External Wall Insulation – A Simple Guide

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External Wall Insulation
Property Type Av Cost Av Energy Saving Av Cost Saving
Mid Terrace House £4 - £6,000 690kg CO2/year £160
Semi Detached House £8 - £10,000 1,120kg CO2/year £255
Detached House £12 - £14,000+ 1,870kg CO2/year £425

*Figures taken from the Energy Saving Trust

What is external wall insulation?

External wall insulation, also known as EWI and external solid wall insulation, is a type of insulation that is attached to the outside of your walls. Two layers of material are used - one to offer insulation, and an outer layer of mineral or synthetic render to protect against weather and provide a decorative finish.

Installing EWI will add around 100mm to your house, depending on the thickness of insulation used.

What buildings can external wall insulation be used on?

EWI is suitable for any building with solid walls. These can be:

  • Older properties built before 1930 that have solid brick walls
  • System built (concrete) properties that tend to have been built between the 1930s and 1980s
  • Solid wall properties with timber or steel frames

Houses built with cavity walls do not need EWI because they already have a space within the walls that insulation can be injected in to. Many properties with cavity walls have already been insulated apart from what are known as “hard to treat” cavity walls that require more specialist treatment.

How do I tell if I have solid walls?

If your walls are brick, then the best way to tell if they are solid walls or cavity walls is to look at the brick pattern on the outside of the house.

If the bricks are all rectangular and laid end-to-end then the wall is probably cavity. If some of the bricks are laid with the square end facing out then it is probably a solid wall.

Solid walls?

If the brick work is covered up (e.g. with a pebble dash render) then it can be harder to identify.

Properties of a non-traditional construction, such as system built or timber framed houses, are likely to have solid walls.

If you have an Energy Performance Certificate (EPC) for your home, then you will be able to see what type of construction the walls are. This is typically listed on the second page of the document.

Check the Landmark register to see if your home has an EPC

What is external wall insulation made up of?

EWI creates a barrier that stops heat escaping through your walls. This barrier is attached to the outside of your walls in several layers. The main insulating component is normally made of expanded polystyrene (EPS), phenolic boards or mineral wool. EPS is most commonly used.

The components of EWI:

1. Insulation layer
This is the main insulation - it is normally made of either expanding polystyrene or mineral wool. Technologies such as phenolic resin insulation are also available. 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.

2. Primer
The primer helps waterproof the EWI system and act as a barrier to moisture passing from the outside in. Some primers also have inherent insulation properties adding to the overall efficiency of the system.

3. Glass fibre mesh & fixing anchor
This part is used to hold the primer in place whilst it dries. Another layer of primer is then applied over the top of it.

4. Render finish
This is the final layer of EWI and has a decorative function, enhancing the look of your home. There are various different renders available.

How does EWI insulation work?

Heat is lost from a building in three ways:

  • Conduction
  • Convection
  • Radiation

It travels through the walls, roof, floor, windows and doors. EWI reduces heat loss through your walls.

How does EWI stop:

By using an insulating material that is bad at conducting heat and choosing the right thickness for this material. This helps lock the heat inside your home.

The insulating material has small voids or air pockets to contain air movement. EWI normally uses an expanding polystyrene slab which contains thousands of trapped air pockets.

EWI systems use surfaces that emits little radiant energy and absorb a very small percentage of the radiant energy falling on to it.

Can EWI help prevent condensation?

EWI helps to prevent condensation because it traps heat inside rooms so that internal walls are not as cold. When walls are colder they attract water in the air which is how condensation appears.

How much heat is saved?

What is a U-value?

A U- value measures how effective a material is as an insulator. The lower the U-value the better insulator the material is. So lower U-values also mean that more heat is saved.

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

Below are estimate improvements from Ofgem:

Average U-value improvements for a typical domestic property when EWI is installed:

Insulation Thickness U-value Improvement
50mm From 2.1 to 0.6
100mm From 2.1 to 0.35
150mm From 2.1 to 0.25
200mm From 2.1 to 0.18

Figures taken from Ofgem data, based on properties built in England and Wales before 1967 or Scotland before 1965 of a solid brick construction.

Please note that 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 whist providing better U-value savings. However, these tend to be more expensive.

How long does it take to install?

It can take a few days to fit EWI depending on the size of your property. Most installations are completed within 5 – 7 working days.

How much does it cost?

EWI can cost anything from £4,000 for smaller terraced properties to £14,000+ for larger detached homes. Prices are dependent on the size of your house and how many walls you need insulating.

Can I get a grant for EWI?

There is funding available for EWI on domestic properties via the Government’s Energy Company Obligation (ECO) programme. Depending on your situation you may qualify for either a grant.

Homeowners or private renting tenants that receive certain state benefits (such as income support, tax credits or pension guarantee credit) can qualify for funding through the Government's ECO programme. The amount of funding available is calculated by how much money an average household (of the same type) would save on fuel bills by having EWI fitted. ECO funding will only cover a small cost of the installation.

YES Energy Solutions has HHCRO funding in place and can check to see if you are eligible.

The Green Homes Grant Scheme

YES Energy Solutions can assist residents looking to receive funding from the Government's Green Homes Grant scheme.

Due to unprecedented demand, we are unable to process any new enquiries at this current time.

The Green Homes Grant Voucher Scheme will close to new applications from householders on Wednesday 31 March 2021 at 5pm.

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 access a voucher up to a value of £10,00 to cover all the costs.

YES Energy Solutions may be able to help you receive funding if you live in Yorkshire & Humberside, North West, West Midlands, and central areas of the South East, East of England and the South West only.

The Government’s Green Deal scheme, which offered ‘pay as you save’ loans, is no longer available. (What was Green Deal all about?)

Are there are any other forms of funding available?

It is unlikely that funding through the ECO programme (CERO or HHCRO) will ever fully fund a domestic EWI install. However, many Local Authorities have developed schemes that use a blend of different funding streams to help residents make installations more affordable. Depending on where you live, there may be other grants or loans available that can work in conjunction with ECO.

YES Energy Solutions work in partnership with many Local Authorities. We can check to see if there are any alternative funding routes or schemes running in your area.

Are there any issues with installing EWI?

Fitting EWI is very weather dependant and can only really be installed during dry spells due to the chemical cure in the render and topcoat. Sometimes installations may take longer to complete during the winter or if the forecast is poor.

Is planning permission required?

In some instances you may need planning permission from your local council; however, this tends to be rare. If you live in a listed building or a conservation area then you will need to seek advice from your local planning office before an installation is scheduled.

Should I get external wall insulation or internal wall insulation?

Visit our comparison table.

In general, EWI is more versatile than internal wall insulation (IWI) in terms of thickness, and therefore also in terms of energy saving and warmth provided. IWI will shrink the size of your rooms, so you’re restricted as to how thick you can have it.

As a rule, IWI is no worse at insulating than EWI, however it can work out more expensive after re-decorating and re-plastering. It also won’t provide the added weather protection that EWI offers. EWI is less disruptive to your everyday life during installation.

Find out more about our EWI installation service

T: 01422 880100 (office hours: 9am - 5pm, Mon - Fri)

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