What it costs: Insulated renders

Externally applied insulated render systems provide an effective means to insulate walls while offering an enormous palette of colours. Issues such as thermal bridges and disruption to occupants are minimised. However, detailing and application are critical.

The main types of insulation include:

• Mineral fibres – inorganic, synthetic batts such as glass or stone wool to EN 13162
• Synthetic or organic panels, including closed-cell foam polyurethane and polyisocyanurate to EN 13165. Expanded polystyrene to EN 13163
• Natural, organic panels for example wood fibre boards to EN 13171.

These standards describe insulation material properties; it is important that the correct physical properties are specified. The choice of insulation depends on suitability for the building shape and substrate, method of moisture control, performance requirements and cost. There is usually an optimum insulation thickness to achieve the shortest payback from energy savings, typically between 100-150mm; thicker insulation incurs higher material and installation costs.

Renders can be grouped into three categories:

• Traditional cementitious renders – with sand and/or lime to EN 13914-1. The
• British standard 5256 has now been withdrawn. These are applied as two or three-coat systems, 16-25mm thick but are rarely specified
• Polymer modified cementitious renders or proprietary renders – these incorporate fibres in the mixes which are manufactured in a factory, ensuring consistency. They can be supplied dry or premixed and are applied in one or two coats, 6-16mm thick. Proprietary renders generally have improved bond strength, increased resistance to water absorption and greater freeze-thaw resistance compared with traditional cementitious renders
• Thin render systems – these comprise a polymer modified base coat 4-6mm thick incorporating a reinforcing glass fibre mesh and a finish coat 1.5-4mm thick typically based on an acrylic, silicate or silicone formulation. They all offer improved performance compared with cementitious renders. Silicone renders have improved flexibility and watershedding properties.

Key design and detailing issues

The system must be able to resist expected wind suction forces and driving rain. Insulation should be applied without gaps and provide an even surface for the render.

Direct applied insulated render systems on masonry substrates is standard practice in the UK.

Direct applied insulation systems on timber frame buildings are common in continental Europe. The poor performance of direct applied insulated render systems on timber frames in Canada and the US may have influenced take-up in the UK. Investigation of these cases has shown that in the vast majority of cases failure is the result of poor detailing and installation.

Insulated render systems incorporating a cavity between the insulation and structure provide a belt-and-braces approach. This solution affects thermal performance and may require more complicated detailing.

There is increased interest in direct-applied insulated render systems on timber and steel-framed buildings in the UK using either vapour resistant or vapour diffuse construction techniques.

Standards and maintenance issues

For assurance of performance, specify complete insulated render systems which have been certified by technical approval organisations or systems which meet the European Technical Approval ETAG 004. Ensure application is by approved or competent installers. Expected service lives for third-party-assured, polymer-modified and thin render systems are 30 years.

Insulated render systems correctly installed should not be a great maintenance burden; however, experience of past defects due to the risk of moisture penetration at junctions suggests that regular inspections are a wise precaution.

Cementitious renders may crack due to expansion and contraction with temperature or in response to movement in the underlying substrate.

Mould and algae growth take place on the render depending on the microclimate – sheltered northern facades tend to be affected. Ironically, cleaner air resulting from reduced emissions of sulphur dioxide, an effective biocide, may contribute to this growth. Polluted rainwater and concentrated rainwater run-off may lead to facade discolouration. Maintenance may involve washing down or over painting.