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How Much Does External Wall Insulation Cost?

This article will dive into the various aspects of how much external wall insulation cost, factors to consider before and during installation, applying for grants and assess if investment is justified.

In recent decades, energy costs have been spiralling upwards, and this is set to continue for the foreseeable future (especially in the UK). One of the ways to overcome this is through external wall insulation (EWI) which can significantly reduce an individual’s energy expenditure. 

adding render to a wall

What is external wall insulation?

External wall insulation can be a way of boosting your home’s insulation to achieve higher energy ratings. It’s a system consisting of insulation which is applied on the exterior walls of your home which not only insulates, but protects the exterior as well. It can be covered with a render cladding of your choice.

In addition to improving your home’s insulation, you can improve its appearance by adding external wall insulation and an outer layer of render or another material.

The process generally involves the installation of an insulating layer to the external fabric of an existing, or new building, and is usually finished with a render coat, brick slips or clad with timber boards or tiles.

How much does external wall insulation cost?

External wall insulation cost can vary based on various factors:

  • Whether you have solid walls
  • Whether you want to insulate your walls internally or externally
  • The type of insulation components required
  • Whether you need scaffolding

External wall insulation cost per m2

External wall insulation cost is usually around £150 to £200 per m2. Therefore, The price range for a typical insulation project can vary depending on the size, complexity of the job and other factors mentioned above. It usually ranges from £10,000 for an end of terrace house to £30,000 for a large detached house.

Additionally, the average cost of hiring a scaffolding contractor to insulate a semi-detached property is around £1,800 to £2,500.

External Wall Insulation Grants

Government’s Energy Company Obligation (ECO) programme

If you receive certain state benefits (such as income support, tax credits or pension guarantee credit), you may qualify for funding through the Government’s ECO programme. How much funding you get depends on how much money an average household of the same type would save. However, ECO funding will only cover a small cost of the installation.

The Green Homes Grant Scheme

The Government’s Green Homes Grant scheme offers funding for certain insulation and low carbon heating measures. Households in England only can claim vouchers up to £5,000 to cover two thirds of the costs. Low income households on certain benefits may be able to claim up to £10,000 to cover all the costs as well.

Other grant schemes for External Wall Insulation

Depending on your local area, many local authorities have developed schemes that use a mix of different funding schemes to help residents make installations more affordable. Additionally, there may be other grants or loans available that can work in conjunction with ECO. 

If you want to learn more about grants available in South London, do get in touch with our team at Ralph Plastering.

Can external wall insulation be used on all buildings?

In general, EWI is suitable for a wide variety of properties, not just new-builds. EWI can be installed (and are more suited) on buildings that have solid walls. These include:

  • Properties built before 1930 that have solid brick walls;
  • Solid walls built with timber or steel frames; and
  • System built properties built between 1930s and 1980s

However, some cavity walls do not need external wall insulation because they already have space inside that can be filled with insulation. Many houses with cavity walls have been insulated except for those difficult cases (usually referred to as “hard to treat” that require additional work.

How do I tell if I have solid walls?

The easiest way to tell if a wall is solid or cavity is to look at the brick pattern on the outside of the house. If the bricks are all laid end-to-end then it’s likely that the wall is cavity, but if some of the bricks are laid with their square end facing out, then it’s likely that it’s a solid wall.

In some instances, the brick work can be covered with a pebble dash render. This can make it harder to identify the difference. However, the walls of a non-traditional construction, such as system built or timber-framed houses, are likely to be solid.

If you’re unsure about your building being suitable for EWI to be installed, do get in touch with one of our team members for a professional consultation. Alternatively, you could check your Energy Performance Certificate (EPC) which will have the wall type listed.

What is external wall insulation made up of?

With EWI, a barrier is created to stop the heat escaping through the walls. The main insulating component of EWI is normally made of expanded polystyrene (EPS) which is most common. Other components include phenolic boards or mineral wool. The choice of components used can determine the external wall insulation cost.

There are generally the components of an external wall insulation:

Insulation layer

This is the main layer that is made of either expanding polystyrene or mineral wool. All of these materials are used to prevent heat escaping through walls. The insulation is typically fastened to the building using direct fixings such as self-tapping screws. In turn, a layer of insulation is generally placed around the frame as an additional safety measure.


The primer helps prevent water from passing through the EWI system, as well as serving as a barrier to outside moisture. In other words, primers can help prevent moisture from seeping in through the walls. It also has some insulation properties that add to the overall efficiency of the system.

Glass fibre mesh & fixing anchor

These are generally put in place to hold the primer together during the drying process. Once dried, another layer of primer can be added on top of it.

Render finish

This is the final layer of EWI and usually has an aesthetic appeal. This helps enhance the look of your home. There are various render types available for you to choose from.

How does External Wall Insulation work?

There are there generally 3 ways in which heat is lost in a building; conduction, convection and radiation. They travel through walls, windows, doors, roofs and floors. The purpose of EWI is however to only reduce the heat lost through the walls.

Let’s look at each of the 3 ways in turn:

  • Conduction: Using the right insulating material and selecting the right thickness will ensure that heat can’t easily escape from your home.
  • Convection: The insulating material has several small sized air pockets to contain air movement. An EWI typically uses an expanding polystyrene slab that contains thousands of trapped air pockets.
  • Radiation: EWI systems use surfaces that reflect almost no radiant energy and absorb less radiant energy than other surfaces.

How much heat can External Wall Insulation save?

What is U-value?

U-values measure how effective materials are as insulators, with lower values meaning more effective insulators. The lower the U-value, the more effective a material is as an insulator. Because less heat escapes through the material, you use less energy to keep warm.

The insulation properties of an EWI system are influenced by the following factors:

  • The material used
  • The thickness of the insulation used
  • The thickness of your current walls

Thicker insulation boards can sometimes be impractical and compromise access to windows and doors. As a general rule, 100mm insulation boards are typically installed. Some EWI systems are manufactured to be thinner whilst providing better U-value savings.

How long does it take to install?

Installing EWI can take a few days to complete, but most installations are done within 5 – 7 working days. You’ll need to hire someone in order to work on the installation while you’re away.

What are the issues with installing EWI?

Installing EWI is  actually quite weather dependent, and has to be installed during dry spells, because the chemicals in the render and topcoat need to cure.

Do we need planning permission to install EWI?

If you live in a listed building or conservation area, you’ll need to seek permission from your local planning office before you can get an installation scheduled.

Should I get external wall insulation or internal wall insulation?

Unlike internal wall insulation, external wall insulation can be applied on a home’s surface, which gives you greater freedom when it comes to the thicknesses you apply. Consequently, this gives EWI greater potential as a rater of warmth and energy.

However, EWI will prove more economical in the long term since it can be retiled and re-plastered once installation is complete. Likewise, you can make repairs at any time, something that’s not possible with an IWI roof.

Final thoughts

As discussed above, external wall insulation cost can vary based on various factors. Additionally, you can also consider the option of applying for grants which can help cover some of the costs. If you’re not sure about how to go about choosing the right type of insulation for your wall, get in touch with our rendering team at Ralph Plastering.

Click here to view our external wall insulation services and feel more than welcome to request a free quote by contacting our team today.

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Ralph Plastering is a highly recommended plastering company in South London and Surrey providing all aspects of plastering, dry lining, rendering and external wall insulation services.

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