Heat release and flame spread assessment of insulation in External Thermal Insulation Composite System (ETICS) façades

Abstract

External Thermal Insulation Composite Systems (ETICS) represent a popular modern façade for reducing energy consumption, particularly in retrofit applications. Insulation – typically in the form of either EPS (expanded polystyrene) or mineral wool – is applied to the exterior of buildings using mortar adhesive, and then coated with a final layer of sand-cement render and glass fibre reinforcement. The materials are relatively cheap, and the labour can be easy and inexpensive compared to other solutions.The system does however mean that highly flammable EPS insulation is protected by only a thin layer of render, normally 3–8mm. The addition of this combustible material to the façade of the building represents a change in the fire risk, as the typical fire safety strategy involving compartmentation does not anticipate vertical fire spread on the exterior of a building. It is therefore necessary to characterise the fire risk, and ensure that an optimal fire barrier – namely, the render layer – is defined adequately.In this work, the insulation component of an EPS ETICS façade is tested using micro- to small-scale methodologies. The objective of this is to characterise some of the fundamental thermal material properties, ignitability, heat release, and flame spread of the specific components. The aim is to then extend this to a larger-scale methodology for testing of complete systems, which will be complemented by numerical modelling to enable scaling. Previous attempts at testing ETICS in a small-scale have been largely unsuccessful, and efforts have pointed to large-scale apparatuses – generally 4–8m high – as the only viable solution at present. These are however expensive and time consuming, and deliver little information on how the façade performs. Testing is especially problematic for EPS, which melts and shrinks at low temperatures, leading to difficulties in attempting to extract global flammability properties.MCC (Microscale Combustion Calorimeter) testing has been performed to benchmark the different EPS insulation that can be used in buildings. It is found that black EPS containing expanded graphite used in façades has substantially improved performance compared to ordinary white EPS used as building insulation. The peak heat release was reduced from 1160 W g−1 down to 490–740 W g−1, occurring at a temperature of 430–440 °C. An extra additive to improve the moisture performance of the system also enhances the fire performance slightly by reducing the heat release further.Previous attempts of testing ETICS and EPS using LIFT (Lateral Ignition and Flame spread Test) have in many cases been unproductive. For this experimental series, a sheet of paper has been applied to the surface of the first 100 mm of EPS in order to force ignition. Results for the flame spread velocity near to ignition have then been discarded to minimise the influence of this method. Flame spread results for the white EPS were not satisfactory due to the fact that the flame spread velocity had a weak dependence on the incident heat flux, and that that minimum critical heat flux for flame spread was 0.0 kW m−2, that is, no external energy was required to sustain burning. For black EPS specimens, the addition of the expanded graphite was effective in improving the flame spread performance. The minimum heat flux for flame spread was increased to 0.75–1.09 kW m−2, and the velocity was sufficiently moderate that a reasonable value for the flame spread modulus could be obtained.In future, attempts will be made to correlate the flame spread results of these individual EPS products to the flame spread as part of a complete ETICS façade. This would then provide a more meaningful method to compare different ETICS solutions as a first step without the need for expensive large-scale testing during the main development phase of systems.