## 2 Experimental Validation
### Numerical Model  
When developing the numerical models, different hypotheses were considered about the air cavities of the constructive model: M1--model with air mesh (considered as a solid model that allows heat conduction inside the cavity); M2--model with radiation effect due to the introduction of a new finite element SURF151, inside the cavity; M3--model with radiation and convection boundaries in the cavity, but introducing a test curve Tf-Test 4, representative of the temperature evolution inside the cavity; and M4--model with radiation effect with the finite element SURF151 and the application of convection with LINK34 element. In the other parts of the constructive model, the two-dimensional (2D) element PLANE55 was used. All these models are shown in Figure 2 representing the developed meshes and their boundary conditions.  
The finite elements used were:  
* PLANE55--2D Thermal Solid Element--can be used as a plane element for thermal conduction capacity. The element has four nodes with a single degree of freedom, temperature, at each node. The element is appropriate for a 2D thermal analysis in steady-state or transient [23,24].
* SURF151--Surface Thermal Effect--used for various load and surface effect applications. It may be overlaid on the face of any 2D thermal solid element. It allows the radiation between the internal surfaces of the cavity and any point inside it. The SURF151 element consists of two nodes, associated with an extra node, located inside the cavity.
* LINK34--Convection Link Element--is a uniaxial element with the ability to conduct heat between its nodes. The element has a single degree of freedom, temperature, at each node point. The convection element is valid for 2D or 3D thermal analysis in steady-state or transient. The element is defined by two nodes, a node on the surface and another inside the cavity, to represent a convection surface area [23].  
Figure 1: Model in study: (**a**) Wooden constructive model; (**b**) Measuring points; (**c**) Dimensions.  
To elaborate the finite element mesh, perfect contact between all the materials was assumed, to allow the thermal energy conduction between them, with the element edge dimension equal to 10 mm.  
The thermal characteristics associated with wood GL24H, the gypsum board type A and air are following references [13; 25; 26], to approximate the materials involved in Test 4 [7]. On the surface exposed to fire, the boundary conditions are radiation and convection, with the effect of increasing temperature through the introduction of the experimental fire  
Figure 2: Mesh and boundary conditions. Cavity with (**a**) M1—air mesh; (**b**) M2—radiation; (**c**) M3—heating curve; (**d**) M4—radiation and convection. The light blue color represents the wood, the violet color represents the gypsum board, and the red color represents the cavity.  
curve Tf. The initial conditions of the model correspond to an ambient temperature of 20 \({}^{\circ}\)C. The relative emissivity properties of the wood material were 0.8 and 0.85 for gypsum board. On the unexposed face, only the convection was considered. At the lateral edges of the numerical model, an adiabatic condition was assumed, with no heat exchange between these zones. All these conditions comply with EC1-1-2 [20]. To satisfy the non-linear conditions of the problem, the program uses the Newton-Raphson iterative method, with a convergence criterion based on the heat flux with a tolerance equal to 0.9. The total time of each simulation corresponds to a fire exposure of 120 min.