# Thermal and Energy-Efficiency Assessment of Hybrid CLT-glass Facade Elements
## 2 Thermal and Energy Efficiency Assessment of Facade Components
### Reference Thermal Performance Indicators  
For the present study, the EN ISO 13788 standard was followed [32]. This standard is focused describing the assessment of the hygothermal performance of building components and building elements. This standard recommends calculating the well-known temperature factor at the internal surface \(f_{\text{Rsi}}\) as  
\[f_{\text{Rsi}}=\frac{T_{si}-T_{out}}{T_{int}-T_{out}} \tag{1}\]  
where \(T_{si}\) is the temperature of the internal wall surface, \(T_{int}\) is the internal air temperature, and \(T_{out}\) is the external air temperature. \(T_{si}\) strictly depends on the features of the investigated building system and can be especially sensitive to thermal bridges. As is known,\(f_{\text{Rsi}}\) should be close to the unit to represent optimally insulated buildings. To ensure the occurrence of mould growth and surface condensation in dwellings, the \(f_{\text{Rsi}}\geq 0.75\) values are commonly accepted in practice. In addition to the EN ISO 13788 recommendations, a series of National guidelines are available in several countries that recommend minimum limit values for \(f_{\text{Rsi}}\). These are in the range of \(f_{\text{Rsi}}\geq 0.52\) (France), \(f_{\text{Rsi}}\geq 0.65\) (The Netherlands), or \(f_{\text{Rsi}}\geq 0.7\) for Germany [33].  
Another relevant parameter is the internal surface resistance, which depends on convection and radiation coefficients, air movements in the room, the air and surface temperature distribution in the room, and the surface material's properties. Accordingly, refined and time-consuming numerical models of a given room as a whole are essential to account for several aspects such as the thermal resistance of the surrounding envelopes, the environmental temperature, the air distribution in the room, and the room's geometry. However, simplified calculation methods or input values recommended by the existing guidelines can be used for preliminary estimates.  
In an outdoor climate (temperature and relative humidity), four main parameters control the surface condensation and development of fungi [32]:  
* The \"thermal quality\" of the peripheral elements of a building is represented by thermal resistance, thermal bridges, geometry, and internal surface resistance; these are defined by the temperature factor on the inner surface,\(f_{\text{Rsi}}\);
* The internal humidity, influencing the dew point in the air;* The indoor air temperature: A lower room temperature is generally more critical for rooms with reduced intermittent heating, or unheated rooms where water vapour can escape from adjacent heated rooms;
* And heating systems, that affect air movement and temperature distribution.  
However, many problems can arise and should be properly assessed for novel envelope systems, such as:  
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2. Mould that can occur when the surface humidity is higher than 80% over several days [33; 34];
3. Troubles due to danger of material corrosion: For each part of a heated building, it should be checked that the expected \(f_{\text{Rsi}}\) value on the interior surface is less than the maximum allowable limit [35; 36];
4. A surface temperature that is too low, as resulting from contacting exterior surfaces of heated spaces, poor external thermal insulation of building sections, possible thermal bridges, or furniture arrangement [37; 38];
5. An increase in humidity, as resulting from an increase in the air humidity (at the air contact position with the wall) above the recommended design values (i.e., inappropriate use of spaces and airflows, see for example [39; 40]).