Against the environmental impact of the construction sector – responsible for 40% of the total primary energy consumption of the European Union
Against the environmental impact of the construction sector – responsible for 40% of the total primary energy consumption of the European Union – reduce both the energy embedded in the materials in the manufacturing phase, as well as the energy consumption of the buildings in their phase of use are urgent tasks. Wood, agricultural waste, and materials of biological origin are local renewable resources that can be used to promote the circular economy and reduce the environmental impact of the construction sector. This article shows an example of a prefabricated structural insulating panel, made with materials of biological origin, for new buildings of almost zero consumption.
ISOBIO New Build Panel
The prototype of the panel that was monitored measures 1.95m x 1.95m, with a total thickness of 33.2cm in 8 layers with 9 different materials (Figure 1). It is composed by an exterior plaster of 25mm thick lime and hemp, applied on a rigid 50mm hemp thermal insulation, mechanically fixed to the 145mm thick red pine wood structure. Among the timber structure there is hemp, cotton, and linen insulation, followed by a 12 mm OSB 3 board. On the OSB an airtight and dynamic vapor control membrane has been fixed, followed by a 45mm thick service void filled with thermal insulation of hemp, cotton, and linen, between wooden battens, rotated 90º from the timber structure to mitigate the thermal bridge through the wooden elements. The void is closed with a thermo-compressed straw board 40mm thick, revoked inside with a clay and hemp compound, applied in 3 layers, 15mm thick.
Installation and monitoring of demonstrators
Figure 3 and Figure 4 show the installation of the panels in the demonstrators in Wroughton and Seville. A monitoring system was installed with a weather station recording the external conditions, a temperature probe on the outer face of the panel, a heat flow sensor and a temperature probe on the inner face, in accordance with ISO 9869 . Additionally, temperature and relative humidity probes were installed in 3 interstitial points (Figure 2), to measure the dynamic hygrothermal behavior inside the panel and compare the results with the WUFI model, according to EN 15026 . Data were measured at an interval of 5 minutes. The indoor temperature was maintained at an average temperature of 25.5 ° C throughout the period, with an electric air heater.
Results of monitoring and validation of calculation models
Table 1 shows the calculation results of the U of the ISOBIO panel according to ISO 6946  in steady state. For the thermal conductivities of the materials, the values measured in the laboratory were taken (for the dry material at 10 ° C, with a water content w = 0), and were recalculated with a model developed by the University of Rennes 1, for the material at a relative humidity of 50%, being a more realistic water content. For the comparison with the experimental data, the thermal effect of the structural elements of wood was neglected, since their incidence in the measurements of heat flux, temperature and relative humidity were considered negligible.
The results of the period 02/24/2018 to 03/14/2018 are presented in the HIVE demonstrator, United Kingdom, for a total of 432 hours, or 18 days, with 5,184 data points. Table 2 and Figure 5 show the results of the thermal transmittance measured in situ according to ISO 9869, and its comparison with the value calculated in steady state, according to ISO 6946. Figure 6 shows the thermal transmittance measured in situ, compared to the dynamic value calculated with the WUFI tool. Figure 7, Figure 8 and Figure 9 show the temperature and relative humidity measured and calculated with WUFI, inside the panel, in the 3 positions indicated in Figure 2.
The result of the average thermal transmittance measured in situ (Figure 5), is 7% higher than the value calculated in steady state, being a minimum difference, within the range of uncertainty of the measurement. The results indicate reliable behavior and a close correlation between the calculated and measured. It highlights the importance of taking into account a realistic moisture content in the materials, when making a simplified calculation of thermal transmittance, where the only parameter is thermal conductivity.
The hourly thermal transmittance measured in situ and the dynamically calculated values with the WUFI tool (Figure 6), show an even better correlation, with a difference of 4% between the average of the U measured in situ and the U calculated with WUFI . It indicates that the coupled calculation of heat and humidity transfer of the WUFI tool accurately reflects the dynamic thermal transmittance for such a building element, with materials of biological origin.
Finally, the results of the temperature and HR measured and modeled with WUFI in positions 2, 3 and 4 (Figure 7, Figure 8 and Figure 9), show that dynamic temperature variations are very well reflected in the model. Short-term variations in relative humidity are not reflected with the same precision in the model, possibly because of the assumption that the content of water in equilibrium in the materials is instant, when in reality, there is a hysteresis . However, the results show a very good correlation between the measured and the calculated, demonstrating that the materials of biological origin in such a composite panel can contribute to the reduction of the energy consumption of a building in its operation phase, with a minimum amount of energy embedded in the materials in the manufacturing phase.
The ISOBIO project was carried out thanks to grant No. 636835 granted by the European Union. http://isobioproject.com
Based on the article de N. Reuge, F. Collet, S. Pretot, S. Moisette, M. Bart, O. Style, A. Shea, C. Lanos 2019, Hygrothermal transfers through a bio-based multilayered ISOBIO wall – Part I: Validation of a local kinetics model of sorption and simulations of the HIVE demonstrator. Laboratoire de Génie Civil et Génie Mécanique, Axe Ecomatériaux pour la construction, Université de Rennes, 3 rue du Clos Courtel, BP 90422, 35704 Rennes, France.