Solid wall retrofit insulation:
Render on insulation (insulated render)
• Insulated render is usually applied as part of an overall refurbishment scheme.
• Where external walls are poorly insulated.
• Where external walls are deteriorating or are insufficiently weathertight, causing damp, draughts and heat loss.
• Where wall cavities are bridged or blocked, making them unsuitable for cavity fill insulation.
• The external appearance needs updating as part of a rehabilitation scheme for high-rise or system-built housing.
• Installing internal lining insulation would be disruptive, would alter critical internal dimensions or make room sizes too small.
• Greater thickness of insulation is required compared with what is usually achievable with internal linings
• The design and installation of insulated render is a specialist job.
• Though not always, insulated render systems usually come in the form of proprietary products. In these instances, standard details and construction methodology are determined by the system manufacturer.
• The products are accredited by a third party such as the BBA or BRE for use in specific situations.
• They are normally guaranteed when installed by the manufacturer’s approved contractors.
• Though construction technique remains the same, systems differ through their use of components such as types of insulation or render finishes.
A typical insulated render system includes:
- The attachment of standard-size insulation boards to the existing wall using
- Mechanical fixings (sometimes accompanied by adhesive) through to the wall
- Mesh reinforcement to form a key backing to the
- Render base coat
- Finish coat
|Insulation material||Thickness of insulation|
|Mineral wool slab||0.31||0.22||0.17|
|Expanded polystyrene (EPS) slab||0.33||0.23||0.18|
|Polyurethane (PUR) slab||0.22||u/a||u/a|
|Phenolic foam slab||0.20||0.14||0.11|
Fixings: avoiding thermal bridging
Insulated render systems usually include a series of mechanical fixings – normally made of metal or plastic. When used to fix the insulation, they form a series of point repeating thermal bridges analogous to the effect of metal cavity wall ties.
Thermal bridging can be largely avoided by:
• Using adhesive fixed insulation. This is a common method, however to ensure that there is no air movement behind the insulation, it is recommended that the adhesive fully covers the insulation – rather than the more usual ‘ribbon’ of adhesive. Note that this method is not suitable where there are doubts over adhesion quality eg painted or rendered walls.
• Using a mechanical fixing ‘track & rail system’ where the insulation is supported by metal or plastic tracks attached to the wall. Though this system has the advantage of placing the fixing behind the insulation, a small space is created resulting in air movement between the insulation and the wall surface.
Traditionally mixed directly on site, but now more often supplied pre-mixed with additives to improve application and curing. Consists of of 2 or 3 coats of render (total 16 – 25mm). Used in particularly exposed climatic conditions or in situations where damage might occur. The reinforcement is either stainless or galvanised. Cement render is prone to cracking, so movement joints should be used as required. aka 'Heavy-weight render'
Polymer-modified cementitious render (PMCR)
In use in the UK since the 1980s, PMCR consists of 1 or 2 coats of render (total 6-12 mm). Used in average exposed climatic conditions and where there is no particular risk of damage. The reinforcement is either mineral fibre or glass mesh. The added polymer reduces the overall weight of the render and makes it more feasible to be used in both low and high-rise applications. Polymer content varies considerably between systems according to application requirements. aka 'Medium-weight render'
Acrylic render uses acrylic rather than cement as the binder and combines with higher quality aggregates / sand. It usually consists of 2 coats of render (total 4 - 6 mm). Used in situations of light exposure and low risk of damage. The reinforcement is either mineral fibre or glass mesh. Acrylic render's advantages are that it is has a high level of elasticity combined with weather resistance and breathability. aka 'Light-weight render'
Hydraulic lime renders
There is a least one lime-based insulated render system provided by collaboration between Unilit and Foamglas. The Foamglas insulation is rendered with a 10 – 12mm coat of hydraulic lime render containing glass fibre mesh reinforcement together with a 3mm decorative layer.
Insulating render is traditionally a render that is mixed with an insulating 'aggregate' which might include polystyrene balls, perlite or vermiculite. It is not commonly used in the UK and does not offer a significant increase in the thermal efficiency of the render. Its most useful application is in areas such as window and door reveals where dimensions preclude the use of other insulation materials - in these instances, insulating render can contribute to reduction of thermal bridging.
Hemp - lime render
Along with the maturing of the hemp insulation market insulation there comes the development of traditional lime render blended with hemp. Though not yet widely specified, manufacturers claim a modest improvement in the performance of a solid brick wall, citing the increase in u-value from an uncoated 1.71 for a 215mm brick wall up to 1.00 for the same wall coated with 50mm of hemp-lime render. Combined with a lime plaster finishing coat, the overall system represents a more sustainable alternative to the other, more synthetic, systems listed here.
An example of coloured pre-mixed acrylic top and base coat renders (Envirowall) on rockwool insulation
• Silicone and Acrylic renders: suitable for application onto thin coat or thick coat render systems, they provide resistance to cracking. Additionally, the inclusion of silicone in any render adds a high water-repellant quality, whilst allowing water vapour to pass freely through the render. This property makes for a dry surface to the render which helps it resist algae growth and lime bloom. A wide range of textures and finishes are available. Additionally, a variety of finishes is possible by using different trowels and rollers – though spray applications are becoming increasingly popular. Care should be taken when applying a textured coating over a large area to ensure consistency of finish.
• Aggregate dash (aka Spardash or Pebbledash): applied as a background for dry dash or roughcast finishes. Normally applied onto proprietary base coats. To conform with BS5262 Type 3.
• Scratch plaster or Scraped finish: a pre-coloured specialist render with a large grain size that will provide a textured finish when scratched.
• Roughcast render: A traditional finish in Scotland, it is a cement-based, polymer-modified, self-coloured render incorporating an aggregate of small, evenly-sized pebbles, to provide a textured finish. Applied over a base coat and dash receiver. To conform to BS 5262 Type 2.
• Spray render or Tyrolean finish: Hardwearing and weatherproof, Tyrolean render is a traditional finish in the UK, it is achieved by repeated spraying using a spatter machine, building-up to a minimum coat thickness of 4mm applied onto a smooth, coloured, polymer-rendered layer.
• Coloured Smooth / Flat render: Usually available in a variety of colours, it provides for a smooth, clean finish
• Cementitious, particularly traditional sand:cement renders, are prone to cracking through temperature changes and moisture variations. Thinner, more flexible, acrylic-based are much less inclined to crack.
• It is generally recommended that movement joints should be provide at no more than 5m intervals – both horizontally and vertically, or areas of render not exceeding 42 sq m.
• Check for requirements from the specialist subcontractor / system provider.
Brick slips are an established method of replicating the impression of a traditional brick wall. In recent years proprietary systems have evolved to replace the demanding task of aligning and adhering slips to a simple board backing. Modern mounting systems come in a variety of formats including profiled polystyrene and wire mesh backings.
Typical details (render)
Window head detail (window frame retained)
Window head detail (new window frame)
Cill detail (under-cill)
Cill detail (over-cill)
• Other critical details that might be included will be: Junctions with eaves; Junctions with verges (consider extending the roof covering); Raised features, particularly around doors and windows; Copings and cappings; Changes in finishing materials and colours.
• Pay particular attention to potential instances of thermal-bridging and design to exclude them.
Insulated render and the RIBA plan of work
|RIBA Work Stage||Design Team Tasks|
|A Appraisal||• Survey existing building.
• Determine existing structural integrity, thermal performance and ventilation.
• Determine existing SAP performance.
• Identify issues arising from damp and condensation.
• Determine insulation strategy.
• Determine air-tightness target and strategy.
• Establish a wall performance target in conjunction with other building elements and services and the SAP performance target.
• Assess any planning constraints.
• Identify any unique local factors that might effect durability of cladding system (eg susceptibility to damage, particularly inclement weather conditions etc.)
|B Feasibility / Briefing||• Determine the exposure zone of the site in accordance with BS 8104
• Calculate the wind suction loading in accordance with BS 6399:Part 2
• Determine relevant surface classification(s) in accordance with Approved Document B, ‘Fire Safety’.
• Identify defective areas of existing building and determine procedures for rectification.
• Determine any risks involved in specifying an insulated render system.
• Identify procedure for review and testing
• Establish durability requirements of the proposed render system, including installed lifetime expectation, maintenance requirements and reliability.
|C Outline proposals||• Consider performance issues in relation to decisions about types of render– including air-tightness, suitability of insulation and minimisation of cold bridging.
• Consider aesthetic issues including factors of texture and colour.
• Assess the environmental impact of proposed construction.
|D Detailed Proposals||• Select render system. Try to use local materials where appropriate.
• Identify requirement of additional consultants / design by specialists
• If using a proprietary system, ensure that it is BBA / BRE certified and suitable for the proposed application.
• Develop and apply detail design methodology for openings, penetrations, abutments, corners, and terminations.
• Confirm that the structure is adequate for the total weight of the render system as installed, and for the calculated wind loading and any other relevant loading information.
• Ensure compliance with Building Regulations, particularly:
- Approved Document E ‘Resistance to the passage of sound’.
- Approved Document C ‘Site preparation and resistance to moisture’.
- Approved Document F ‘Means of ventilation’.
- Approved Documents L1B or L2B ‘Conservation of fuel and power in existing dwellings / existing buildings other than dwellings’ as appropriate
• Ensure compliance with British Standards quoted in Approved Documents
• Determine wall element rating in accordance with the BRE Green Guide
• Ensure that environmental issues and targets are on the agendas of all design team and progress meetings.
|E Final Proposals||• Ensure co-ordination between the Design Team to ensure air tightness, prevention of cold bridging and minimisation of penetrations.|
|F Production Info||• Select sub-contractor if required for specialist work
• Careful specification of components, membranes and insulation
• Ensure that existing external fixings (eg rainwater goods, satellite dishes) are removed and re-attached to stand clear of the proposed render. New brackets should be fixed back to the substrate using a tube sleeve passing through the insulation.
• Emphasise responsibilities in specification for dealing with ‘loose ends’ between sub-contractor interfaces.
|G Tender Documentation||• Define Contractors’ responsibilities for coordinating work sequences|
|J Mobilisation||• Preparation of samples, training, testing and QA procedures|
|K-L Site Works||• Co-ordinate inspection with Building Control if required
• Ensure inspection of areas to be covered
• Ensure design changes do not compromise performance
|M Post Completion||• Carry out remedial work as required at end of DLP.|
Building Research Establishment (BRE)
• Thermal Insulation: Avoiding Risks, C.Stirling, BRE Press, 2001
• Installing Thermal Insulation, BRE Press, 2006
British Standards Institute (BSI)
• A list of applicable British Standards (.doc)
• Insulation materials compared
• The Insulated Render and Cladding Association (INCA)
• National Insulation Association
• British Urethane Foam Contractors Association
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