In this guide we take a comprehensive look at ETICS (External Thermal Insulation Composite System) that are often described as external wall insulation, or EWI.
As well as explaining what it is and the many benefits it can bring to a dwelling, we will look at the different types of insulation that can be used as part of an EWI system. We will explore the options available when insulating an external wall, whether planning permission is likely to be required and what grants are available if you’re considering insulating your property using external wall insulation.
What is external wall insulation?
External wall insulation, as the name suggests, refers to an insulating material that is installed on the outside of the external walls of a property. This differs from cavity wall insulation, where the cavity between the internal and external skins of a wall are insulated.
EWI is commonly installed on solid wall properties. The walls are either made of solid brick with a thickness of around 215mm or solid stone that can vary greatly in thickness. This compares to cavity walls with an inner and outer layer. Or blocks or bricks with an air space in between where the thickness is between 270mm to 300mm. The insulation is first fixed to the outer face of the wall with adhesive and mechanical fixings. Then a protective coating is applied to the insulation usually in the form of a render to provide the insulation with a barrier against the ingress of moisture.
EWI adds extra thickness to the existing wall, so door and window reveals generally increase in depth and items fixed to the existing wall, such as downpipes, will need to be reconnected with appropriate fixings. As EWI alters the external appearance of a property, this may require planning permission to be sought.
According to a report by INCA ‘Why Fabric First is Right’[1] just 5% of England’s 8.9m pre-1945 dwellings with solid walls have insulation, 17% have cavity wall insulation and 14% have cavity walls without insulation. This shows that there are many properties that could benefit from installing external wall insulation.
Which insulation type should you choose for external walls?
There are many types of insulation that can be used for EWI, and which ones are used depend on the priorities of the project.
The following factors are influenced by the choice of insulation for an EWI project:
- Diffusion open or diffusion closed system
- Final thickness of the EWI system
- Thermal performance
- The project’s sustainability goals
For most EWI systems the focus is on U-value performance, as this is what needs to be achieved in order to comply with Building Regulations, but there are many more factors which play a critical role such as: cost, diffusion open construction, internal temperatures and overheating, thickness and an effective solution should address all these issues.
Diffusion open or diffusion closed insulation for EWI?
Whenever the fabric of a building is insulated, it has the potential to affect the existing system of moisture management within the property. The control of moisture is important as, without it, internal condensation and damp can appear. Also trapped moisture within the structure, known as interstitial condensation, can cause long term damage to the building fabric.
Both diffusion open and diffusion closed insulation can be used for EWI. Strategies for coping with moisture control are listed in the standard BS 5250 ‘Management of moisture in buildings.’ In some older and heritage solid wall properties, the walls are constructed of materials that allow the wall to breathe. In such cases diffusion open insulation may be preferred by EWI specifiers as it most closely matches the existing behaviour of the building fabric.
Diffusion open EWI insulation types:
- Mineral Wool
- Wood Fibre boards
Diffusion closed insulation types:
- EPS (Expanded polystyrene)
- XPS (Extruded polystyrene)
- PIR (Polyisocyanurate)
- Phenolic
Insulation thickness of EWI system
The thermal performance of each insulation type will vary. The lower the thermal conductivity, or lambda value, the better the insulation will be at keeping the building warm during the winter.
Here are examples of typical lambda values for some different insulation types used in EWI. Individual lambda values should always be verified with the manufacturer.
Insulation type used in EWI | Typical lambda values (W/mK)[2] |
PIR | 0.020 – 0.027 |
XPS | 0.029 – 0.036 |
EPS | 0.030 – 0.045 |
Mineral wool | 0.032 – 0.044 |
Wood fibre | 0.037 – 0.043 |
Therefore, thinner EWI solutions are possible when using insulation with lower lambda values to achieve the required thermal performance.
Insulation that is diffusion closed can present the thinnest options for EWI. However, where breathability is paramount, a slightly thicker solution may be a more suitable option for the long-term performance of the building.
Projects with a focus on sustainability
Where a project has a focus on sustainability the contribution of products to embodied and operational carbon can come under scrutiny. In some cases, the source of the materials can also feature as a potential selection criterion. Although insulation can help to contribute to the energy efficiency of a building, the energy used in its manufacture means that it comes with its own embodied carbon footprint. This can be measured by Environmental Product Declarations (EPDs) and these EPDs can be used as a way to specify one EWI insulation solution over another.
Wood fibre insulation from STEICO is made from more than 80% natural wood sourced from PEFC certified forestry and the associated product EPD's reflect the bound carbon dioxide equivalent content and also detail the amount of biogenic carbon that is stored within the products. For example, STEICOprotect L dry binds 160kg CO2eq[3] and can thus contribute to climate protection.
All year-round performance of external wall insulation
External wall insulation can help to keep the heat in the building during the winter and help to keep the heat out during the summer. Insulation can also affect the flow of moisture within external walls by either acting as a barrier to moisture flow or allowing moisture to pass through.
Wood fibre insulation allows moisture to pass through and can be used as part of a breathable EWI system.
This can help ensure the long-term moisture management of solid walls is sustained for the lifetime of the building. Using wood fibre insulation also adds thermal mass to the walls, helping them to keep the building cooler in the summer.
Whatever insulation sits within EWI, it should always form part of a tested system. This ensures that all products in the system – insulation, adhesives, fixings and external weather proofing – are compatible and that the EWI system should deliver its designed performance.
What are the benefits of external wall insulation?
Installing external wall insulation brings a number of benefits.
EWI can help to lower energy bills
Installing EWI can help improve the U-value of the walls of buildings, which in turn can help save heating energy and can lead to reduced heating costs. The cost savings vary depending on the house type. According to the Energy Savings Trust[4] it is possible to save between £160 (mid-floor flat) and £550 (detached house) per year
EWI can help to improve energy efficiency
EWI can help to improve the energy efficiency of a property. As well as helping to lower fuel bills, it can also assist with reducing the dwelling’s carbon emissions when assessed using SAP[5] for new buildings or RdSAP[6] for existing buildings. This will improve the Energy Performance Certificate (EPC) of the property, which in turn can help to increase its value. A study carried out by Rightmove[7] of 300,000 properties shows that an improved EPC rating brings about an increase in value of between 3% and 15%. The amount depends on the scale of the improvement in EPC value.
EWI can help to improve thermal comfort
As external wall insulation is effectively wrapping the walls in an insulated blanket, the whole interior of the dwelling – including the walls themselves – can be kept warm. Therefore, areas of thermal bridging in the fabric where cold spots and condensation and mould can appear are less likely to occur. This can help to improve the thermal comfort for the residents and contributes to a healthier indoor environment.
EWI can help to enhance property aesthetics and durability
Fitting EWI results in the outside of the property getting a new, refreshed appearance. This aesthetic enhancement can add value to the property. The new EWI system also helps to protect the original wall surface from external weathering.
EWI can improve sound insulation
EWI can help to improve the soundproofing of walls to reduce noise pollution within the property. The level will depend on the density and type of insulation used.
What are the different ways in which an external wall can be insulated?
The most appropriate way to insulate an external wall will depend on the type of wall in the property.
If it is a cavity wall, then cavity wall insulation is likely to be the most appropriate solution. However, for solid walls, the choice comes down to either internal wall insulation (IWI) or EWI.
IWI adds insulation to the inner surface of the external walls. This results in a reduction in internal rooms space and can be a disruptive process. Furniture needs to be moved and internal wall fixtures, including skirting boards, need to be removed and later replaced. It is an option for helping to improve the energy efficiency of the property and is a good alternative where EWI is not viable.
EWI provides the least disruptive way to insulate an external wall because all the work takes place outside the property. However, depending on the age and type of property and local regulations, in some instances, EWI may require planning permission before it can be installed.
What is a good approach when carrying out an EWI installation?
As with any insulation measure, the key to a successful EWI project is to first get a professional survey carried out to check a property’s suitability for the measure. Then use a tested EWI system that is fitted by qualified and experienced installers to ensure that the work follows best practice guidance.
The key parts in the EWI process are as follows.
1. Assess the property for EWI suitability
This should include checking the type of walls on the property to ensure there are no existing issues such as damp or damage that need to be rectified before work can begin. If the solid wall has an existing render, this should be assessed to make sure it is of a type that is appropriate to leave in place. If not, it will need to be removed or repaired.
2. Preparing the property for EWI installation
External fittings such as downpipes, wall lighting, pipework and meter boxes should be removed or adjusted. Doors and windows should be protected from dirt and damage from the installation process. Any repair work identified in the survey should be carried out to make sure the walls are in a good condition before insulation is applied.
3. Fitting the insulation in an EWI system
The insulation will be fitted to the wall, usually with adhesive and mechanical fixings. The precise method will depend on the EWI system that is being used. The installers should be familiar with the system and always follow the system providers guidelines.
4. Applying the external protective finish in an EWI system
Render is commonly used as the protective finish for EWI. A basecoat is applied, followed by a reinforcement mesh and another basecoat layer is added over the mesh. This provides a stable surface for the final render finish. A primer may be required to enhance adhesion before the final render is added. The type of render will depend on many factors including whether the system needs to be breathable, as well as the colour and decorative finish required by the resident.
The exact process of building up the external protective finish will depend on the EWI system.
While render is the prevalent choice for the external finish, there are EWI systems where the render is replaced by cladding or façade panels.
5. Replacing external fittings after installing EWI
When the external finish is ready, any external fixtures that were removed before the work commenced should be replaced. EWI fixings are available and can be used for lightweight connections such as lights or downpipes but where heavier items are required the fixings may need to penetrate through to the original wall.
6. Inspection and maintenance guidance for EWI
The installer should carry out a final inspection to check the quality of the work. In some cases, especially for government funded schemes, this may be carried out by an independent third party to audit the competency of installers who are part of a quality assurance system. Information should be passed to the resident on best practice cleaning and maintenance of the EWI system.
Is planning permission required for external wall insulation?
As installing external wall insulation does not count as an extension or enlargement of a property it is classed as permitted development. Therefore, the vast majority of EWI installations do not require planning permission. There are a few guidelines to follow, including that the external appearance of the new EWI should closely resemble the original wall and EWI should not cross the boundary line with a neighbour’s property. If the EWI is on a wall close to a neighbouring property, the insulation must meet fire regulations designed to prevent fires spreading between buildings.
There are some instances where planning permission is required, and these are based upon the type or location of the property. Permission will be required if it is a listed building, or is in a conservation area, National Park or area of Outstanding Natural Beauty.
If you’re unsure if you do need permission, always contact your local planning authority for their advice and guidance.
What grants are available for external wall insulation?
There are two government schemes where grants for external wall insulation are available – the Great British Insulation Scheme (GBIS) and the fourth iteration of the Energy Company Obligation (ECO4). The schemes are set to run until April 2026.
Both schemes have eligibility criteria and offer different insulation measures including EWI. To be able to fit EWI measures under GBIS or ECO4, installers must be registered with the Trustmark government endorsed quality scheme. Installers must also follow retrofitting standards PAS 2030 and PAS 2035 and their work can be inspected for its quality. Failure to meet the required standards can result in installers no longer being eligible to carry out work under the schemes.
Where can I find out more information about wood fibre insulation solutions for external wall insulation?
STEICO makes wood fibre insulation made from more than 80% natural wood sourced from PEFC certified forestry. STEICOprotect L dry is a render carrying, diffusion open insulation board ideal for external wall insulation. It can be combined with a number of render systems to deliver an EWI solution that can help to support the energy efficiency of buildings.
To enquire about using our external wall insulation on your next project, please contact our expert team who will be happy to help.
[1]www.inca-ltd.org.uk/why-fabric-first-is-right/
[2] Values stated in the table are typical and may vary from manufacturer to manufacturer – always check the declared product performance with the manufacturer. Values can vary due to different densities, formulations, and manufacturing techniques.
[3] Storage refers to CO2 equivalent in life cycle phase A1 according to EN 15804
[4]energysavingtrust.org.uk/advice/solid-wall-insulation/
[5]bregroup.com/expertise/energy/sap
[6]files.bregroup.com/SAP/RdSAP10-dt13.02.2024.pdf
[7]www.rightmove.co.uk/news/articles/property-news/green-premium-epc-ratings/