On the Fire Performance of Exterior Wall Materials and Assemblies

Abstract

Recent attempts at combating climate change have urged the developers, public authorities, and other building stakeholders to employ enhanced energy conservation initiatives as Canada’s building sector accounts for 13% of national greenhouse gas emissions. These initiatives are often realized by using energy-efficient envelopes on the building exterior. As a physical barrier separating the controlled interior spaces of a structure from the exterior surroundings, the building envelopes are designed to provide effective vapour control while simultaneously being constructed with high thermal resistant materials in the façade. Consistent with this, modern energy-efficient envelopes are composed of four distinct layers (from outermost to innermost): decorative surface siding, exterior continuous insulation, weather resistant barrier, and exterior sheathing. Each layer addresses a specific design objective, and when assembled into the envelope, contributes to the overall performance of the exterior wall in meeting energy efficiency and hygrothermal requirements. Nowadays, multi-layered exterior envelopes are taking centre stage in construction, gradually replacing conventional monolithic concrete and masonry façades. Predicting fire performance of such assemblies, however, is challenging and should be carefully addressed to ensure occupant and firefighter safety as well as minimize property damage in the event of fire. The importance of this has been tragically demonstrated through recent massive fires around the globe in Turkey (Polat Tower, 2012), Australia (Lacrosse Building, 2014), England (Grenfell Tower, 2017) and even twice for the same residential building in the U.A.E. (Torch Tower, 2015 and 2017). It has become evident that while much emphasis is placed on improving energy and hygrothermal performance of exterior assemblies, next generation exterior wall designs must also exhibit superior fire resistance. For this, understanding the fire performance of exterior wall materials, alone and in combination with underlying layers, is critical since a broad selection of material combinations are possible when designing an envelope.

Assessment of the fire response of exterior wall assemblies usually involves expensive large-scale testing, that can only be conducted in a limited number of facilities, and which are often outfitted with the minimum instrumentation required by a given test standard. This has led to a dearth of detailed data, with consequent knowledge gaps, regarding the high temperature properties and fire behavior of many building materials. Further, it still poses a significant challenge to properly assess the effect of building envelope design features on the fire performance of exterior wall assemblies. Careful and detailed assessment of fire behaviour of such assemblies is therefore needed to advance performance-based engineering of new and innovative exterior wall designs.

The goal of the present research is to address these challenges through developing a consistent set of test methods where exterior wall materials are subjected, at small- through larger-scale, to temperature and heat flux conditions similar to those encountered in realistic fire exposures. More specifically, experiments are aimed toward characterizing and advancing current understanding of the impact of different exterior continuous insulation products on the fire performance and bulk-path heat transfer in wall assemblies. To achieve these objectives, representative samples of the main layers forming an energy-efficient, exterior building envelope in Canada were first identified. These layers are vinyl siding panels, stone wool insulation, polyisocyanurate insulation, extruded polystyrene insulation, non-woven house wrap, and oriented strand board. Their fire performance was then characterized via two complementary avenues of investigation. In Phase 1, small-scale fire testing of the selected materials was conducted by instrumenting representative specimens of each material with thermocouple probes and subjecting them to varying levels of radiant heat flux exposure to determine key parameters such as time variations in mass loss and heat release rate per unit area, time-histories of surface and bulk sample temperatures, as well as thermal degradation and damage (shrinking, charring, melting, self-heating). In Phase 2, 2,438 mm × 2,438 mm large exterior wall envelopes constructed using the same materials were instrumented with multiple sets of thermocouple probe rakes positioned across each layer of the test structure and subjected to temperature and heat flux from a realistic, but contained, fire. This allowed capture of the thermal response of the wall assemblies to the prescribed exposure and assessment of the observed thermal degradation phenomena relative to the temperatures measured in the degrading walls. Results show that temperatures measured in the small-scale fire tests may provide a good indication of temperature evolution and heat transfer within full-scale wall assemblies during large-scale fire tests. Further, comparable levels of fire damage and thermal degradation (shrinking, charring, melting, self-heating) of the studied building products were obtained across the two scales of fire testing. Thus, the utility of studying thermal degradation of building materials and construction assemblies in the context of energy-efficient building envelopes at both the small- and large-scales was demonstrated. In combination, the novel set of experimental data obtained via Phase 1 and 2 of this research may guide formulation of advanced numerical simulation and design tools for predicting fire performance of individual building materials and their interactions in exterior wall assemblies when subject to realistic fire exposures.