Last edited 16 Dec 2020

External wall insulation

Thermal insulation can be used to reduce the tranmission of heat between the inside and outside of an enclosed space such as a building.

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:

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.

[edit] 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:

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.

[edit] 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.

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