External wall insulation
An external wall insulation system (or EWIS) is a thermally insulated, protective, decorative exterior cladding system which consists of expanded polystyrene, mineral wool polyurethane foam, together with a reinforced cement based, mineral or synthetic finish. plaster.
The thickness of thermal insulation is dependent on the type selected to create a partition with a heat transmission factor of U=0.25-0.3 W/m2K. Calculations need to be used to determine the actual insulation requirements to meet current Building Regulation standards. Consideration must also be given to exposure and durability if subjected to vandalism etc. In many older properties, elements such concrete beams or lintels act as thermal bridges providing poor insulation which need to be insulated.
[edit] Types
External wall insulation systems generally comprise an insulation layer to achieve the requisite thermal performance protected with a weatherproof finish, usually a render, although brick slips, tiles, and decorative boards can also be used. Insulating render can also be an advantage in certain locations. Fixing types and sizes depend on the substrate and design exposure requirements.
Dry finishes are usually fixed to the substrate by means of timber battens independently fixed to the substrate.
Any system selected and installed, should be certified by BBA/BRE
[edit] Traditional finishes
A selection of traditional finishes are currently utilised within the external wall insulation industry. Dry-dash render is a traditional render application used throughout the whole country[vague], dry dashing aggregate is thrown onto the wet render to create a natural aggregate finish. Available in a wide variety of colours, sizes and textures, the practice is relatively cheap. Manufactured aggregates such as ceramics and glass are also used for more specialised projects as the supply costs are considerably greater than for natural aggregates.
Scratch plaster render is a coloured cementitious render scratched while the surface is still workable, but after the initial setting has taken place. The surface of the render is removed by the action of the scratching tool, and approximately 2 to 3 millimetres (0.079 to 0.12 in) of render are removed exposing the open matrix of the aggregate mix. The true colour of the render is exposed with a light even texture.
Rough-cast render consists of a top-coat render and aggregate mix thrown onto a backing coat in a slurry form, the aggregate being totally encapsulated within the cementitious slurry. The aggregate may be any hard stone of an equal graded size to suit the particular application and creates a “lumpy texture” finish. This method is traditionally widely used in Scotland.
A new innovation is the inclusion of silicone water-proofers in pre-blended and pre-packed proprietary renders. This development increases the specification and capabilities of polymer renders, particularly for exposed or coastal areas. It is applied in the conventional manner and now readily available in all the usual colours.
Tyrolean finishes are sprayed cementitious mixes, pre-coloured and applied by a hand-held machine. This finish is widely used throughout the UK as an economical, easily applied colourful finish for all forms of building type. It has medium-term durability under average conditions.[vague]
Smooth/painted finishes conist of masonry paint, and are applied to a good rendered surface to give a smooth coloured effect, free of the imperfections that the more natural aggregate finishes can sometimes deliver. A very wide selection of light colours are available.
Textured coatings are applied by roller or trowel to an approximate thickness of 1.5 to 3 millimetres (0.059 to 0.12 in), and are usually acrylic[disambiguation needed 
] or silicone-based for waterproofing and long term durability. They produce an even, flat-textured, finish.
Acrylic variants are easily applied, and are considered “high performance” finishes. Some cheaper variants can include a tendency to lose opacity and brightness over a relatively short life span. High quality acrylics can provide a longer term durability, colour stability, and crack resistance compared to polymer cement finishes. Acrylic variants are available in a very wide range of colours.
Silicone variants are more resistant to marine environments than acrylics, as they have superior water resistance, but they can be more costly, and come in a smaller range of colours.
Brick slips are a thin facing applied over an insulant to provide a traditional “brick” wall finish. Slips can either be manufactured by extrusion or cut from real bricks. Specialist waterproof colured pointing mortars are used to complete the effect. Commercially available brick slip systems have been developed to include support for the brick slip finish, certified by BBA/BRE.
Simulated brick renders can also be replicated in coloured polymer to a high standard. Two coloured layers of polymer-modified external cementitious render are applied in 3 to 4 millimetres (0.12 to 0.16 in) layers onto a coloured backing. The “brick” pattern is cut into the top layer, exposing an under-layer of differing colour representing the cement joints.
Simulated stone renders are made from coloured polymer. Two coloured layers of polymer-modified external cementitious render are applied in 3 to 4 millimetres (0.12 to 0.16 in) layers onto a specified backing. The “stone” pattern is cut into the top layer exposing an under-layer of differing colour to represent the cement joints.
Terracotta tiles are usually clay burnt material imported from Europe. These proprietary systems are secured by a system of extruded aluminium rail systems fixed securely to the substrate with insulation material inserted within the void. Special pressed metal profiles and cills are added to make them waterproof.
Timber boards, aluminium or PVC “sidings” can be installed over insulation to provide additional alternatives to the traditional render and applied finishes. The boards are usually of Ship-Lap profile, and either cedar or treated softwood. Aluminium or PVC systems are also available as alternatives.
Traditional tile hanging finishes can be applied onto conventional timber support structures and insulated with the required type and thickness of insulation. Suitable consideration has to be given to ventilation passages to avoid interstitial condensation.
[edit] References
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- National Building Specification Section M21 Insulation with rendered finish
- BSI British Standards and Code of Practice
- BRE Report 135: EWI, Fire performance for multi-storey buildings
- BRE Report 262: Thermal insulation avoiding the risks
- INCA Insulated render and cladding Specifiers/property owners briefing
- Technical Drawings: Weber UK EWI Systems
- The Complete Guide to External Wall Insulation by C J Pearson (now on Kindle)
