Housing Retrofit: Insulation: Timber, slate & tile cladding


• Wall cladding is usually applied as part of an overall refurbishment scheme.
• 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 is required compared with what is usually achievable with internal linings


Supporting framework

cladding support systems

The framework supports the cladding. A number of variations are possible, but the guiding principles are that the framework should:
• Be capable of carrying the cladding load.
• Be capable of resisting wind loading and movement. Bracing or racking may need to be added to the framework for greater thicknesses of insulation.
• Provide sufficient depth for the insulation required. Framework members should be best spaced to suit the width of the insulation material.
• Be optimised to reduce thermal bridging through the framework. This can be achieved through minimising the conduction route by cross-battening, bracketing or using narrow framework members of low conductivity.



Ideally, insulation should be fixed back to the existing wall using a complete coat of adhesive so as to avoid the possibility of air gaps occurring. Some designers express a preference, when using a frame system, for using insulation batts that are more flexible than rigid – so as to form a tight fit around the frame.
Alternatively, by adding a vapour-permeable sheathing board between the frame and the breather membrane, a kind of container is formed into which loose fill insulation can be poured (see image top right).


Typical U-values*


Insulation material Thickness of insulation
  100mm 150mm 200mm
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
Foamglas slab 0.34 n/a n/a
Wood fibre board insulation n/a n/a n/a
Cork n/a n/a n/a

* based on a notional 215mm thick solid brick wall (existing U-value: 2.2), insulation and render



Cladding materials


Tiles and slates

• Tile hanging using concrete or clay tiles, natural or fibre cement slates provides for a durable and low-maintenance cladding method.
• Tiles or slates are fixed to horizontal timber battens (min 38mm) which are themselves attached to the supporting framework.



• Timber is the most sustainable cladding material: renewable, recyclable and with low embodied energy.
• Timber cladding can offer a very wide variety of textures, colours and patterns.


Designing with timber cladding

• All timber cladding should be designed along with the presumption that water will penetrate the cladding. Hence consideration should be paid to detailing using a drained cavity backed up by a breather membrane along with carefully designed flashing around openings.
• In most circumstances, cladding is fixed to either horizontal or vertical timber battens (min 38mm). Horizontal boarding has a built-in cavity, but vertical and diagonal boards require the addition of further vertical battens (min 19mm) attached to the supporting framework.
• Water absorption and movement should be allowed for in selecting the cladding type. Systems dependent on interlocking elements (eg t&g), should be used with care to avoid distortion.
• At ground level, ensure that there is an adequate distance between the ground and the bottom of the cladding to avoid the ‘splash line’. The timber should terminate at least 150mm above where risk is minimal, but up to 250mm provides better protection.
• The staining and coating of timber can be avoided by using durable timber (see below)
• All untreated timbers go grey over time – ensure the client understands this.
• Generous eaves: where possible, provide generous eaves – 600mm is the optimum depth.
• Allow for drainage and ventilation. Water penetrating the cavity should be given ample opportunity to drain from the foot of the cavity or evaporate through provision of ventilation.
• Timber boards should be at least 150mm wide
• Battens should be fabricated from durable timber to avoid use of timber treatment.



Types of timber cladding

• Horizontal boards: Square Edge, Feather Edge, Rebated Feather Edge, Shiplap and Horizontal T&G profiles
• Vertical boards: Overlap or T&G profiles are most suitable
• Diagonal boards: Shiplap profile
• Shingles: Cedar or Oak

shingle cladding


A species natural durability is rated by BS EN 350. Naturally durable species suitable for untreated cladding are: • Western red cedar is a ‘durable’ timber from North America. It may be coated with oil to maintain its appearance, with the associated regular costs of re coating.
• European Larch is ‘moderately durable’. UK-grown larch, larch from Siberia or regions where trees are slow-growing and older than 60 years have a similar expected service life with a similar age and growth pattern
• European oak is a ‘durable’ hardwood. Oak is more expensive and needs to be specified carefully to minimise its tendency to warp

Cladding and the (old) 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, 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’.
• Identify defective areas of existing building and determine procedures for rectification.
• 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.
• 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.





External timber cladding, Paddy Hislop, TRADA, 2013

Building Research Establishment (BRE)

Thermal Insulation: Avoiding Risks, C.Stirling, BRE Press, 2001
Installing Thermal Insulation, BRE Press, 2006


Further information

• Timber cladding
• Timber preservation
• Insulation materials compared
• Insulated Render & Cladding Association
• National Insulation Association
• British Urethane Foam Contractors Association



External insulation products on GreenSpec





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