In general terms, it is appreciated less thermal conductivity fluctuation when Radiata-pine is protected with chemical substances or heat-treatment. However, the lowest conductivity value obtained by test box is registered for the natural wood sample, when gradient between surfaces and material average temperature are the lowest. After conditioning the samples to 23°C temperature and 50% relative humidity, the density of chemical and heat-treated wood is slightly lower than natural wood density, which may imply that they have less cellulose or lignin.
The fluctuation along the tests of the natural wood thermal conductivity is twice higher than the fluctuation of the chemical treated sample and four times the heat-treated one. The heat-treated wood is the most stable, as its conductivity difference is only 0.0034 W/(m∙°C), while treated wood with chemical substances has a difference of 0.0065 W/(m∙°C) between the highest and the lowest conductivity value, and natural wood has a difference of 0.0143 W/(m∙°C).
Test box method based on the thermocouples and UNE-EN 12664 [44] procedure were assessed. In both cases, the conductivity of natural wood (1) increases more drastically than the other samples. In case of chemical treated wood (2), it has less conductivity variation than the previous one. The heat-treated wood (3) represents the sample with the least variation of its thermal properties.
However, the resulted thermal conductivity was different as both method represent different thermal situation. The method stablished by test box represents the material exposed to air and the thermal behaviour of both elements is linked. In case of UNE-EN 12664 procedure, it does not contemplate nor convection coefficient nor its behaviour depending on material surface.
In case of using transmittance detector equipment, this is closer to test box method, as it also uses the test box to produce the necessary temperature gradient between surfaces. However, it considers that the convection coefficients are constant, which it is not as it was demonstrated by thermocouples. Both exterior and interior convection coefficients vary, although the exterior one varies less as the temperature in this side is more constant.
Heat-treated wood represents the least variation in terms of convection, which indicates that it is more insulated. In case of natural wood (1), its superficial resistance is the highest which in principle would indicate that it is the least insulated. However, the temperature cycles inside the test box are as long as the cycles of the heat-treated wood (3), while chemical treated wood (2) has shorter cycles. That means that the temperature inside the box reaches the operational temperature at the same time. That is, both samples (1, 3) requiring the same time duration to heat the inner box until reach the operational temperature, as it was mentioned in section 3.2.2.2.
In order to guarantee the correct use of test box, the risk of Radiata-pine condensation was also estimated for each test (a, b, c, d). By these estimations, it is demonstrated that the lower is the gradient of temperature, the lower is this risk. However, at the same time, the higher is the error in the conductivity values as the flux is minor and other effects can become more impact.
Due to that, it is validated that these tests are not recommendable for less than 3 °C gradient of temperature between material surfaces. That is, for this material, Radiata-pine, both tests “a” and “d”, which corresponds to 40 and 28 °C inside the test box, are more susceptible to have an error, the first one due to the higher influence of transient convection inside the test box, and the second one due to the estimated condensation risk.
On the other hand, there is a relation between transmittance and permeability, as the sample with most resistance to water diffusion (3) has similar results by test box and UNE- EN 12664 procedures, while the other samples (1, 2) have higher error. This implies that the hygrothermal behaviour in natural and chemical treated wood is more evident, because their conductivity highly varies depending on temperature and humidity. However, heat-treated wood is more stable which means that the difference of temperature and humidity do not influence in the same way.
The thermal mass and inertia of natural and chemical treated Radiata-pine are quite similar. Natural wood has more density and less specific heat than chemical treated wood. These values got compensated and the resulted thermal mass and inertia become similar. On the other hand, heat-treated wood represents the lowest volume, density, specific heat
and thermal conductivity. As a result, this implies that it has the lowest thermal mass and inertia.
To sum up, in terms of water vapour flux, a further research is required in order to analyse its influence in the material thermal conductivity, as well as the measurement of their porosity.
“I am among those who think that science has great beauty”
Marie Curie 1867-1934
4 Energy simulation
Currently, tendencies around sustainability linked to building façades make us to develop new systems, which apart from presenting a low environmental impact with relation to both production and waste of material, also allow energy savings during the whole building life cycle. In this regard, wood is characterized for being a renewable and ecological resource whose production helps to mitigate the CO released to the atmosphere, cutting down the carbon footprint.
In this case, different exterior claddings made of joined wood strips will be carefully analyzed, in terms of energy savings. Different shapes and configurations will be addressed. For this purpose, a case study will be carried out by the simulation of different cross-sectional areas through the THERM v 7.3© software, which allows us to study the thermal behavior of the material through its thermal resistance as a whole.
A simulation with WUFI Pro is also performed. This software enables a one-dimensional investigation of the thermal behaviour of façades, depending on the climate data, in this case Bilbao weather data were selected [32].
Apart from the energy performance analysis of material and façade, the test box simulation was also conducted. In this case, the DesignBuilder v. 4 software was used. This tool is based on the EnergyPlus software combined with user-friendly interface [62]. In this case, CFD simulation was realized in order to study the air behaviour inside the test box and energy losses through it.