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Housing : June 2009
Technical Feature Photo courtesy MasterWall process which combines styrene molecules with other molecules (details are a bit outside the scope of this article) gives us polystyrene. Flame retardant properties For the polystyrene cladding that we use in the cladding of timber- and steelframed housing we look at two forms of this plastic: XPS (eXtruded rigid cellular PolyStyrene) and EPS (Expanded PolyStyrene). Both these materials – when used for cladding – are modified to include flame-retardant additives. Water resistant The first form, XPS has a homogeneous and hydrophobic (repelled by water) closed-cell structure – making it ideal for applications that require high resistance to moisture intrusion. Both XPS and EPS are the same plastics but processing is different. EPS also has a closed-cell structure and is hydrophobic, but its beads are surrounded by voids. This is because it is made by expanding small polystyrene beads and then fusing them as where XPS starts life as a continuous mass of molten material. Insulating properties EPS is a lightweight cellular plastic material consisting of small, spherical shaped particles containing about 98 per cent air. The microcellular closed-cell construction provides both XPS and EPS with excellent insulation characteristics. So polystyrene cladding is lightweight, waterproof, is a very good insulator, is strong and flexible, energy efficient, cost effective and last but not least, quick and easy to install. Polystyrene is dimensionally stable and will not settle over time when used and installed correctly (always according to manufacturers’ instructions). It does not deteriorate with age, and, as we know, it does not rot, meaning it will have a constant R-value for the life of the product. It should be remembered that this product has been used for cool stores and R-values of some common building materials* Brickwork freezers for a long time; this supports any insulation properties claim, i.e. thermal resistance (R-value) of more than R1.2 per 50mm of thickness claimed. So is it safe to use?VariousMSDSs show us that polystyrene is not a known carcinogen, that there is no known effect from chronic exposure to this product, and auto-ignition occurs at 440o C. Its flashpoint (the temperature at which an ignitable mixture with air is formed) is greater than 200o C.When exposed to high temperature such as this, polystyrene will release irritating vapours to the respiratory system and eyes, but this is apparently less dangerous than the gases released when burning timber. When burning it produces carbon oxides (CO and CO2 ) as well as soot. Polystyrene starts to melt at around 130oC, and when it does it begins to shrink to its original density, i.e. in the case of EPS, prior to its expansion. Polystyrene is not considered to be dangerous to humans. However, please note that the above figures apply to polystyrene, not polystyrene cladding which has been modified by flameretardant additives. Fire indices as per AS 1530.3 Scale between Expanded Ignitability Spread of flame Heat evolved Smoke produced 0–20 0–10 0–10 Extruded 90 1 (Source: Auslite Products) HOUSING JUNE 2009 0–10 12 03 5 4 Fire ratings and coatings There are wall systems available that use polystyrene cladding that are able to offer a 90/90/90 fire rating certificate. (See box on page 53.) The key to improving the fire rating of polystyrene cladding clearly lies in the surface coating(s) applied to the material.And of course, the joint between the panels should also be constructed in such a manner that access to the polystyrene through joints is prevented. Each manufacture would have its own solution to these issues and close adherence to their installation manuals would be essential. When properly finished with the special proprietary coatings available on the market for polystyrene claddings, these buildings look no different from any other rendered and painted house. If used in accordance with manufacturers’ instructions/manuals it can make a costeffective and safe alternative. For further information see the Building Commission’s Inform e-newsletter. Go to ‘publications’at buildingcommission.com.au H 55 Weatherboards Cement sheeting Concrete slab Plasterboard EPS/XPS (50mm) R0.18 R0.08 R0.019 R0.069 R0.059 R1 to 1.74 (depending on EPS or XPS) NB: At 100mm thick an R-value of up to 3 is claimed. * Note this is the cladding alone, not the complete wall system.