge plate with a thermal conductivity of 0.034 W/m\(\cdot\)K. It was concluded that the experimental error of the wall's thermal transmittance was 5.97% relative to the reference value. [PERSON] et al. (2017) [15] carried out an experiment to determine the thermal properties of masonry walls in old buildings in northern France. The masonry wall had dimensions of 0.06 m \(\times\) 0.11 m \(\times\) 0.22 m and was completely instrumented with thermocouples [15]. The hot-box method was used to control the thermal load on one surface of the wall while the other surface of the wall was open to the environment (cold chamber). A small piece of hot-box apparatus was developed by [PERSON] et al. (2016 and 2018) at the University of Perugia (Italy) to measure thermal conductivity using two different methods: the hot-box method and the heat-flux method [16; 17]. The apparatus had a volume of 0.4418 m\({}^{3}\) (0.94 m \(\times\) 0.50 m \(\times\) 0.94 m) and was used as a hot chamber. The chamber wall was constructed with a sandwich structure consisting of two wood panels with a thickness of 2 cm surrounding a middle layer of polyurethane foam with a thickness of 20 cm. The cold chamber was controlled by the laboratory environment using commercial air conditioning. They used the specimen of foam polyurethane, polistyrol, polystyrene, and wood with dimensions of 0.3 m \(\times\) 0.3 m for calibration and validation of the apparatus. The maximum temperature fluctuations on the surface of the specimen for the hot chamber and the cold chamber were reported to be 0.6 \({}^{\circ}\)C and 0.4 \({}^{\circ}\)C, respectively. An experiment was conducted by [PERSON] et al.