# The Ability to Control VOC Emissions from Multilayer Building Materials
## 1 Introduction
### State of Knowledge-Emission Modelling
The model validation results [28; 29; 30; 31; 32] showed that the multilayer material exhibits the same or similar emission properties as the top layer material. The top layer strongly retards the emission of VOCs from the bottom layer material. A multilayer material has a much longer VOC emission time than a single-layer material. [PERSON] et al. [32] investigated the influence of the multilayer material system's parameters on VOCs emission. The results showed that the inner layer could act as a VOC adsorbent or emission source to the top layer depending on the initial VOC concentration in theelayer's material. In the case that the inner layer is an emission source for the outer layer, the outer layer becomes a barrier layer reducing the rate of VOC emission from the source. The emission profile characteristic determined experimentally for the layered building material was presented by [PERSON] et al. [33]. TVOC emissions from OSB wood-based boards and the same boards covered with gypsum fibreboards (GKF boards) were investigated. VOC emissions from uncoated OSBs were higher in the beginning. In the presented case, the maximum (TVOC concentration peak in emission profile) was reached after 3 days, followed by a sharp decrease in the emission profile curve in the time period until the seventh day of the measurement, in which the second decay curve breaking down and a slow but continuous decrease of emissions until the end of the experiment could be read. Considering both curves of the emission profile, it can be stated that the gypsum covering the surface of the OSB board caused a reduction and significant delay in the first peak of TVOC concentration. Another approach to building a model of emissions from many coexisting emitting surfaces in one space, which also applies to layered building materials systems, is presented by [PERSON] et al. [34]. When different building materials release VOCs at the same time, the indoor VOC concentration increases and vice versa decreases due to the mutual inhibition of the emissions released from the material. Therefore, the whole process is dynamic, and the final indoor VOC concentration cannot be obtained by simply superimposing the concentrations of pollutants emitted from the individual layers of dry building materials [31]). An equation describing the equilibrium of mass transfer through all thin layers of material [34] would be Equation (3):  
\[V\frac{\partial C_{a}}{\partial t}=-QC_{a}-A_{1}D_{m,1}\frac{\partial C_{m,1}} {\partial t}\bigg{|}_{y=\delta_{1}}-A_{2}D_{m,2}\frac{\partial C_{m,2}}{ \partial t}\bigg{|}_{y=\delta_{2}}....-A_{i}D_{m,i}\frac{\partial C_{m,i}}{ \partial t}\bigg{|}_{y=\delta_{i}};\;\;i=1,2,3 \tag{3}\]  
where \(V\) is the chamber volume (m\({}^{3}\)), \(C_{a}\) is the concentration of compound a in the overlying air (mg/m\({}^{3}\)), \(Q\) is the amount of ventilating air, m\({}^{3}\) h\({}^{-1}\), \(A_{i}\) is the emitting surface of the \(i\)-th building material, m\({}^{2}\) and \(D_{m,1}\) is the diffusion coefficient of the compound transfer through the material \(a\) of the lower layer with a thickness of \(\delta_{1}\) of the layer system, \(D_{m}\), and diffusion factor of the compound \(a\) through the material of any layers \(i\). However, this equation does not take into account the mutual inhibition effect of emissions and assumes constant values of \(C_{m,i,j}\); therefore, it would only be valid at very short intervals. For the numerical determination of the time course of the increase in the concentration of VOC in the chamber caused by the emission of VOC from building materials coexisting in the chamber (in the floor layers), [PERSON] et al. [34] used the finite difference method (Saul'ev finite difference method). They validated the model in the testing of chamber VOC emissions from wall and floor sandwich systems. Interestingly, for both models, in the [PERSON] physical model completed in 2004 [15] based on mass transfer equations and the [PERSON] et al. model [34], the same assumptions were made.