Aug. 25, 2025
Construction & Real Estate
External wall insulation (EWI) refers to a layer of insulation fixed to the outside face of an existing wall. The insulation can be finished with render or clad with an alternate facing material to protect it from the weather. It is commonly installed to thermally upgrade solid masonry walls, but it can also be used to improve cavity walls.
Goto Langfang Clear Chemical to know more.
While adding EWI to existing homes can help reduce our carbon emissions, it’s not without its challenges – chiefly, the build-up of condensation which can lead to damp and mould.
Here, we outline the main considerations for external wall insulation which we hope will help you plan your project.
We are in a climate emergency and huge amounts of carbon dioxide are emitted into the atmosphere by heating our homes. Heating our housing stock is responsible for 14% of annual national emissions. Older properties are by far the worst polluters.
Over a third of a building’s heat is lost through the walls. EWI is the most effective insulation method of retaining this heat.
Aside from the environmental benefits, EWI, as part of a wider retrofit, can help:
s house remodel by Dow Jones Architects. Above: finished building with external wall insulation and standing seam zinc cladding; Below: existing building.
What are the disadvantages of external wall insulation?
EWI will change the appearance of a building. If the building is of low architectural quality, it offers the chance to improve its looks. However, if the building is made up of good quality materials and detailing, it’s likely to have a detrimental impact.
For this reason, EWI is generally not accepted on the principal elevations of homes in conversation areas. It’s also unlikely to be accepted on listed buildings.
Other potential downsides of EWI include:
EWI is mainly suited to older properties, particularly those built before with solid external walls.
It’s also suitable for houses built between the s and s which feature uninsulated cavity walls, when combined with cavity insulation and good levels of moisture protection and ventilation.
EWI is not suitable for modern homes with partial fill cavity insulation – a common feature in properties from the late s, but only compulsory in the s. Here, internal wall insulation (IWI) is the more suitable option.
Post war housing retrofit with insulated render cladding (existing house type on the left) by Aron Coates of Designs in Detail.
EWI systems typically incorporate one of the following types of insulation:
Plastic foam insulations such as EPS, PIR, and phenolic boards are typically the most thermal efficient. However, being ‘closed cell’, they are impermeable to water vapour which means they can prevent condensation from leaving the building, potentially increasing the risk of damp unless other measures are in place. Plastic foams also have a high carbon and ecological footprint.
Timber insulation products made from wood fibre or cork are ‘open cell’ and allow water vapour to pass through, making them permeable, or ‘breathable’. They are typically made from waste material from the timber industry and have a much lower carbon footprint than plastic insulation. However, timber EWI is less thermally efficient (you will need thicker boards to match the performance of plastic foam) and can be expensive.
When specifying EWI it’s important to consider factors beyond thermal performance to make sure the system you choose is right for your individual project.
PIR external wall insulation on an existing rendered wall, photo by Earthwise.
The most common way to finish EWI is with render. Just like the insulation types listed above, render systems can either be both permeable and impermeable, so careful consideration is needed when choosing the finishing material.
Other potential cladding materials include (non-exhaustive):
When paired with a high quality finish, EWI offers many exciting design possibilities and can really transform a low quality building’s architecture.
External insulation with zinc cladding to an existing s house by Dow Jones Architects.
If properly detailed and installed, EWI will not cause damp.
EWI provides additional protection from the weather and can prevent outside moisture from the entering the building. The main risk of damp comes from within the building.
Damp can occur in buildings through ‘interstitial condensation’ in the colder months when warm moist air penetrates the building fabric and hits the surface of a material that is cold enough for it to release its moisture. The point at which condensation occurs is known as the dew point.
When a good thickness of insulation is added to the exterior of an uninsulated solid wall it creates a dew point outside the existing masonry. This is a good thing because it means any trapped vapour is likely to condense away from the inside, keeping the wall dry. However, in areas where there is insufficient insulation – say, at window or door opening, or behind an external waste pipe – cold bridges can occur allowing vapour to condense further within the wall which can lead to damp patches forming on the inside.
Further, some types of external wall insulation can act as a barrier to trapped moisture evaporating from the wall. This can lead to a build-up of damp behind the insulation boards, causing them to rot which can then result in damp problems on the inside.
To prevent damp from occurring, it is important to avoid cold bridges and add a good, consistent layer of insulation across the entire wall’s surface. Vapour open 'breathable' EWI systems will allow vapour trapped within the wall to evaporate to the outside. Where closed cell, non-breathable systems are used, internal vapour barriers will reduce the amount of moisture entering the wall. Above all else, the building should have proper heating and ventilation to keep internal moisture levels under control.
Existing uninsulated solid walls do not have a dew point, but they suffer from excess heat loss. When external wall insulation is added a dew point occurs outside the existing masonry. If there are gaps within the insulation, or areas of low insulation, the dew point can locally occur within the masonry leading to damp.
Timber based EWI is generally best for traditional solid wall buildings.
Older buildings, especially those built before , were designed as ‘breathing’ structures which enable moisture to freely pass between the inside and outside environments. Insulation systems that work with the building’s existing technology can reduce the risk of damp and other problems building up over time.
EWI systems that use wood fibre or cork as an insulator provide excellent weather protection but also allow warm moist air to safely evaporate out of the building during the colder months. A permeable internal finish, such as breathable paint on lime plaster, will aid the transfer of moisture and allow vapour to evaporate inside the building in the summer.
Wood fibre and cork EWI systems also have a low ecological and carbon footprint, making them a more sustainable choice.
Closed cell plastic foam insulation panels have excellent thermal and weather protecting properties, but they can impede moisture leaving the building. Good home ventilation therefore becomes even more essential to reduce the risk.
If you are considering installing EWI, seek expert advice to make sure the system you choose is best suited to your particular needs.
Wood fibre external wall insulation by Steico.
EWI is not suitable for homes with modern cavity walls – mainly those dating from the s but also some properties as early as the late s. Essentially, EWI is rendered useless if the walls are insulated. It would also compromise the building’s ventilation.
It can be installed, however, on homes with uninsulated cavity walls – typically those built from the s to the late s.
For the external insulation to be effective on homes with uninsulated cavity wall construction, the cavity must be fully filled with insulation, otherwise heat will escape through the air gap before it reaches the external insulation, rendering it largely ineffective.
Blown glass mineral wool, like Knauf Supafil, is probably the most effective type of insulation to use and provides consistent coverage. For a greener alternative, mineralised wood chips, such as Cemwood CW, can be blown into the cavity, but a layer of synthetic insulation is still required at the base as the material must be clear of the ground.
As noted previously, insulation can act as a barrier to internal moisture escaping from the building which can cause damp in the walls. A suitable vapour control layer and good interior ventilation will alleviate this risk.
Insulating cavity walled buildings can be technically challenging and requires greater site monitoring to ensure the works are properly installed. Again, you should seek professional advice before carrying out this type of work.
Uninsulated cavity wall with EWI. Image by LETI.
External insulation increases the overall wall thickness which can have an effect on other parts of the building. This is most apparent at the eaves and external openings.
It’s very important to check the eaves are deep enough to accommodate the new insulation. If there is insufficient space, you will need to extend the roof overhang, which can be costly. This could be a dealbreaker if you are on a tight budget.
You will need to return the insulation into window and door reveals – the space between the frame and the external wall – to prevent cold bridging. There is usually insufficient space to accommodate the same thickness panel as the main wall, so thinner, higher performance insulation boards are required. Alternatively, the glazing unit can be brought forward to eliminate the need for reveal boards altogether.
In addition to the above, external flues, vents, pipework and any other external fittings should be adjusted to enable a consistent thickness of insulation to be installed across the entire wall.
Service pipes repositioned outside of external wall insulation. Photo by Be Constructive.
The answer depends on your target performance and the type of EWI you want to use.
You will obviously need a lot more insulation if you are aiming for EnerPHit Passivhaus accreditation than minimum building regulation standards.
Plastic foam insulation offers a better R-Value – the materials’ ability to resist heat – than wood fibre insulation and can achieve good U values – the measure of the flow of heat through a thermal element – with quite thin boards. The lower the U-value the better the material is as an insulator.
For single brick solid walls with an internal plaster coat measuring about 250 millimetres overall, 70 millimetres of PIR, 90millimetres of EPS, or 100 millimetres of wood fibre insulation will achieve a U-value of about 0.3 – the building regulations standard.
When EWI is installed alongside full fill cavity insulation, 50 millimetres of EPS cavity bead insulation and 50 millimetres PIR external insulation will achieve a U-value of around 0.18.
A U-value of 0.15 is possible if an airtight layer is provided, such as a 12mm thick lime plaster to the masonry.
A U-value calculation should be carried out to establish insulation thickness prior to installation.
External wall insulation (EWI) is a much more effective method of insulating a building's walls than internal wall insulation (IWI) as it provides a continuous layer of insulation over the entire building envelope. For solid walled buildings, it also exposes the thermal mass of the masonry to the interior allowing spaces to heat (and cool) more efficiently.
IWI, on the other hand, involves adding insulation to the inside of the building's external walls, typically by installing insulated plasterboard or insulation boards between studs and covering with plasterboard (or other interior linings). This method requires stopping the insulation layer at wall, floor and ceiling junctions which breaks the thermal layer and reduces overall performance. It also lessens the effectiveness of the building’s thermal mass which effects thermal efficiency.
EWI is therefore the preferred method for insulating external walls, whenever practical. IWI is a consideration if there are restrictions that prevent the use of EWI. For example, on listed buildings or buildings within conservation areas, or if there’s insufficient space or obstructions on the outside of the building.
External and internal wall insulation graphic by LETI. External wall insulation is the most effective method of reducing heat loss through a wall.
Featured content:
For more External Wall Insulation Phenolic Boardinformation, please contact us. We will provide professional answers.
If you live in a house that is not on designated land and the proposed cladding materials are similar in appearance to the existing building, EWI could fall under permitted development rights, meaning it may not require planning permission.
Planning permission is required for:
You are unlikely to gain planning permission for EWI on the front facade of a building in a conservation area, as the visual appearance of streetscape is protected.
EWI is unlikely to not approved on listed buildings.
If you think permitted development applies to your home, seek confirmation before starting the building work by applying for a Certificate of Lawful Development.
A Victorian house in a conservation area in Brighton with external wall insulation on the sides and rear and internal wall insulation to protect detailing at the front. Image courtesy of Earthwise.
All external wall insulation is notifiable with Building Control which means your local council needs to be made aware that the works are taking place. A registered installer can self-certify that their work meets building standards, making the process more efficient.
If the EWI works affect more than half the wall, or more than a quarter of the overall building envelope, the building regulations state you will need to add insulation to improve the standard of the whole of that individual thermal element and achieve a U value of no more than 0.30.
U-value table from Part L of the building regulations.
If you want to install EWI on a shared boundary wall, also known as a ‘party wall’, you must inform your neighbours and obtain written consent through a party wall agreement before carrying out the work.
If the walls of your house abut directly onto the public highway (which includes the pavement) and the insulation will project over it, you will need an oversail licence. An oversail licence can be sought from the Highways Authority in your local council for a nominal fee. A highways officer will visit site before and after the works to ensure the public highway is still fully accessible.
The cost of external wall insulation in the UK can vary depending on several factors, such as the size of your home, the type of insulation material used, and the complexity of the installation process.
On average, external wall insulation in the UK can cost between £100-200 per square metre, including finishing materials and installation. Rendered EPS EWI being the most cost effective solution and wood based EWI with a high quality cladding material the more expensive.
Costs can also vary depending on location, the type of property, condition of the existing wall and any additional work required to facilitate the installation – adjusting the eaves or windows, replacing pipework etc. Therefore, it's recommended to get quotes from multiple installers to get a more accurate idea of the cost for your specific project.
Post war housing retrofit by Aron Coates of Designs in Detail.
There is currently one grant available in the UK for external wall insulation at the time of writing. The Energy Company Obligation (ECO) scheme provides funding for external wall insulation (and other home improvement works) for eligible low-income households and those living in properties that are hard to heat.
If you are not eligible for the ECO Scheme, the Parity Trust is currently working in partnership with 16 local authorities to provide secured loans to homeowners for a range of home improvements, including external wall insulation.
External wall insulation is best undertaken by a qualified contractor. Check that the insulation system they are using either has a British Board of Agreement (BBA)certification or European Technical Approval (ETA).
To gain ECO funding the insulation will need to be accompanied by a SWIGA (Solid Wall Insulation Guarantee Agency) or equivalent guarantee.
Companies such as Back to Earth and Mike Wye supply sustainable insulation systems and are excellent places for advice.
Registered EWI installers can be found at The Installation Assurance Authority.
How to install wood fibre external wall insulation, video by Back to Earth.
External wall insulation (EWI) is the most effective way to improve the energy efficiency of homes built with solid walls, and homes with early cavity walls when paired with full-fill cavity insulation.
If your home is poorly insulated, EWI can reduce heat loss and lower energy bills, making it a worthwhile investment in the medium to long term. Additionally, the UK government's Energy Company Obligation (ECO) scheme provides funding for low-income households and those in fuel poverty to install insulation and other energy-saving measures.
However, EWI can be a significant upfront investment and you may need to carry out other building work at the same time which can increase expenditure. It will also radically change the appearance of your building which may not be desirable, or possible. An architect will be able to assess your specific situation and advise if EWI is a good idea for your home.
Courtyard House by Yellow Cloud Studio has external wall insulation with dark cement board cladding at the rear.
About the author: Aron Coates is an architect with over two decades of experience, encompassing projects ranging from the restoration of historic buildings to the design of contemporary homes.
The information provided in this article is intended for general guidance and educational purposes only. At Designs in Detail, we advise you appoint a skilled residential architect to provide specific expert advice for your project.
As many of you will know, I am having External Wall Insulation (EWI) applied to my house.
As closer confidantes will confirm: I am obsessed with the project. Why? Because based on my calculations, it is the single-most effective thing one can do to an old house to improve its thermal performance and reduce carbon dioxide emissions.
And yet very few people seem to be doing it. My hope is that by simply talking about it – and by measuring how effective it really is – more people will consider it as an option.
The idea of EWI is simple – “just stick insulating materials to the outside of a house“. But the reality of doing this reliably and leaving the house weatherproof and looking good is complex.
There are some nice videos out there, such as this one below showing the Be Constructive team working on a previous house. There are more videos here. And if you want details, then check out the extensive EWIPro Complete Guide (pdf) and all the materials are available at the EWI Store.
But partly for my own satisfaction I thought I would outline each step with pictures rather than video. Also, the video shows the application of expanded polystyrene boards and the procedure for the polyurethane foam boards that I have used is a little different.
So here is my description the process. There is a gallery of photographs at the end of the article.
Step 1: Preparation
The job began by protecting all the working surfaces – the patio and the front and rear gardens – with protective plastic, and then all the windows were covered with a transparent adhesive film.
For my house, the Be Constructive team demolished an old chimney which no longer had a reason for existing, and removed almost 2 tonnes of loose render from the side wall. So much render was removed that the wall had to be roughly re-rendered before they could begin applying the EWI.
They then moved the boiler exhaust, external electrical fittings and drain pipes to take account of the fact that the house was about to grow by about 120 mm in all directions. This stuff is rather tedious – but essential.
Next came the preparation of the outside walls and the painting of a ‘stabilizing primer’. This penetrates porous surfaces and binds them, creating a surface to which adhesive can stick. This is particularly important for some building blocks which can be quite powdery.
Step 2: Boarding. Kingspan K5
Next the team installed so-called ‘starter track’. This plastic support is screwed into the wall at the level of the first layer of insulating boards – usually just above the damp proof course – and makes sure the boards are horizontal, and supports them while the adhesive mortar dries.
Normally EWI utilises either expanded polystyrene (sometimes abbreviated as XPS or EPS) or Rockwool™, and boards made from these materials are available in a wide range of thicknesses.
However I had asked to use a board made by Kingspan called K5. I chose this because I could only put about 100 mm thickness around the house – and for a given thickness, K5 will give the best insulation.
I limited the insulation to 100 mm because that amount would still keep the walls underneath the existing ‘soffit’ under the eaves. Also – if the insulation were much deeper – I felt the windows might seem to be too recessed.
For some reason, 100 mm thick boards of Kingspan K5 were not available and so the Be Constructive team glued pairs of 50 mm thick boards together to achieve the required thickness.
The ‘double’ boards were stuck to the wall using several thick blobs of adhesive mortar. Using a big blob of mortar perhaps 10 mm deep allows the outer surfaces of the boards to be made parallel even when the underlying wall is not.
In my illustrations I have deliberately drawn the boards as being not parallel. In fact the Be Constructive team actually took a lot of care into making the final surfaces vertical and smooth. This is important because it is very difficult to compensate for this after the fact.
The boards are ‘overlapped’ at corners and cut to shape around windows and other architectural features. Any gaps are filled in with expanding foam.
Step 3: Mechanical Fixing.
Once the boards are stuck to the wall and the mortar has set, the boards are mechanically fixed in place. To achieve this a hole is drilled through the boards and into the wall. Then a plastic fixing is pushed into the hole. Finally a metal nail is hammered into the plastic fixing which locks the plastic fixing in place – like a rawlplug – and holds the boards against the wall.
Using metal nails adds a heat leak directly through the boards: each fixture increases the thermal transmittance of the board by about 3%. However there is not much that can be done about that. It would be unwise to rely solely on the mortar or just plastic fixings.
Step 4: Base Coat Layers
Now the boards are attached to the wall and functionally insulating the house. But they are neither weatherproof nor attractive.
So the next step is to coat the boards with an adhesive mortar (called a ‘base coat’) in which a glass-fibre mesh is embedded. This mesh is essential to prevent cracking due to building movement.
For polystyrene insulation this is a simple process: the boards are rasped to create a smooth surface; a layer of base coat is applied; the mesh is pressed into place; and then the mortar is smoothed. This forms a surface on which the the final render can be applied.
For K5 insulation, the process is more complicated because the surface of the boards should not be abraded. So:
The base coat also meshes with the corner and reveal ‘beads’, and with extra fibre-glass mesh placed around the corners of windows.
Step 5: And finally
And finally we come to the point where render is applied.
The render is a mixture of stone with a specifiable particle size: 1 mm, 1.5 mm or 2 mm , together with a mortar and a silicone polymer. It can be coloured in a very wide range of colours.
Additionally, my house will have ‘faux’ bricks called ‘brick slips’ applied to match architectural details on neighbouring buildings.
I’ll be sure to post pictures when we have finished.
Photo Gallery – click for a larger version
Does it work?
But does it work? Well, of course it works! It would be physically impossible for it not to work!
The question is “How well does it work?“. And specifically, “Does it work as well I anticipated in my modelling?”
These are complicated questions to answer definitively – and they are especially difficult to answer quickly.
I will not have a definitive answer until later in the winter, but I will explain how I will answer the question in a follow-up article. For now I will just tease you with the answer that the data look ‘promising’.
Keep warm
Previous: A252 Reinforcement Fabric Mesh Sheet 3.6m x 2m
Next: A Complete Guide to Insulation Boards (2025) - GreenMatch
If you are interested in sending in a Guest Blogger Submission,welcome to write for us!
All Comments ( 0 )