Thermal imaging showing how external wall insulation can improve heat loss. Image by Bere Architects.
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.
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:
1980s house remodel by Dow Jones Architects. Above: finished building with external wall insulation and standing seam zinc cladding; Below: existing building.
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 works best for older homes, especially those built before 1930 with solid walls. It can also suit houses from the 1930s–1970s with uninsulated cavity walls, if combined with cavity insulation, moisture protection and good ventilation. EWI is less suitable for modern homes with partial-fill cavity insulation, common from the late 1970s and compulsory from the 1990s. In these cases, internal wall insulation (IWI) is usually the better choice.
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 are the most thermally efficient. However, as ‘closed-cell’ materials, they do not allow water vapour to escape, which can trap condensation and increase the risk of damp unless managed carefully. They also carry a higher carbon footprint.
Natural insulations like wood fibre and cork are ‘open-cell’, meaning they are breathable and let vapour pass through. They are often made from timber industry by-products and have a lower carbon footprint. However, they are less thermally efficient (requiring thicker boards to achieve the same performance) and can be more costly.
When specifying EWI, it is important to look beyond thermal performance to ensure the system is the right fit for your project.
PIR external wall insulation on an existing rendered wall, photo by Earthwise.
The most common finish for EWI is render. As with insulation materials, renders can be either permeable or impermeable, so it is important to choose carefully.
Other potential cladding materials include (non-exhaustive):
With a well chosen finish, EWI not only improves performance but can also transform the look of a building, giving even poor quality homes a fresh architectural character.
External insulation with zinc cladding to an existing 1980s house by Dow Jones Architects.
When fitted properly, EWI should not cause damp. In fact, it helps protect walls from rain and stops moisture entering from outside. Most damp problems actually come from inside the home. Damp often happens in winter through a process called condensation. Warm, moist air from inside the house can get into the walls and meet a colder surface. When this happens, the moisture in the air turns into water. The point where this change happens is called the dew point.
By adding insulation to the outside of a wall, the dew point is pushed further out, away from the inside of your home. This is good because it helps to keep the inner wall dry. But if the insulation is patchy – for example around windows, door frames or pipes – “cold spots” can form, allowing condensation and damp patches to appear indoors.
Some insulation boards also hold moisture in the wall instead of letting it escape. Over time, this trapped moisture can cause problems, both to the insulation and inside the house.
To prevent damp, it is important to cover the whole wall with insulation and avoid gaps. Using “breathable” systems that let vapour pass through can help moisture escape to the outside. If less breathable (closed-cell) systems are used, extra measures such as vapour barriers may be needed. Above all, good heating and ventilation inside the home are key to keeping damp 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.
For traditional solid wall buildings, timber-based insulation systems are usually the best option.
Older homes, especially those built before 1930, were designed to “breathe”, allowing moisture to move freely between inside and outside. Using an insulation system that works with this natural process helps reduce the risk of damp and other long-term issues.
Wood fibre and cork EWI provide strong weather protection while still letting warm, moist air escape safely during colder months. Pairing this with a breathable internal finish, such as lime plaster with vapour-open paint, allows moisture to evaporate inside the home during summer. These natural products also have a much lower ecological and carbon impact, making them a sustainable choice.
By contrast, closed-cell plastic foams (like EPS or PIR) are more thermally efficient and highly weather-resistant, but they block moisture from leaving the wall. In these cases, good ventilation indoors becomes even more important to avoid damp.
If you are considering EWI, it is always best to get expert advice to make sure the system chosen suits your specific home and needs.
Wood fibre external wall insulation by Steico.
EWI is not suitable for homes with modern insulated cavity walls – typically those built from the 1990s, though some had partial insulation from the late 1970s onwards. In these cases, external insulation adds little benefit and can even disrupt the building’s ventilation.
EWI can, however, be used on older homes with uninsulated cavity walls, usually built between the 1930s and 1970s. For it to work properly, the cavity itself must first be fully filled – otherwise heat will escape through the air gap before reaching the external insulation.
Blown glass mineral wool, such as Knauf Supafil, is one of the most effective cavity fill products, giving good coverage. A more sustainable alternative is mineralised wood chips, such as Cemwood CW1000. In this case, a layer of synthetic insulation should still be installed at the base of the wall, as the wood chip material cannot sit in contact with the ground.
As with any insulation, it is important to prevent trapped internal moisture from causing damp within the walls. A suitable vapour control layer, combined with good indoor ventilation, can help manage this risk.
Insulating cavity walls can be technically challenging and requires careful site supervision to ensure the work is carried out correctly. Always seek professional advice before starting this type of project.
Uninsulated cavity wall with EWI. Image by LETI.
Adding external insulation increases wall thickness, which can impact other building elements, especially at the roof eaves and around windows and doors.
The eaves must be deep enough to cover the added insulation. If not, the roof overhang may need to be extended – a potentially costly adjustment that can be a dealbreaker on tight budgets.
Insulation should also be returned into window and door reveals (the gap between the frame and external wall) to prevent cold bridging. Because space here is usually limited, thinner, higher‑performance boards are often used. Another option is to move the glazing unit forward so full‑thickness boards are not needed.
Finally, external flues, vents, pipework and other fittings may need to be repositioned so the insulation layer can be applied at a consistent thickness across the wall.
Service pipes repositioned outside of external wall insulation. Photo by Be Constructive.
The required thickness depends on your performance target and the type of insulation chosen. A home aiming for EnerPHit Passivhaus standards will need far more insulation than one looking to simply meet building regulations.
Plastic foam boards (such as PIR or EPS) have higher thermal resistance (R‑value) and achieve good heat loss figures (U‑values) with relatively thin panels. Natural materials like wood fibre need greater thickness to reach the same performance. Remember: the lower the U‑value, the better the insulation.
As a guide for a single solid brick wall with plaster (around 250 mm thick overall): - 70 mm of PIR - 90 mm of EPS - 100 mm of wood fibre Each of these achieves a U‑value of roughly 0.30 W/m²K, which is the common target. For homes where external insulation is combined with full‑fill cavity insulation, using 50 mm of EPS cavity beads plus 50 mm of PIR externally can achieve a U‑value of around 0.18 W/m²K. With an added airtight layer (e.g. 12 mm lime plaster on the masonry), a U‑value of 0.15 W/m²K is possible.
Because every property is different, a U‑value calculation should always be carried out before installation to determine the exact insulation thickness required.
External wall insulation (EWI) is generally a much more effective way to insulate a building’s walls than internal wall insulation (IWI) because it provides a continuous layer of insulation over the entire building envelope. For solid-walled buildings, EWI 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 external walls, typically by installing insulated plasterboard or insulation boards between studs and covering these with plasterboard or other interior finishes. This method requires the insulation to stop at wall, floor and ceiling junctions, which breaks the continuity of the thermal layer and lowers overall performance. It also reduces the effectiveness of the building’s thermal mass, impacting thermal efficiency.
For these reasons, EWI is the preferred method for insulating external walls wherever practical. IWI may be considered if restrictions prevent the use of EWI, such as on listed buildings or in conservation areas, or where there is limited 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.
If you live in a house that is not on designated land and the proposed cladding materials closely match the existing appearance, EWI may fall under permitted development rights and not require planning permission.
Planning permission is required for:
You are unlikely to gain permission for EWI on the front façade of buildings in conservation areas, as street scene is usually protected. EWI is also rarely approved for listed buildings.
If you believe your project qualifies as permitted development, it is advisable to seek confirmation by applying for a Certificate of Lawful Development before starting work.
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 work must be notified to Building Control, meaning the local council needs to be informed that the installation is taking place. A registered installer can self-certify that their work meets building standards, which streamlines the process.
If the EWI affects more than half of a wall or more than a quarter of the total building envelope, Building Regulations require the insulation to upgrade the entire affected thermal element. The upgraded wall should aim to achieve a U-value of 0.30 W/m²K to comply with energy efficiency standards.
U-value table from Part L of the building regulations.
If you want to install EWI on a shared boundary wall, known as a ‘party wall’, you must notify your neighbours and obtain written consent through a party wall agreement before starting work.
If your house walls directly abut the public highway or pavement and the insulation will extend over it, you will need an oversail licence. This licence is issued by the Highways Authority at your local council for a small fee. A highways officer will visit your site before and after the work to ensure that the public highway remains fully accessible during and after the installation.
The cost of external wall insulation in the UK varies depending on factors such as home size, insulation material, and installation complexity. On average, EWI ranges from approximately £100 to £200 per square metre including finishing materials and installation.
Rendered expanded polystyrene (EPS) is usually the most cost-effective option, while timber-based EWI with high-quality cladding tends to be more expensive.
Prices also vary by location, type of property, condition of existing walls, and any additional work needed, such as adjustments to eaves, windows, or pipework. For accurate pricing, it’s best to get quotes from multiple reputable installers.
Typical total costs for external wall insulation projects can range from £6,000 for smaller terraced homes up to £25,000 or more for large detached houses. Energy savings of around £300-£700 per year are often achieved with good installation.
Post war housing retrofit by Aron Coates of Designs in Detail.
In the UK, there are two main government schemes offering help with the cost of EWI: the Energy Company Obligation (ECO4) and the Great British Insulation Scheme (GBIS).
These schemes provide free or discounted insulation to eligible households, especially those on low incomes, receiving certain benefits, or living in properties that are hard to heat. To qualify, homes usually need to have solid walls and an Energy Performance Certificate (EPC) rating of E, F, or G — meaning they are less energy efficient.
If you don’t qualify for these grants, the Parity Trust offers secured loans for home improvements, including EWI, in partnership with some local councils.
To apply, you’ll generally need a home assessment by a qualified professional to check if your property and household qualify. It’s worth getting quotes from approved installers to help with the application process and to ensure quality installation.
External wall insulation should always be installed by a qualified contractor. Ensure the insulation system they use holds either British Board of Agrément (BBA) certification or European Technical Approval (ETA), which guarantee the product’s quality and performance.
To qualify for ECO funding, the insulation must come with a Solid Wall Insulation Guarantee Agency (SWIGA) or equivalent guarantee.
Companies like Back to Earth and Mike Wye specialise in sustainable insulation systems and can offer expert advice.
You can find registered EWI installers through the The Installation Assurance Authority which maintains a list of certified, approved contractors.
How to install wood fibre external wall insulation, video by Back to Earth.
External wall insulation (EWI) is one of the most effective ways to improve the energy efficiency of homes with solid walls, as well as homes with older cavity walls if combined with full-fill cavity insulation.
If your home is poorly insulated, EWI can significantly reduce heat loss and lower your energy bills, making it a worthwhile investment over the medium to long term. Additionally, the UK Government’s Energy Company Obligation (ECO) scheme provides funding to help low-income households and those in fuel poverty install insulation and other energy-saving measures.
However, EWI can require a significant upfront investment and may involve additional building work, such as adjustments to eaves or windows, increasing overall costs. It will also substantially change the appearance of your building, which might not be desirable or feasible in some cases.
An architect or qualified professional can assess your specific property and advise whether EWI is a suitable solution for your home.
Courtyard House by Yellow Cloud Studio has external wall insulation with dark cement board cladding at the rear.
The information provided in this article is intended for general guidance and educational purposes only. For specific expert advice on your project, consult a skilled residential architect featured on Designs in Detail.
About the Author
Aron Coates is a qualified architect with over 20 years of experience. He has worked on a wide range of projects, from restoring historic buildings to designing contemporary homes.
