Overpressure induced by fires in airtight buildings

The need for sustainability and smaller ecological footprint leads to the construction of more airtight building envelopes with better thermal insulation in order to reduce the pollution and increase the energy efficiency, according to the Energy Performance of Buildings Directive (EPBD 2010/31/EU). The specific fire hazards possibly associated with such structures raised questions amongst fire community in Belgium. Consequently, in 2010 the Belgian Ministry of Interior funded a study about the fire hazards possibly associated with Passive Houses in comparison to traditional houses. The University of Mons and ISSeP used zone modelling for investigating how much the characteristics of a passive house such as airtightness, ventilation and thermal insulation could affect the fire development. A significant difference in pressure rise (due to thermal expansion of fumes) was observed because of the airtightness in the passive house. However, due to a lack of validation from experimental data at large scale, the scientific monitoring committee of the project decided to not mention the potential problem that the forces of pressure could block the occupants during a certain period due to inwards-opening doors. It was decided in 2015 to build a full-scale experimental facility in the region of Mons with the support of the KCC centrum and the Régie Provinciale du Hainaut. This facility was designed with the support of the Ghent University. The inner dimensions of the construction are the same as the inner dimensions of a 40-foot shipping container, but the outer shell consists of 20 cm concrete blocks and the inner shell is made of plaster board. The volume is divided into two rooms, which simulate two different rooms of a passive house. In one room air is extracted, while in the other one fresh air is supplied. In this study, we present the tests carried out in this facility by burning pallets stack and wood cribs. Two different ventilation duct configurations were tested: one with mechanical ventilation on, the other with the ducts closed with an airtight metal cap (the mechanical ventilation being off). Measurements were made for gas pressure, mass loss rate, gas temperature, volumetric flow rate in the ducts and for some tests O2, CO2, CO and THC concentrations were also quantified. Over pressure peaks from 850 to 2035 Pa were measured without mechanical ventilation (ducts closed), while values from 420 to 750 Pa were observed with the mechanical ventilation on. Both the reverse flow of the fan supplying fresh air and the extra flow rate of fumes in the extraction duct due to the overpressure inside the apartment were not sufficient to prevent overpressure inside the rooms. These tests confirm there may be problems of overpressure in very airtight houses such as passive houses in case of fire. Especially, the occupants may not succeed in escaping during a period of several minutes due to the impossibility to open inward opening doors. This experimental study confirms the observations carried out during the reconstruction of the fire which occurred during the night of the 5 February 2013 in an passive apartment in Cologne, the occupant being blocked for about 2 minutes. This experimental campaign was also used for validating the zone model CFAST predictive capability. The way to take into account the effective leakage which depends on the overpressure inside the building as well as the mechanical ventilation in CFAST for obtaining satisfactory simulation results are briefly presented in this paper. The use of validated software could be helpful to take into account the fire-induced overpressure in confined dwellings in fire safety design.