FAQs

External wall insulation (EWI) is the application of an insulation layer to the outside wall of a building in order to improve the building’s energy efficiency. By minimising heat loss in cold weather and heat gain in warm weather, EWI creates a much more comfortable living environment and reduces heating costs.

EWI is a composite system made up of three basic components: an insulant, a fixing, and a render or cladding. The insulant is secured to the wall using either special adhesives or purpose-made fixing screws/dowels. The final surface, which provides both the aesthetic and weatherproofing functions of the wall, is usually a coloured render but could also be a brick effect or other specialist finish. A range of EWI systems are available allowing the building owner to prioritise between cost and technical performance.

EWI has a number of benefits over other more traditional means of insulating buildings such as cavity fill or insulated dry lining. As EWI is applied to the outside of the wall, there is minimal disruption to the occupants of the building and virtually no limit to the thickness of insulation that can be applied. The combination of insulant and protective finish ensures that the entire thickness of the supporting wall remains dry and warm, avoiding structural damage and minimising the risks of condensation or mould on the walls.

The result is a very warm environment for the winter months whilst the building is kept cool in the summer. There may also be in an increase in the value of the property.

Oakwood Estate (Before) Oakwood Estate (After) Halewood Estate (Before) Halewood Estate (After)
Oakwood Estate (Before) Oakwood Estate (After) Halewood Estate (Before) Halewood Estate (After)

In a poorly insulated property, it is estimated that up to one-third of the heat produced by a central heating system is lost through the walls and the roof. The Energy Saving Trust estimates that heating costs can be reduced by up to 40% by installing EWI to solid walls.

The U-value is a measure of heat loss through the walls, floor or roof of a building. The lower the U-value, the greater the resistance to heat loss and the less it will cost to heat the building. A solid brick wall will typically have a U-value of 2.0W/m²K and installing EWI can reduce this to 0.35W/m²K or better which could cut heating costs by up to 40%.

EWI should be installed by a competent Specialist Contractor trained by an approved system designer. The Insulated Render and Cladding Association (INCA) is the recognised trade association for the EWI industry, representing the major system designers, a nationwide network of specialist installers and the key component suppliers. Promoting the best in design solutions and quality of application, INCA members are required to operate in accordance with the INCA Code of Professional Practice, ensuring customer satisfaction and high quality standards.

By choosing an INCA member, you can be confident that you are engaging with a reputable Specialist Contractor. However, in the unlikely event that anything does go wrong, there is protection in the form of the INCA Complaints Procedure. INCA also works with insurance administrator Quality Assured National Warranties (QANW) to provide access to insurance-backed guarantees for EWI works, which protect against any defects in systems or workmanship arising after the works have been completed for up to 25 years.

EWI is normally used to insulate solid wall properties, both brick and concrete, but it can also be used to further improve the performance of cavity walls, and non-traditional properties. Solid walls are found in most houses built before the 1920s so, if your property dates back to Edwardian, Victorian or Georgian times, its walls are likely to be solid.

You can check the wall type by either consulting your deeds, or manually drilling a test hole in the wall. Another test would be to measure the wall thickness at an opening. Solid walls measure in the region of 225mm thick. The majority of terraced housing in the UK are of a solid wall construction.

If you live in a mid-terraced property, it is possible to install EWI even if your neighbours on either side choose not to by providing a ‘stop end’ at the edge of your boundary.

If there isn’t enough room under the sills and roof overhang on your property, overcills and roof profiles can be used or it may be possible to extend the roof.

If your house is of a non-traditional construction, it is highly likely to be suitable for EWI. There are many different types of non-traditional houses and it will be a case of selecting the most appropriate fixing solution for attaching the EWI system. The house wall construction must be able to support the weight of the EWI system and transfer the weight back to structurally supporting parts of the construction. It is advisable to consult someone experienced in this type of construction and, if in doubt, seek professional advice from an experienced Structural Engineer or Building Surveyor.

As EWI covers the whole of the outside of your property, it is likely to change its appearance and you should ask your local authority planning department whether planning permission is required to install EWI. Planning permission is not required and is acceptable under permitted development if the final wall covering reflects that of the existing building. i.e. if the existing is a dry dash finish, the new finish should be dry dash. If the new finish varies from that on the existing building, then it is likely planning permission is required. Planning permission is generally required if the property is withing a conservation area or a designated area such as the green belt or countryside.

In a local conservation area, the decision will depend upon the planning constraints of the particular conservation area. Your local authority planning department will be able to advise on any special requirements, particularly relating to the necessary appearance of the finished works. There are a range of finishes which can be used to emulate the surrounding properties and samples can be provided to satisfy local authorities.

Further information on planning permission can be found on the Planning Portal website at www.planningportal.gov.uk.

The length of time taken to install EWI will depend on the size of the property but, as a guide, a typical 3-bedroom semi-detached house will take approximately 3 to 4 weeks. This will include preparatory works such as extending flues and overflow pipes etc. and reinstatement of the rain water pipes.

As EWI is applied to the outside walls, there is minimal disruption and it should not be necessary to vacate the property whilst EWI is installed. Although it can be a little noisy due, for example, to drilling of the walls, there should be little or no disturbance to the inside of the house and no need for redecoration. The Specialist Contractor will require good access to the walls from the outside in order to carry out the work.

Your choice of insulation should be based upon your budget and any particular requirements you may have; there is no single ‘best’ type of insulation. There are various insulation types:

  • Expanded Polystyrene (EPS)
    One of the most cost effective solutions to give good insulation performance, combined with the benefit of being lightweight and easy to work with.
  • Mineral Fibre (Rock Fibre)
    Offers good insulation performance at a mid-range cost and is the best choice where fire resistance is top of the list of requirements.
  • Polyisocyanurate (PIR)
    Generally a mid-range price board which provides an improved thermal performance compared to Mineral Wool and EPS, however it has a lower thermal value than phenolic.
  • Phenolic
    Normally comes at a higher cost than EPS or Mineral Fibre but gives a very high insulation performance, typically requiring in the region of 60% thickness of comparable insulations.

Other ‘Green’ insulation boards are available such as Cork, and Wood Fibre.

Your Specialist Contractor will be able to advise on the most suitable type of insulation for your property.

The thickness of insulation may be dictated by the Building Regulations, which require a particular thermal performance through the wall. Your Specialist Contractor will be able to calculate in conjunction with the system designer a thickness based on the existing U-value of the wall construction and the required U-value of the completed installation. In the absence of Building Regulation requirements, any minimum thickness will be dictated by practical considerations. Very thin boards will be difficult to handle, particularly when a large format board, and boards will rarely be used at less than 20mm thickness.

A client should check that the system that they have chosen can demonstrate that it has achieved recognised accreditation such as a BBA (British Board of Agrement) certificate, has an ETA (European Technical Approval) certificate and has accompanying LABC (Local Authority Building Certificate) approval. To obtain these approvals, a system has been rigorously tested and has passed the recognised European test ETAG004 and other relevant fire classification tests.

EWI systems can be bonded or mechanically fixed, or a combination of both, to the walls using either special adhesives or purpose-made fixing screws/dowels. Generally, if the existing wall is painted or the surface is in a poor condition, the system will be mechanically fixed. It is possible to insulate over existing render with a new system depending on the condition of the existing render and whether it is sound and well adhered to the wall. The condition of the existing render can be determined by the Specialist Contractor through hammer testing. If the render falls off, it is recommended the existing render is removed entirely.

Most walls are suitable for EWI and your Specialist Contractor will be able to advise accordingly after doing a site survey.

Adhesive Fixing Mechanical Fixing - Rail Supported
Mechanical or Adhesive fixing Mechanical fixing – Rail Supported

EWI can transform the appearance of a property and there is an extensive range of different product, textured and coloured finishes to choose from.

Mineral renders can be finished with dry dash aggregate or the final surface can be scraped back to leave a textured finish
Synthetic finishes come in silicone, acrylic or silicate plasters in a variety of colours and textures
Brick slips or brick effect render offers a more natural finish.

Halewood Estate (After) Private House Private House

The finish is a personal choice taking into account the desired texture and colour. Consideration should be given to the durability and toughness of the surface needed and this will often be related to the type of building, how it will be used and the likelihood of impact/physical damage being made on the surface. Your Specialist Contractor will be able to advise on the best choice of finish for your property. Changes to the final appearance of the property may require planning permission.

Variations in both building and insulation type make it difficult to provide an estimated cost for professionally installed EWI. As a guide, it will be in the region of £70 – £90 per square metre for detached and semi-detached housing, giving a total cost of £5,600 – £7,200 for an average semi-detached house of 80 square metres. For larger properties where economies of scale are possible, the cost per square metre is likely to be less.

The final cost will be influenced by a number of factors including:

  • Insulation thickness and type
  • Size of the wall area
  • Number of windows and doors
  • Condition of the exterior surface
  • Ease of installation access and any scaffolding requirements.

Payment terms should be agreed before a contract is signed with the Specialist Contractor which may include a deposit for a substantial part of the works.

Financial assistance in the form of grants or interest-free loans is available from the Government, Local Authorities and energy companies for installing EWI. Individual schemes and eligibility vary according to regional area and specific details can be obtained by visiting the Energy Saving Trust.

VAT is payable on repair, maintenance and improvement works. A reduced rate of VAT (currently 5%) is applicable to EWI installation.

INCA members are required to operate to the highest quality standards in accordance with the INCA Code of Professional Practice. In the event of any problems with their work, they will be required to investigate and take any appropriate remedial action, and INCA offers a Complaints Procedure in the unlikely event that problems cannot be resolved.

INCA also works with insurance administrator Quality Assured National Warranties (QANW) to provide access to insurance-backed guarantees for EWI works, which protect against any defects in systems or workmanship arising after the works have been completed for up to 25 years.

The Office of Fair Trading (OFT) has published guidance offering tips to homeowners who are having improvement works on their property including advice on budgeting, finding a competent supplier and what to do if things go wrong – OFT Guidance.

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Worcester High Rise, Worcester - Domestic Refurbishment - High Rise: Winner

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