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Last edited 16 Dec 2020
External wall insulation
External wall insulation (EWI) is the application of thermal insulation to the external walls of buildings - generally referring to the application of thermal insulation material and a finish system to the outside face of the external walls of an existign building to improve its thermal performance. The analogy that can be made is that of a tea cosy placed around the building.
Benefits of EWI can include:
- Reduced heat losses, lower heating bills and improved carbon footprint.
- Reduced the risk of condensation (if installed correctly).
- Allows a choice of colour and texture of finish that can enhance a building.
- Can reduce solar heat gains in the summer.
- It does not reduce internal floor area.
- Improved weather resistance.
- Improved internal comfort.
- Preserves the substrate (eg old brickwork), extends its life and reduces moisture content.
- Work can be undertaken without disruption to the building’s interior.
However, external wall insulation makes walls signicantly thicker. This is noticeable at door and window openings, and may be difficult to accommodate under roof overhangs, or may make roof overhangs less attractive. In addition, it can make walls more difficult to access, and can conceal problems cuased by the insulation, or problems that were not properly rectified before the insulation was applied.
 Types of external wall insulation
Traditionally, thermal insulation and weather protection could be imparted to an external wall by fixing clay tiles, slate or brick slips. Alternatively, a cheaper and faster method was to apply a one-coat render that may include aggregates and painted for an attractive textured finish.
Rough cast render is widely used in Scotland and comprises a top-coat render and aggregate (or graded hard stones) applied as a slurry-type consistency and resulting in a lumpy finish. Such renders may include waterproofing agents for better moisture resistance
Today, there are numerous types of advanced external wall insulation (EWI) systems available for solid external walls (ie, those without a cavity) and many are based on the same three-layer concept, namely:
- Insulating layer: traditionally this was a render (typically cement and sand with an insulating material) trowelled over the substrate. More popular today are insulation boards of expanded/extruded polystyrene, polyurethane, phenolic, isocyanurate or other lightweight material. The insulating boards can be fixed relatively quickly to the substrate either by physical connectors or adhesive, or both.
- Reinforcing layer: once in place, the boards are given impact protection and resilience with a layer of metal or plastic mesh.
- Weatherproof finishing layer: the assembly is finished with a final layer of render that combines weather protection and decoration. Alternatively, tiles or brick slips held on a supporting backing plate can be used as the final top layer (see below). The thickness of the total build-up applied to the wall will depend on the heat transmission requirements (U-values) prevailing at the time in the relevant building regulations.
The UK-based Mortar Industry Association states that specialist renders available from its members may be used for external thermal insulation composite systems (ETICS). It states they will usually contain relatively large amounts of polymeric material and resist thermal shock, have acceptable impact resistance and have good adherence qualities.
The thermal properties of such renders are improved by additives that may include a lightweight mineral aggregate such as perlite, vermiculite or expanded or sintered clay, or an organic product such as expanded polystyrene.
 Other types
Slips provide a traditional brick finish and can be used in place of a top-coat render. A backing plate attached to the insulation layer supports the slips and allows them to be placed quickly in horizontal rows. Once set, the gaps between them are filled with mortar to give a traditional 10mm brick joint.
Simulated brick renders: a brick effect is cut into the top layer of a polymer-modified external cementitious render (3-4mm thick) while it is setting. Cutting-in the brick effect exposes the coloured backing to give the effect of cement mortar joints.
Terracotta tiles: they can be supported on aluminium rails fixed to the substrate in a rainscreen-type application. Insulation is laid in the void. An alternative to terracotta is timber boards that have a shiplap profile.
Composite cladding systems: usually for larger commercial or residential buildings. An aluminium framework is fixed to the building's exterior. Insulated cladding panels are fixed to the new framework to provide better thermal insulation and possibly better aesthetics. This is similar to the system used on the ill-fated Grenfell Tower.
 Related articles on Designing Buildings Wiki
- ACM cladding.
- Approved Document B.
- Brick veneer.
- BS 8414 Fire performance of external cladding systems.
- Building a safer future: an implementation plan.
- Celotex RS5000 PIR insulation.
- Cladding for buildings.
- Consultation on banning the use of combustible materials in the external walls of high-rise residential buildings.
- Evacuating vulnerable and dependent people from buildings in an emergency FB 52.
- Fire performance of external thermal insulation for walls of multistorey buildings, third edition (BR 135).
- Fire risk in high-rise and super high-rise buildings DG 533.
- Grenfell fire door investigation.
- Grenfell Tower articles.
- Grenfell Tower independent expert advisory panel.
- Grenfell Tower Inquiry.
- Independent review of the building regulations and fire safety.
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