Caractérisation et analyse des risques incendie dans les toitures végétalisées

En l'absence d'études scientifiques sur le comportement au feu des toitures végétalisées, des inquiétudes se posent quant à leur sécurité et à l'adéquation des mesures de protection existantes contre la propagation du feu. La province de Québec a publié le guide technique sur l'installation de ces toitures où la section sur la sécurité incendie est élaborée de manière très détaillée, en recueillant les règles de différentes sources, ce qui dans certains cas pose des limites pour la conception. Cette recherche visait à étudier les risques d'incendie que ces toits présentent et à analyser leurs performances dans des conditions extrêmes pour une meilleure compréhension des possibilités du feu et de l'importance des mesures de sécurité incendie. La conductivité thermique effective de substrat de croissance en fonction de la température pour les toitures végétalisées a été déterminée pour une utilisation dans des simulations numériques d'analyse de transfert de chaleur. Une série d’essais en laboratoire a été effectuée afin d'ajuster un modèle existant de calcul de la conductivité thermique des sols minéraux pour l'application sur des substrats de toitures végétalisées. Les résultats montrent une petite différence dans les valeurs mesurées et calculées, spécifiquement 1.07 et 0.9 W/(m·K), ce qui confirme la validité du modèle pour les substrats de toitures végétalisées. Le modèle a permis le calcul de la conductivité thermique en considérant séparément les matériaux organiques et inorganiques, ce qui facilite la détermination de cette propriété à différentes températures compte tenu de la décomposition de la matière organique entre 250 et 700 °C. L'analyse de décomposition thermique a également été effectuée pour obtenir des proportions de composants minéraux et organiques. Les résultats satisfaisants du test de validation et des simulations numériques montrent l'applicabilité de la conductivité thermique effective déterminée en fonction de la température pour les problèmes de transfert de chaleur. L'analyse de la propagation de la chaleur à travers le toit végétalisé a été réalisée afin d'évaluer le risque d'endommagement de la structure du toit par la chaleur lorsqu'une surface d'un toit végétalisé est exposée à des températures élevées. Plusieurs simulations numériques ont été effectuées pour déterminer les conditions dans lesquelles la défaillance du toit se produit. Un toit végétalisé extensif avec le substrat de croissance sec contenant 5% de matière organique a été pris pour la modélisation. Il a été constaté que le substrat de moins de 10 cm iv d'épaisseur à l'état sec peut protéger efficacement le platelage de toit, retardant la propagation de la chaleur. Un retard d'au moins 30 min est obtenu avec la couche de substrat de 3 cm d'épaisseur. L'effet de la porosité du substrat (entre 0.5 et 0.7) sur le temps de défaillance était faible et observable uniquement sous une charge thermique de 200 kW/m². Les caractéristiques d'inflammabilité des toitures végétalisées, telles que la vitesse de dégagement de chaleur, la densité de la charge combustible et le temps d'allumage ont été déterminés. Des mesures en laboratoire à l'aide d'un calorimètre à cône ont été effectuées sur les substrats de croissance de toit vert à l'état sec et humide. Les résultats ont montré que même à l’état sec, le substrat libère beaucoup moins d'énergie qu'une couverture de toiture typique en bitume modifié. Généralement, la performance des toits végétalisés en feu est meilleure qu'un toit typique en bitume. Le risque d'incendie des toits végétalisés pour les bâtiments adjacents a été analysé en termes d'exposition à la chaleur rayonnante produite par un incendie sur un toit vert. Les résultats des calculs montrent que la présence d'humidité dans les plantes réduit considérablement la distance sécuritaire entre le toit et la façade d’un bâtiment voisin et est le principal facteur de réduction des risques d'incendie. Il a également été montré qu’en raison du fait que le vent a un fort effet sur la propagation du feu, il est important de le considérer lors de la conception d'un toit vert.In the lack of scientific studies on fire performance of green roofs concerns arise about their safety and the adequacy of existing protection measures against the spread of fire. Province of Quebec issued the technical guide on installation of such roofs where fire safety section is drawn up in considerable detail, collecting the rules from different sources, which in some cases poses limitations for design. This research aimed at investigating the fire risks that such roofs present and analyzing their performance in extreme conditions for better understanding of fire possibilities and the importance of fire safety measures. The effective thermal conductivity as a function of temperature of green roof growing media was determined for using in numerical simulations of heat transfer analysis. A series of laboratory measurements were conducted in order to adjust an existing model of calculation of thermal conductivity of mineral soils for the application to green roof substrates. The results show small difference in measured and calculated values, specifically 1.07 and 0.9 W/(m·K), that confirms the suitability of model for green roof substrates. The model allowed the calculation of thermal conductivity considering organic and inorganic materials separately, which facilitates the determination of this property at different temperatures considering the decomposition organic matter between 250 and 700 °C. The thermal decomposition analysis was also performed to obtain proportions of mineral and organic components. Satisfactory results of validation test and numerical simulations show applicability of predicted effective thermal conductivity as a function of temperature for heat transfer problems. The analysis of heat propagation through the green roof assembly was conducted in order to assess the risk of roof structure being damaged by heat when a surface of a green roof is exposed to elevated temperatures. Multiple numerical simulations were performed to determine conditions at which the roof failure occurs. Extensive green roof with dry growing media containing 5% of organic matter was taken for the modeling. It was found that the substrate of less than 10 cm thickness in dry state can effectively protect the roof deck, retarding the heat propagation. At least 30 min of retardation is achieved with the substrate layer of 3 cm thickness. The effect of porosity of the substrate (between 0.5 and 0.7) on time to failure was found to be small and noticeable only under heating load of 200 kW/m². vi Flammability characteristics of green roofs, such as heat release rate, fire load density and time to ignition was determined. Laboratory measurements using a cone calorimeter were conducted over the green roof growing media in dry and moist state. The results showed that even in dry condition the substrate releases much less energy compared to a typical roof covering made of modified bitumen. Generally, the performance of green roofs in fire is better than a typical bitumen roof. Fire risk of green roofs to adjacent buildings was analyzed in terms of exposure to the radiation heat produced by a fire on a green roof. The results of calculation show that the presence of moisture in plants greatly reduces safe distance to façade and is the main factor in reducing fire hazard. It was also shown due to the fact that the wind has a strong effect on fire spread it is important to consider it when designing a green roof

Heat transfer behavior of green roof systems under fire condition : a numerical study

Currently, green roof fire risks are not clearly defined. This is because the problem is still not well understood, which raises concerns. The possibility of plants catching fire, especially during drought periods, is one of the reasons for necessary protection measures. The potential fire hazard for roof decks covered with vegetation has not yet been fully explored. The present study analyzes the performance of green roofs in extreme heat conditions by simulating a heat transfer process through the assembly. The main objective of this study was to determine the conditions and time required for the roof deck to reach a critical temperature. The effects of growing medium layer thickness (between 3 and 10 cm), porosity (0.5 to 0.7), and heating intensity (50, 100, 150, and 200 kW/m2 ) were examined. It was found that a green roof can protect a wooden roof deck from igniting with only 3 cm of soil coverage when exposed to severe heat fluxes for at least 25 minutes. The dependency of failure time on substrate thickness decreases with increasing heating load. It was also found that substrate porosity has a low impact on time to failure, and only at high heating loads

Flammability characteristics of green roofs

Assessing the fire risk of vegetated roofs includes the determination of their possible contribution to fire. Green roof components such as plants and growing media are organic materials and present a fuel that can catch and support the spread of fire. The flammability characteristics of these components were analyzed and compared to a typical roof covering. Growing media with 15% of organic matter were tested using cone calorimeter apparatus. The fuel load and heat release rate of the growing media were measured in both moist (30%) and dry conditions. It was observed that growing media in a moist condition do not present a fire risk, reaching a maximum heat release rate of 33 kW/m2. For dry substrates, a peak heat release rate of 95 kW/m2 was recorded in the first minute, which then rapidly decreased to 29 kW/m2 in the second minute. Compared to a typical bitumen roof membrane, the green roof showed a better fire performance. The literature data report more severe results for plant behavior, reaching peak heat release rates (HRRs) of 397 kW/m2 for dried and 176 kW/m2 for a green material. However, a rapid decrease in HRR to much lower values occurs in less than 2 min. The results also show that extensive and intensive types of green roofs present 22% and 95% of the additional fire load density when installed on a modified bitumen membrane, 19.7 and 85.8 MJ/m2, respectively

A conceptual framework for modelling the thermal conductivity of dry green roof subsrtates

The fire performance of green roofs has never been assessed numerically. In order to simulate its fire behavior, the thermal conductivity of a growing media must be determined as an important input parameter. This study characterized the thermal conductivity of a dry substrate and its prediction as a function of temperature, considering temperature effects on soil organic and inorganic constituents. Experimental measurements were made to provide basic information on thermophysical parameters of the substrate and its components. Thermogravimetric analysis was conducted to consider the decomposition of organic matter. An existing model of the thermal conductivity calculation was then applied. The results of calculated and measured solid thermal conductivity showed close values of 0.9 and 1.07 W/mK, which demonstrates that the model provided a good estimation and may be applied for green roof substrates calculations. The literature data of a temperature effect on soil solids was used to predict thermal conductivity over a range of temperatures. The results showed that thermal conductivity increased and depended on porosity and thermal properties of the soil mineral components. Preliminary validation of obtained temperature-dependent thermal conductivity was performed by experiments and numerical simulation

A Parametric Study of Fire Risks of Green Roofs to Adjacent Buildings

The susceptibility of plants to burn raises concerns about fire hazard that green roofs may pose to buildings. Main concerns relate to cases when such roofs are poorly maintained or stressed by drought conditions which leads to drying out of plants and the accumulation of dead organic material, greatly increasing the availability of fuel load. Existing standard safety measures aim to prevent the spread of fire through the vegetation cover. However, fire spread by thermal radiation is not considered. In this study, fire risk of exposure of adjacent buildings to radiant heat flux produced by fire on green roofs was assessed. Based on generally accepted maximum tolerable radiant heat flux to exposed facades of 12.5 kW/m2, the minimum safe separation distances were obtained for different conditions. Wildland fire behavior model was used to determine flame lengths which is the necessary parameter for a radiation model. Several vegetation types, moisture content scenarios and wind speeds were taken as variables. It was found that by providing the vegetation with reasonably high moisture content the fire risk can be greatly reduced, especially for grass-covered roofs. Since wind also has a strong effect on flame size, considering the exposure of a green roof to wind can bring better understanding of fire risk to adjacent buildings. At no-wind condition and at extremely low moisture content separation distances are as short as 3.1 m for dense shrubs and 2.4 m for tall dense grass