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‘For those projects where the highest performance in relation to fire is seen as being of paramount importance specifiers might be advised to consider the fire performance of the insulation material
alongside its thermal performance characteristics’
undetected and will result in the building exhibiting much higher rates of heat loss than the design might suggest. The use of EWI, on the other hand, places the insulation
externally so that thermal continuity is easier to achieve. Furthermore, there is usually more scope for introducing thicker insulation layers to the outer face of the wall than into a cavity and therefore higher levels of thermal resistance, and hence lower wall U-values, can be achieved. The use of EWI on solid masonry walls is therefore a favoured approach to achieving the high thermal performance requirements for walls of passivhaus designed buildings. All is not necessarily straightforward, however, and specifiers
of EWI should understand some of the other factors that should be considered before settling on a specific EWI system. EWI systems can incorporate a number of different insula-
tion material types, including synthetic foam materials such as phenolic, polyisocyanurate (PIR) and expanded or extruded polystyrene (EPS and XPS respectively) or mineral-based mate- rials such as stone wool. Generally the synthetics are lower thermal conductivity materials than their stone wool counter- parts and may therefore be advantageous in terms of their thermal performance, but the mineral/stone fibre types offer superior fire performance and may therefore be looked upon more favourably by the building insurer. For those projects where the highest performance in relation to fire is seen as being of paramount importance specifiers might be advised to con- sider the fire performance of the insulation material alongside its thermal performance characteristics. Insulation materials for EWI generally take the form of rigid
boards or batts, and these can be fixed to the substrate using a number of different methods. Ideally the insulation material should be fixed to the substrate so that it is in intimate contact with the substrate. Such an arrangement will reduce the phe- nomenon of thermal bypass whereby heat losses occur through the action of air movement behind the boards and through gaps in the insulation or at abutments. On uneven substrates it may be necessary to use adhesive to bond the insulation to the walls. Here the adhesive provides several functions: The bonded insu- lation has better resistance to wind suction forces since the
action of wind is transferred to the wall through a greater contact area that would be the case if the boards were only mechanically fixed. The adhesive also provides a levelling func- tion, enabling the unevenness of the substrate to be overcome, as well as blocking the path for convective heat losses behind the insulation boards. In some cases, however, the incorporation of a drainage cavity
behind the insulation layer is a requirement of the building insurer. A cavity placed behind an insulation layer is counter intuitive since it invites heat loss by convection. It is essential, therefore, for any such cavity to be carefully designed so that a free flow of air through the cavity cannot occur. The cavity may have drainage holes at the base but openings elsewhere must be avoided if the thermal performance of the insulation layer is not to be compromised further. If cavities extend over several storeys provision will have to be made to prevent the risk of the spread of fire through the cavity, while maintaining the drainage requirements of the building insurer. The incorporation of intu- mescent cavity fire barriers can provide a way of achieving this. In terms of the aesthetics of EWI systems, synthetic renders
are now available is a very wide range of colours and textures and now include metallic or lustrous effects. Specifiers should be mindful of the fact that not all colours, and indeed not all render types, have the same durability, in particular in relation to colour fastness and fade resistance. All exterior coatings achieve their colour through the incorporation of coloured pigments. These might be synthesised organic materials or inorganic mineral based pigments. Generally, those pigments which have their origins in naturally occurring minerals will have superior resistance to the potentially damaging effects of ultra violet light and their pigments will exhibit better colour retention and a lower tendency to face that synthetic pigments. The resistance to UV of the binder component will also affect the colour fastness of the decorative finish, and therefore specifiers should look carefully at the type of product that are specifying and should ensure, when selecting colours for render finishes, that the colour required can be achieved in a product that has the highest resistance to fading, both in terms of the pigment content and the binder.
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