The Knowledge Hub From INCA - EWI FAQs

Are you considering the use of external wall insulation whether this be in a retrofit thermal upgrade of an existing property or incorporating EWI into a new build specification to achieve or surpass current regulations?

It’s always going to generate questions and, for that reason we’ve listed the questions we’re asked the most below and included links to further information as appropriate.

This is an evolving list – more will be added as they come in and so, if your question isn’t featured, please submit it to the experts at INCA using the form below and we will answer it directly and publish for the benefit of others.

All your EWI questions answered:

- What Is External Wall Insulation (EWI)?

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 has been recognised throughout Europe since the 1960s. It is a composite system made up of three basic components:

  1. An insulant – typically expanded polystyrene or mineral fibre of varying thickness
  2. A fixing to secure the insulation to the existing wall – size and type project specific
  3. A finish – typically a render, brick / stone effect finish or a 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.

Further information: Introduction To EWI

- Why Should I Choose EWI?

External wall insulation is one of the most effective ways of improving the energy efficiency of your property and 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. Heating costs will reduce dramatically, EPC will improve and there may also be in an increase in the value of the property.

Further information: Homeowners Guide To EWI

- How Much Will I Save On My Heating Bills By Installing EWI?

The Energy Savings Trust estimate that in a poorly insulated house, up to 1/3 of the heat generated internally can be lost through the walls and roof. Therefore, by insulating the loft and wrapping the walls externally with insulation heating costs can be reduced by around 40%.

Further information: The Benefits Of EWI

- What Is A U-value?

A U-value is the measure of heat loss through the walls, floor or roof of a building. In the case of EWI and represented in watts per square metre, per degree Kelvin (W/m2K) it’s the sum of the thermal resistances of the layers that make up the walls after taking into account any gaps etc.

In simple terms this means that the lower the U-value, the greater the resistance to heat loss and the less it will cost to heat the building.

To give you an example, a solid brick wall will typically have a U-value of 2.1W/m²K and installing EWI can reduce this as follows (approximate OFGEM numbers only):

  • 50mm insulation – from 2.1 to 0.6 W/m2K
  • 100mm insulation – from 2.1 to 0.35 W/m2K
  • 150mm insulation – from 2.1 to 0.25 W/m2K
  • 200mm insulation – from 2.1 to 0.18 W/m2K

More information: EWI – Specifying Insulation

- Is EWI Suitable For My Property?

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.

A simple visual check will often suggest whether walls are solid or cavity – if all the bricks are laid lengthways there’s likely to be a cavity. But if some are laid lengthways and some end to end this usually means solid wall.

You can also 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.

Further information: The Suitability Of EWI

- Do I Need Planning Permission For EWI?

In the majority of cases planning permission is not required. However, you should always check with your local authority planning department before starting works.

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 within 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

Further information: The Suitability Of EWI

- How Long Does EWI Work Take?

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 just a couple of weeks. This includes all preparatory works such as extending flues and overflow pipes etc. and reinstatement of the rain water pipes along with installation of the system, application of materials, the finish and all drying times.

Further information: EWI Installation Guide

- Will I Have To Leave My Home Whilst The EWI Work Is Carried Out?

The short answer is no – there’s no reason why you will need to leave you home when fitting external wall insulation.

EWI is applied to the outside walls so 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 only require good access to the walls from the outside in order to carry out the work.

Further information: EWI Installation Guide

- What Is The Best Type Of Insulation?

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 but there are two main 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 Wool)
    Offers good insulation performance at a mid-range cost and is the best choice where fire resistance is top of the list of requirements.

Other types of insulant may be specified and ‘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.

More information: EWI – Specifying Insulation

- Is There A Minimum Thickness Of Insulation That Should Be Used?

The thickness of insulation may be dictated by the Building Regulations, which require a particular thermal performance through the wall, and your own requirements and targets. Most commonly 100mm insulation boards will be installed and it is unusual to find a system specified much below this thickness.

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.

More information: Specifying EWI Insulation

- What Is The Best Way Of Securing The EWI Sytem To The Wall?

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.

These days the vast majority of systems incorporate a mechanical fixing with the exact type and length specified according to the insulation type and thickness, the property and the existing substrate it will be fixed into. Checks are carried out by way of a “pull out test”.

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

More information: EWI Installation Guide

- What Are The Different Types Of EWI Finish?

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

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

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.

Note: changes to the final appearance of the property may require planning permission.

Further information: EWI Finishes

- Do I Have To Pay VAT On EWI Work?

In the Chancellors spring statement on 2022 it was announced that the installation of energy efficiency measures would be zero vat rated for a period of 5 years. Therefore VAT is not currently applicable to EWI installation.

More information: Homeowners Guide To EWI

- How Do I Know That The Chosen System Is Fit For Purpose?

A client should check that the system they have selected can demonstrate that it has achieved recognised accreditation such as a BBA (British Board of Agrement), Kiwa BDA certificate or UKTAs (United Kingdom Technical Assessment) certificate with accompanying LABC (Local Authority Building Certificate) approval. To obtain these approvals, a system has been rigorously tested and has passed the recognised testing outlined in ETAG004 or EAD 040083-00-0404 and relevant fire classification tests.

The systems of all INCA System Holder members conform to all current regulatory requirements.

More information: Suitability Of EWI

- How Much Does EWI Cost?

As a very general guide only, for a standard EWI system comprising of a 90mm EPS insulation board fixed to a masonry substrate and finished with a lightly textured silicone render the cost will likely be somewhere between £65 and £95 per square meter. Thicker insulation will increase material costs and different finishes will also increase the overall cost.

So, purely for general guidance, you’re looking at anything from around £4,000 – £6,000 for a small terraced property, £8,000 – £10,000 for an average semi and up to £12,000 – £14,000+ for larger detached homes.

Note: with rising prices and supply issues of 2022 these estimates may well be subject to increase.

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

  • Insulation thickness and type
  • Size of the wall area
  • Type of finish i.e., render, wet / dry dash, brick-slips, brick-effect
  • Number of windows and doors
  • Below DPC requirements
  • Will the existing roofline need to be extended?
  • Flue extensions – Gas Safety Certificate or CP12 (Landlords)
  • Condition of the exterior surface
  • Ease of installation access and any scaffolding requirements.
  • Single or multiple dwellings

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.

Whether you require a quick estimate, or an in-depth quotation please contact an INCA member located in your area, to find a member click here.

- Can I Get A Grant Or A Loan For Installing EWI?

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 Simple Energy Advice, the Energy Saving Trust or Trustmark.

Your INCA member contractor will be able to advise you in general terms on any current grants that may be available.

- How Can I Protect Against Defects Or Problems Arising With The EWI Work?

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.

Typically all INCA member EWI systems are guaranteed for a minimum of 25 years but INCA also work with insurance administrator SWIGA 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.

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.

- Does EWI Conform To UK Fire Regulations

The classification system defined in BS EN 13501-1 is the primary recognised standard which applies to external wall insulations systems. All INCA member EWI systems conform to the stringent UK fire regulations.

To understand more about the regulations and how the different EWI systems conform read more at: EWI And UK Fire Regulations

- Can EWI Be Used In New Build Or Just Refurbishment?

EWI can be, and frequently is incorporated into new build as well as the refurbishment of existing properties. It’s widely incorporated into a new build specification in order to help achieve or surpass energy efficiency regulations and targets.

Note a main difference – if a residential building is to be constructed from timber or steel frame it will generally be required (by the NHBC) to incorporate a drained cavity within the EWI system.

- Can I Install EWI Myself

We would never recommend you install EWI yourself – even if you consider yourself a DIYer

An EWI system needs to be professionally designed and specified and installed by system manufacturer trained and carded installers. Guarantees will be voided if installed by non-trained personnel In addition, grants will only be issued if a PAS2030 accredited / Trustmark registered installer carries out the work.

Find An INCA Member

- Why Should I Use An INCA Member?

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 are able to offer advice on insurance administrators who can offer insurance backed guarantees for EWI works which protect against any defects or workmanship arising after the works have been completed for up to 10 or 25 years.

Ask A Question

If you have any EWI related question which has not been covered then please feel free to ask the INCA team of technical external wall insulation experts. Simply fill out the form below and submit. We aim to respond within one business day and will publish in our list of Q&As above.

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