Your Questions About External Wall Insulation Answered

Your Questions About External Wall Insulation Answered
Disclosure: Clicking on links on this page may earn us a small commission. This helps us continue to produce free content and doesn't affect the price you pay.

Making your property more energy efficient brings many benefits, including a more comfortable home and reducing your energy bills. One way to do this is external wall insulation.

It’s an insulation method that’s most commonly used with older buildings that don’t have cavity walls. It’s designed to reduce heat loss, which is a drawback of older building methods, such as solid brick walls.

While it has benefits, there are also drawbacks, as with anything. So to help you decide whether this is the right insulation method for you, we answer commonly asked questions about external wall insulation.

What is external wall insulation?

external wall insulation

External wall insulation (EWI) is a system used to insulate the external walls of a property.

It is usually applied to properties with solid walls. However, it can also be used on properties with cavity walls, though it doesn’t happen often.

EWI works by adding thick insulation boards to the outside of the house, which are fixed to the walls using adhesives or a mechanical method. It is then covered with a finish, such as render or cladding.

There are different materials that can be used, depending on the needs of the individual property. Some materials are breathable (permeable), others are non-breathable (impermeable). Choosing the right material for your home is vital to prevent issues occurring.

Why would you have external wall insulation?

There are several reasons why you might want to have external wall insulation installed on your property, such as:

Other methods aren’t suitable

EWI isn’t the only way to insulate your home, but it might be the only one that is suitable for your property. Cavity wall insulation is only possible if your home has cavity walls, which is unlikely if you live in an older building.

Part of our home was built in the 1850s and has solid walls, so for us only external or internal insulation would be an option.

Internal insulation will reduce your room sizes, as the insulation boards are fixed to the inside walls. If that made your rooms too small, your only option is insulating your property externally.

Improving your home’s thermal efficiency

It’s the main aim of EWI, and can help to reduce your energy bills as you will need to use less heating and hot water to maintain a comfortable temperature inside.

It’ll also improve your EPC, which will be helpful when you come to sell your property.

Reduce noise pollution

The insulation can help reduce the amount of noise that comes into your property from outside, making it more peaceful and quieter.

Improve the appearance of your property

Having insulation can give your property a facelift as it can be finished in a variety of colours and finishes.

What are the advantages of external wall insulation?

external wall insulation improves energy efficiency of a property

External wall insulation has pros and cons – here are some of the advantages:

  • Reduces heat loss and energy bills
  • Fewer draughts
  • No disruption when being installed
  • Does not reduce the internal floor area
  • Insulated walls will contribute to thermal mass (also known as the ‘tea cosy’ effect)
  • Improves weatherproofing and sound resistance
  • Boosts the life of the wall
  • Reduces condensation on internal walls
  • More suitable for the English climate than internal insulation (Xu et al., 2019)

What are the disadvantages of external wall insulation?

Having looked at the advantages, the problems with external wall insulation include:

  • It can be difficult to install
  • If not installed correctly, it can cause damp problems
  • It can be expensive

Common problems include damp patches appearing on walls, mould growth, and cracking or bulging of the render or brickwork.

Problems can also occur if the insulation is not properly protected from the weather, or if it’s installed on a property that is not suitable for it.

To avoid problems with external wall insulation, it’s important to choose a reputable installer who has experience in installing this type of insulation.

It’s also important to make sure that the property is suitable for external wall insulation, and that all precautions are taken to protect the insulation from the weather.

The best way to ensure the installation is done correctly is by getting a competent person to do it. You can use the government’s competent persons scheme to find a professional installer.

What are the common problems with external wall insulation?

External wall insulation can have some disadvantages, such as being difficult to install.

You have probably heard that external wall insulation can come with problems, and it’s true. These are the most common issues:

  • Faulty installation – make sure you get an installer who knows what they are doing
  • Damp and condensation – make sure that the right material is used for your home
  • Cold bridges around windows and doorframes – if left this can lead to condensation, damp and mould
  • Planning permission and building regulations – while not every EWI installation needs planning permissions, they all need to be done according to current building regulations
  • High costs – it can cost several thousand pounds to insulate your home externally

You can read more about the potential issues in our article about the problems with EWI.

How much does external wall insulation in the UK cost?

This is one of the most common questions about external wall insulation, as many people know it isn’t cheap.

The costs will vary depending on the size of the property and the material that is used. But generally speaking, on average, external wall insulation can cost anywhere between £5,000 and £15,000.

A small flat will be cheaper to insulate than a 5-bedroom detached home. This makes external wall insulation an expensive affair.

We would recommend getting at least three quotes before you decide with which installer to go, as you don’t want to overpay on something so expensive as this.

How much could I save fitting external wall insulation?

In the UK, external wall insulation can cost between 5000 and 15000 pounds.

It’s estimated that external wall insulation will increase energy efficiency of a property by up to 35% to 40%. How much this will save you will depend on the size of your home and also which type of energy source you use.

But it could result in a yearly saving of £140 to £700 on your energy bills. The bigger your property, the bigger the saving.

But you also gain comfort, because your home will be warmer and more comfortable.

Does external wall insulation need planning permission?

It’s important that you check with your local planning authority because external wall insulation can change your home’s external appearance.

If it does, you need planning permission, and you should begin this process before approaching insulation firms.

A listed building or a property in a conservation area will also need planning permission before you can install external wall insulation.

However, in most cases, the installation of external wall insulation for a domestic property will be classed as a permitted development, so you should have no problems.

But in either way, your installation has to adhere to current building regulations. So make sure you get a certificate of building regs approval after the work has been completed.

Will external wall insulation cause damp problems?

Yes, if the installation isn’t done correctly and the wrong materials are used. In fact, a study on Swedish properties concluded that the type of external wall insulation even influenced mould growth and instances of moisture damage (Hamid et al., 2024).

The main concern is that the insulation material fits with the building. If your home has breathable walls, which means moisture can pass through from inside to the outside, the insulation also has to be permeable.

Otherwise, the moisture won’t be able to go anywhere, which will eventually lead to damp. In those cases, you have to install additional ventilation to allow moisture to leave the property.

This shows that it’s important to chose the right material. For example, external wall insulation materials such as mineral wool or rockwool are permeable (Vololonirina & Perrin, 2014).

While these materials work to prevent water ingress, they don’t stop any moisture from reaching the wall’s internal surface, so you may need to consider improving your property’s ventilation to move any moist air to prevent damp problems.

Another way damp could occur is through cold bridges. Especially around windows and doors, it can be difficult to fit insulation boards correctly. Any gaps will lead to cold patches that can lead to condensation and damp.

Water ingress is also a possibility, which again can lead to damp.

I have solid stone walls, can I use external wall insulation?

Even if you have solid stone walls you can install external wall insulation.

Yes, solid walls are suitable for external insulation. In fact, it’s the type of building where this method is most commonly used.

Also, some homes with a solid stone wall may have a small cavity between the interior and exterior wall that is filled with rubble.

So, any moisture that forms within the cavity will either evaporate away because of the air movement.

It is also worth considering that a stone wall will usually be a ‘breathing wall’ and you need to maintain the breathability that will ensure the wall operates as it is designed to do.

Essentially, you’ll need to use natural insulation such as cork or wood fibre and use lime as the external render.

For a solid brick wall, which type of external wall insulation is best?

To answer this question, you have to check the bricks of your property. That’s because not all bricks are the same since the permeability and the quality of them will vary.

As we have said before, the material used as insulation has to match your external walls. An experienced installer will be able to advise you which is best to use for your building.

I have cavity walls, can I use external wall insulation?

A cavity wall will usually be ventilated, which is why builders create the cavity in the first place.

So, any warmth from the property that penetrates the internal wall to enter the cavity will then be exhausted by the cavity’s ventilation.

This then means that installing external wall insulation will not have much of an effect since most of the heat lost will disappear in the cavity before reaching the external skin.

Cavity wall insulation would be the better option here, where an insulating material is sprayed in the cavity, preventing the warmth from escaping. This is also a much cheaper option then installing insulation externally.

How long will external wall insulation last?

Another often asked question about external wall insulation. Depending on the installer, most manufacturers offer a warranty of 25 years, but the insulation has to be installed correctly.

This is a minimum term, so you should expect your insulation to last longer than that.

There shouldn’t be any problems with external wall insulation if it has been protected from the weather so a waterproof final render coating will be crucial.

If you need to contact installers of external wall insulation, the National Insulation Association has a list.

What is the best insulation for outside walls?

As far as medical research goes, there are no signs that external wall insulation can be harmful to our health.

Several different types of insulation material can be used, including:

  • Polystyrene (EPS)
  • Polyurethane
  • Phenolic resin (K5)
  • Mineral wool
  • Wood fibre

Each type of insulation has its own benefits and drawbacks, so it is important to choose the right one for your needs.

For example, polystyrene is a good choice if you are looking for a lightweight insulation material, but it is not as durable as other options.

Can too much external wall insulation cause condensation?

If the insulation is too thick or not installed correctly, it can cause condensation to form on the inside of the property. This can lead to damp problems and is something you should be aware of before installing external wall insulation.

Can external wall insulation damage health?

There is no evidence to suggest that external wall insulation can damage health. However, it is important to make sure that the property is well ventilated to prevent condensation and mould growth.

Authors

  • Paul James

    Paul James, is a marketing expert with a passion for property. As well as being a property investor, Paul has also worked within the marketing departments of some of the UK’s leading estate agents. Paul is the founder of Property Road.

    View all posts
  • Property Road logo

    Founded in 2017, Property Road is one of the UK’s leading independent property websites for consumers, focused on educating buyers, sellers, investors, and homeowners on all things property related. It's run by a team of experienced property owners, investors, and a former estate agent.

    View all posts
Checklist - 101 Ways To Sell Your House Faster

101 Ways To Sell Your House Faster eBook

FEATURED DOWNLOAD:

FREE Checklist: 101 Ways To Sell Your Home Faster

When you subscribe to our email newsletter. Plus, receive a 7-day crash course on how to get higher offers on ANY type of property.

You can unsubscribe at any time.
See our Privacy Policy.