ABSTRACT
Fire spreading through the façades is widely recognized as one of the fastest pathways of fire spreading in the buildings. Fire may spread through the façade in different ways depending on the type of façade system and on the elements and materials from which it is constructed. Nowadays the trend in architecture is towards a growing usage of lightweight construction systems that are quick to install, versatile and have a high technical and aesthetic value. The ventilated façade has all these characteristics as well as providing a good performance from hygrothermal point of view.
Ventilated façades are multilayer systems whose main feature is the creation of an air chamber of circulating air between the original building wall and the external cladding. The “chimney effect” in the air cavity is a mechanism that improves the façade’s thermal behaviour and avoids the appearance of moisture from rain or condensation. However, in an event of fire, it may contribute to the quickest spreading of fire, representing a significant risk to the upper floors of a building. This study deals with some aspects of fire propagation through the ventilated cavity in ventilated façade systems. Also we review the provisions stipulated by the Spanish building code (Código Técnico de la Edificación, CTE) [1] to avoid fire spreading outside the building.
The results highlight the importance of the use of proper fire barriers to ensure the compartmentalization of the ventilated cavity, as well as the use of non-combustible thermal insulation materials, among others. In addition, based on the results, it might be considered that the measures stipulated by the CTE are insufficient to limit the risks associated with this kind of façades systems. The study has been performed using field models of computational fluid-dynamics. In particular, the Fire Dynamics Simulator (FDS) software has been used to numerically solve the mathematical integration models.

1. Support (wall, column, slab, etc)  2. Substructure/ fixing system  3. Insulation  4. Air chamber (Cavity)  5. Cladding
1. Support (wall, column, slab, etc)
2. Substructure/ fixing system
3. Insulation
4. Air chamber (Cavity)
5. Cladding

Ventilated façades are multilayer systems whose main feature is the creation of an air chamber of circulating air between the original building wall and the external cladding.

The pressure difference between the air in the cavity and outside leads to the creation of an airflow known as the “chimney effect“, which eliminates humidity in wet conditions and prevents condensation. The use of ventilated façades offers different advantages from the higrothermal point of view. However, in terms of fire safety, the chimney effect poses a risk, because the ventilated cavity may provide a pathway for the fire to spread quickly.

Objectives

This study aims to assess fire behaviour and its propagation through the cavities of ventilated façades systems. In particular, we seek to assess the level of protection provided by the measures stipulated by the CTE. We focus our study on the following aspects:

  • A.Fire barriers in the ventilated cavity.
  • B.The influence of the use of combustible and non-combustible thermal insulation.
  • C.The influence of the size of the cavity and level of ventilation. Three variables are considered: low, medium and high ventilation.

Methodology

Computational domain and scearios 

Computational domain and scearios  INCAFUST

This research is conducted using field models of computational fluid-dynamics to evaluate some aspects of fire dynamics in the different cases studied. In particular the software Fire Dynamics Simulator (FDS) is used.Two computational domains are performed; one is the basic scenario and the other is double the size scenario.

Geometric description of the scenarios and location of the thermocouples
Geometric description of the scenarios and location of the thermocouples

Details of the ventilated cavity elements

The follow graph shows the details of the ventilated cavity elements analyzed.

the details of the ventilated cavity elements INCAFUST

Case studies

In all, eight cases are evaluated according to the mentioned variables.

Eight cases studies are evaluated according to the mentioned variables INCAFUST

Some results

A. Fire barriers

The results show the great influence of the ventilated cavity in fire spread through the façade when fire barriers are not employed.

(Left) Graphics of fire spreading through the façade. (A) Without barriers, at 300 s. (B) With barriers, at time of 300 s.  (Right) (thermocouples 2) Comparative of temperatures between a scenario without barrier and the two types of barriers studied.
(Left) Graphics of fire spreading through the façade. (A) Without barriers, at 300 s. (B) With barriers, at time of 300 s.
(Right) (thermocouples 2) Comparative of temperatures between a scenario without barrier and the two types of barriers studied.

This result shows the ability that may have the fire and smoke to spread through the cavity, even if the insulation is non-combustible.

B. Thermal insulation

(Left)Comparative of HRR and temperature. (Right) (thermocouples 4) between scenarios with combustible and non-combustible insulation.
(Left)Comparative of HRR and temperature. (Right) (thermocouples 4) between scenarios with combustible and non-combustible insulation.

When the insulation is a combustible material the spread of fire is much more intense and the probability of fire spread to the upper floors is increased.

(Left) Graphic of flames spreading through the façade without fire barriers, at 450 s.  (Right) Temperatures recorded by thermocouples located inside the cavity.
(Left) Graphic of flames spreading through the façade without fire barriers, at 450 s.
(Right) Temperatures recorded by thermocouples located inside the cavity.
Graphics of fire spreading through the façade. (A) Combustible insulation, at time of 400 s. (B) Non-combustible insulation, at 400 s.
Graphics of fire spreading through the façade. (A) Combustible insulation, at time of 400 s.
(B) Non-combustible insulation, at 400 s.

Conclusions

  • The ventilated cavity is a potential pathway for fire spreading in fire situations. For this reason the compartmentalization of the cavity on each floor of the building, by using appropriate barriers, is considered essential in order to prevent this type of propagation.
  • The establishment of barriers every 3 floors or 10 meters, as is required by the Spanish building code, is not sufficient: it would be equivalent to protect a part of the building while the other part remains without protection. Likewise, the provision of CTE, in the sense that partitioning barriers are not required in buildings lower than 18 meters, leaves a significant risk without covering.
  • The spread of fire through the ventilated cavity is due to the system configuration and the façade chimney effect that occurs within the cavity, regardless of the used thermal insulation material. However, the use of combustible insulation materials significantly increases the spreading and the fire intensity and raises, therefore, the level of risk.
  • In our opinion, it is important, to inform the professionals involved in building about the risks that remain unfilled by the regulations. Unfortunately the fulfillment of regulations does not guarantee, in all the cases, an acceptable safety level.

María Pilar Giraldo
Architect, Ph. D
Researcher in Timber Construction and Fire Safety

This research was submitted in the International 1st Seminar for ire Safety of Facades held on Nov 14th-15th 2013 in Paris, France.