Thermal imaging showing how external wall insulation can improve heat loss 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.
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 1930 with solid external walls.
It’s also suitable for houses built between the 1930s and 1970s 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 1970s, but only compulsory in the 1990s. 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 1980s 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. Open cell, 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 1930, 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 evaporate out of the building during the warmer months. A permeable internal finish, such as breathable paint on lime plaster, will aid the transfer of moisture and allow moisture to evaporate to inside during the colder months.
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 1990s but also some properties as early as the late 1970s. 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 early cavity walls – typically those built from the 1930s to the late 1970s without existing cavity wall insulation.
For the external insulation to be effective on homes with early cavity wall construction, the cavity must be fully filled with insulation, otherwise heat will escape through the air gap before it reaches the insulation.
EPS beading is probably the most effective type of retrofit cavity insulation and provides consistent coverage. For a greener alternative, mineralised wood chips, such as Cemwood CW1000, 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 vapour barrier 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.
External wall insulation on an existing cavity wall, 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.
EWI changes the appearance of a building. Planning permission for EWI is always required for:
You are unlikely to gain planning permission for EWI to the front facade of a building in a conservation area, as the visual appearance of streetscape is protected.
EWI is generally not accepted on listed buildings.
In some circumstances, EWI may fall under permitted development which does not require planning permission. Excluding the situations listed above, EWI may be permitted development when the proposed cladding materials are similar in appearance to the existing construction.
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 their 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. The 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 building products 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.
