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External Wall Insulation Systems (EWIS)

• Introduction
• A brief explanation of the system components
• Design and detailing issues
• Reference sources

Introduction

• External Wall Insulation Systems, often referred to as insulated render systems or rendered insulation systems, have been used successfully in the UK, generally applied to concrete or concrete blockwork backings, for over 25 years.
• The website of the Insulated Cladding and Render Association (INCA) at www.inca-ltd.org.uk provides a good introduction to the subject. The Technical Properties page, and the associated sub-pages, are particularly useful in understanding the different systems that are available. There is also some good general guidance in NBS Section M21.
• For an in-depth review of the particular concerns and solutions when external wall insulation systems are used in conjunction with Light Steel Framing Systems, refer to the LSFS and EWIS page.
• As far as can be determined, there are no external wall insulation systems currently (November 2008) available in the UK with a BBA Agrément Certificate for use with steel framing systems. All the current Agrément Certificates refer to the use of the systems on a masonry or concrete background only.

A brief explanation of the system components

The systems consist, in principle, of three main components. Specifiers should be aware that system suppliers may offer only a limited range of options when combining the three main components.

Render finish

• Mineral-based : Mineral-based renders are, in general, applied thicker than synthetic renders (two coats, each 5mm thick, is typical) and are therefore heavier. They have a more traditional appearance and are also the "eco-friendly" alternative.
• Synthetic-based : Acrylic-based and silicone-based are the two most common materials. There is some debate about the relative merits of the two polymer types but, in principle, acrylics have better resistance to mechanical stress and damage, silicones offer better vapour permeability and weather resistance.
     Specifiers should be aware that proprietary synthetic renders are generally referred to as "two-coat" although most consist of a 3-4mm thick reinforced basecoat plus a thick (~1mm) paint finish. The reinforcement is intended to eliminate/minimise cracking but many remain sceptical that a 5mm finish over a compressible material can ever be truly crack-free, and cracking is one of the main reasons why these systems fail.

Insulation

• Mineral fibre slabs and batts : Provide a better acoustic performance, but lower compressive strength, when compared with polystyrene and other foam board products. They are inherently more susceptible to mechanical damage than systems using board insulation although they are the only option certified non-combustible - see below.
• Expanded and extruded polystyrene foam boards : The lightest and cheapest of the insulation options. Used as the base insulation material in most systems.
• Polyisocyanurate and phenolic cellular foam boards : The highest thermal performance, and the most expensive, for a given thickness, but not classified as non-combustible.

Insulation choice:

• Education buildings : Zurich Insurance are one of the main insurers of school buildings and publish a design guide, "School and Academy Design", setting-out their requirements for a wide range of building-related issues. External Insulated Systems are covered in Section 5.2.7 which states "The insulation used must be of non-combustible construction eg. Rock mineral fibre type, stonewool or mineral wool.". Section 5.2.7 also precludes the use of external insulated systems where these are susceptible to mechanical damage, and cautions against their use on exposed sites.
• Healthcare buildings : HTM 05-02 Appendix F, although referring to insulating core panels rather than external insulation systems, would appear to suggest that only mineral fibre insulation is suitable for use in the external cladding of healthcare buildings.
• Residential buildings : NHBC advice, if any, to follow..

Fixing system

• Adhesive-fixed : The simplest method of fixing. The insulation boards are adhesive-fixed directly to the substrate. This method is still used extensively when external wall insulation systems are applied to concrete, or concrete blockwork, walls but is not generally the preferred method for fixing to sheathing boards on light steel framing systems. Example: StoTherm Classic K System by Sto Ltd.
• Mechanically-fixed (direct) : The insulation boards are pinned to the substrate using proprietary wide-head fixings. This method is generally only suitable for fixing to concrete, or concrete blockwork, walls. Some systems combine adhesive and mechanical fixing. Example: Swistherm by Alumasc Exterior Building Products.
• Rail-fixed : This method is suitable for situations, such as some refurbishment projects, where the substrate is too irregular (or otherwise unsuitable) to permit direct adhesive or mechanical fixings. Metal rails, shimmed and packed if necessary, are screw-fixed to the substrate. The insulation boards are then pinned to the rails using proprietary wide-head fixings. Examples: StoTherm Classic M System by Sto Ltd.; Swispan by Alumasc Exterior Building Products.
• Rail-fixed (with drainage cavity) : A variation of the rail-fixed method where the rail system is installed to provide a specified air gap, and drained cavity, between the outer face of the substrate and the inner face of the insulation. These systems differ from rainscreen systems in that the insulation is external to the non-ventilated (typically 15-25mm wide) cavity. Example: Swisrail by Alumasc Exterior Building Products.
• Rainscreen : In these systems the insulation is fixed directly to the substrate (as for other types of insulated rainscreen) and the render finish is applied directly to a board product which is held clear of the substrate and the insulation by a proprietary rail system of sufficient depth to provide a fully-ventilated cavity between the outer face of the insulation and the inner face of the render-supporting board. Example: StoVentec by Sto Ltd.

Design and detailing issues

Cavities

• The design of most external wall insulation systems assumes that the outer render coat provides a single barrier defence against water penetration. The introduction of any cavity within the system, when this cavity is intended to provide a means of drainage for water which has passed the external skin, is therefore highly contentious as the implication is that the single barrier is not 100% effective.
• The NHBC has responsed to concerns about the long term viability of insulated render systems by requiring a minimum 15mm drained cavity within the system. This requirement obviously caused some difficulty during the drafting of the SCI publication referred to below as the primary systems of most of the INCA members are adhesive- or mechanically-fixed, without cavities.
• The SCI/INCA response is therefore to restrict their recommendation on the desirability of a cavity to locations with a severe or very severe level of exposure, whereas the NHBC requirement covers all levels of exposure.
• The cavity, if provided, should be drained but not ventilated. Specifiers should be aware that there are very few systems available from INCA members which meet this requirement. Swisrail by Alumasc Exterior Building Products is a good example of an NHBC-compliant product.

Interfaces

• Information to follow.

Reference sources

• Refer to the reference sources on the LSFS and EWIS page.

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