Solid wall retrofit insulation:
• Though not precluding low-rise, Rainscreen cladding is usually retro-fitted to buildings over two storeys as part of an overall refurbishment.
• Where external walls are poorly insulated.
• Where external walls are deteriorating or are insufficiently weather-tight, causing damp, draughts and heat loss.
• Where wall cavities are bridged or blocked, making them unsuitable for cavity fill insulation.
• Installing internal lining insulation would be disruptive, would alter critical internal dimensions or make room sizes too small.
• Greater thickness of insulation and reduced thermal bridging is required compared with what is usually achievable with internal linings.
The Rainscreen principle
Rainscreen systems are comprised of the following key elements:
1 The outer skin or panel (the ‘Rainscreen’). The panel forms the primary rain barrier but does not prevent the passage of air through open joints between the paneling components. The degree of accepted water penetration through the joints defines the kind of rainscreen system. The are two distinctive systems which should not be confused or combined (see below)
2 An air gap that prevents water crossing the gap and penetrating the insulation / backing wall. The air gap provides ventilation and, depending on the design, may provide pressure equalisation across the outer skin. Any water penetrating the cavity is drained away.
3 A backing wall that forms an effective air barrier and frequently includes an insulating layer.
Drained and ventilated
|This system allows water to freely penetrate the joints between panels in adverse conditions such as driving rain, snow or sleet. Water that reaches the cavity is evaporated or drained-off.
Permanent ventilation gaps are required above and below the cladding as well as around window openings and non-perforated cavity barriers.
|This system functions through the nature of the joints between the panels. Air pressure acting on the face of the panel is balanced by the pressure created at the joint. This equalisation provides an ‘air cushion’ within the air gap that allows the passage of air but not of water through the joints. Any residual water that should penetrate the skin is drained-off within the cladding system.
Compartments control the pressure equalization process. Continuous vertical and horizontal separations must be designed into the system to prevent air from flowing from one compartment to another (see 'Pressure Equalization' under 'Downloads' below).
Rainscreen cladding construction & materials
There is a wide range of construction possibilities with rainscreen cladding. At one end of the spectrum lies the simple combination of timber battens and boards typically designed by the architect; At the other end there is the bespoke stone or metal panel systems designed in conjunction with a specialist contractor. The range can include:
• board / panels on battens
• board / panels on a proprietary support system
• a total proprietary panel and support system
• bespoke panels on a proprietary / standard support system; and
• complete bespoke rainscreen cladding
Each support system is designed to carry both the mass of the rainscreen along with the projected wind loading. Support structures vary in their format and complexity according to the nature of the cladding material, drainage, and rainscreen system. Most support systems are proprietary and are often designed together with a cladding system from the same manufacturer.
The support system will usually comprise of vertical or vertical and horizontal fixing rails fixed back to the structure using brackets. Additional brackets may be used to fix the cladding to the supporting rails.
The design of the support system will also effect the appearance of the cladding - for example where the selection of a support rail will define the appearance of the joints between panels and where the method of fixing panels can be either visible or invisible on the facade.
• Timber (see also 'Timber, tile and slate cladding') is likely to be the more green option. Timber cladding is increasingly being designed on rainscreen principles to allow for a ventilated and drained cavity. A number of architects have recently developed facade designs where large joints (even gaps) between timber boards become features (see example below). When specifying materials in the latter format, ensure that the exposed breather membrane is UV -resistant.
• Laminate board
• Fibre cement
• Metal / ACM
• Recycled glass
Examples of cladding material and support systems
• Note that there is no allowance for the effect of the cladding itself because the space behind is fully ventilated.
• Note too that the overall effectiveness of the insulation can vary according to the degree of thermal bridging provided by the method of fixing. Further information on u-value calculation methodology can by consultling 'Conventions for U-value calculations', Brian Anderson, BRE Scotland
|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||n/a||n/a|
|Phenolic foam slab||0.20||0.14||0.11|
|Wood fibre board insulation||n/a||n/a||n/a|
Rainscreen cladding 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 systems (eg susceptibility to damage, wind loading, sea water 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’.
• Determine procedures for rectification of existing damaged fabric.
• Determine any risks involved in specifying an external cladding system.
• Identify procedure for review and testing
• Establish durability requirements of the proposed cladding system, including installed lifetime expectation, maintenance requirements and reliability.
|C Outline proposals||• Consider performance issues in relation to decisions about type of cladding system – including air-tightness, suitability of insulation, minimisation of cold bridging and cavity drainage.
• Consider aesthetic issues including factors of shape, size, colour, texture, material.
• Assess the environmental impact of proposed construction.
• Consider preference for locally obtained cladding materials
• Select a fixing method in accordance with the aesthetic, sustainability and performance criteria.
• Consider designing for deconstruction.
|D Detailed Proposals||• Select cladding system.
• Identify requirement of additional consultants / design by specialists
• Determine design of cladding and configuration of support rails.
• Develop and apply detail design methodology for openings, penetrations, abutments, corners, terminations, ventilation and compartmentalisation (the latter if using pressure equalisation)
• Confirm that the structure is adequate for the total weight of the cladding as installed, and for the calculated wind loading and any other relevant loading information. The support system should be designed to meet BS 6399-2
• 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 drainage, air gap integrity, 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
• 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.|
• Pressure Equalization in Rainscreen Wall Systems, Institute for Research in Construction, Canada, 1998
• Thermal Improvement of Existing Dwellings, Clarke et al, University of Strathclyde, 2005
Centre for Window and Cladding Technology
• Standard for systemised building envelopes, CWCT 2006
• NHBC Standards 2008. Chapter 6.9 - Curtain walling and cladding, NHBC 2008
• External timber cladding, Paddy Hislop, TRADA, 2007
• Thermal Insulation: Avoiding Risks, C.Stirling, BRE Press, 2001
Building Research Establishment (BRE)
• Installing Thermal Insulation, BRE Press, 2006
British Standards Institute (BSI)• British Standards associated with insulation (.doc)
• British Standards associated with wall cladding (.doc)
• Durability - Timber cladding
• Durability - Ceramic wall cladding
• Timber preservation
• Insulation materials compared
• Insulated Render & Cladding Association
• National Insulation Association
• British Urethane Foam Contractors Association
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