Does External Wall Insulation Work?

External wall insulation has sometimes been approached with caution, largely because poorly designed or incorrectly installed systems have led to problems on some properties. However, that does not mean external wall insulation itself is ineffective.

So, does external wall insulation work? Yes. When the right system is chosen for the property and it is designed and installed correctly, external wall insulation can significantly reduce heat loss through the walls and improve the thermal performance of a home.

External wall insulation, often shortened to EWI, involves adding insulation to the outside of a building before covering it with a reinforced protective and decorative finish, such as render. The principle is relatively simple. The insulation creates an additional thermal layer around the home, helping to slow down the movement of heat through the external walls.

Where problems arise, they are often linked to factors such as unsuitable materials, poor preparation, incorrect detailing or installation. This is why the effectiveness of external wall insulation depends on much more than simply attaching insulation boards to a property.

How Does External Wall Insulation Work?

External wall insulation works by creating an insulated layer around the outside of a building.

A typical rendered EWI system may include:

• Insulation adhesive

• Insulation boards

• Mechanical fixings

• Reinforced basecoat

• Fibreglass mesh

• Primer

• Decorative render

Each component performs a different role, from attaching the insulation securely to protecting it from external weather conditions.

We describe external wall insulation as a system in which insulation is fixed to the existing external wall before a reinforced basecoat and decorative finish are applied. 

The finished system helps create a more continuous thermal layer around the property. Instead of heat passing as easily through the existing external walls, the added insulation helps slow down that heat loss.

Does External Wall Insulation Reduce Heat Loss?

Yes. Reducing heat loss is one of the main purposes of external wall insulation.

According to Energy Saving Trust, around 33% of the heat lost from an uninsulated home can escape through the walls. Adding suitable wall insulation can therefore address a significant area of heat loss in homes that currently have little or no wall insulation. 

This does not mean every home will experience exactly the same improvement.

The result will depend on factors such as:

• What the existing walls are made from

• How well insulated the property already is

• Which insulation material is used

• The thickness of the insulation

• How well the system is installed

• How areas such as windows, doors and rooflines are detailed

This is why two properties can have very different requirements even if they look similar from the outside.

Why Does External Wall Insulation Sometimes Go Wrong?

This is where the distinction between the principle of EWI and the execution of an EWI project becomes important.

An insulation board can have excellent thermal properties, but that alone does not guarantee a successful installation.

Problems can occur if:

• The existing wall has not been assessed properly

• Damp or water ingress is covered rather than resolved

• The insulation is unsuitable for the property

• Incorrect fixings are used

• Insulation boards are poorly fitted

• Important junctions are not detailed correctly

• Water is allowed to get behind the system

• Incompatible products are mixed together

• The reinforcing layer is installed incorrectly

• The final render or cladding is unsuitable

External wall insulation should therefore be treated as a complete system, rather than a collection of individual products.

EWI Pro also recommends obtaining a full system specification for EWI works, particularly where warranties or compliance are required. Individually purchased components do not automatically constitute a warranted system. 

What Makes External Wall Insulation Work Properly?

Several factors contribute to an effective external wall insulation system.

1. Choosing the Right Insulation

Not every insulation material is right for every building.

Common EWI insulation options include:

Graphite EPS
A lightweight and thermally efficient insulation commonly used on residential properties.

Mineral Wool
Offers thermal insulation alongside vapour permeability, acoustic properties and an A1 reaction-to-fire classification.

Phenolic insulation
Provides strong thermal performance where the available space for the insulation build-up is more restricted.

Other insulation products may be appropriate for specialist applications.

The choice should be based on the building rather than simply choosing whichever insulation is cheapest or thinnest.

Our buyer's guides cover different system build-ups for EPS, Mineral Wool, Kingspan K5 and other substrates and applications. 

2. Using the Correct Insulation Thickness

The insulation also needs to be thick enough to achieve the required level of thermal performance.

There is no single thickness that works for every house.

The amount required depends on the existing wall construction, the insulation material and the thermal performance the finished wall needs to achieve.

This means that simply asking whether 90mm or 100mm of insulation is enough does not give the full picture. The existing property needs to be considered first.

3. Preparing the Existing Wall

Before insulation is installed, the existing wall needs to provide a suitable background for the system.

The condition of the brickwork, blockwork or existing render should be assessed. Loose material, failing render or other issues may need to be dealt with before installation begins.

Existing damp problems should also be investigated rather than simply covered with insulation.

Existing damp issues need to be resolved before external wall insulation is installed. 

4. Fitting the Insulation Correctly

The insulation boards need to form a consistent layer around the building.

Poorly fitted boards or unnecessary gaps can interrupt the insulated layer and reduce its effectiveness.

The correct adhesive and mechanical fixings also need to be used for the insulation and the wall behind it.

This is why an EWI installation should follow the specified build-up rather than treating every substrate in exactly the same way.

5. Getting the Details Right

Some of the most important areas of an EWI project are not the large, flat sections of wall.

Careful detailing is needed around:

• Windows

• Window sills

• Doors

• Rooflines

• Eaves

• Pipes and vents

• Meter boxes

• Corners

• Ground level

• Damp-proof courses

These areas are where insulation has to meet other parts of the building.

For example, the top of the EWI system needs to be appropriately protected where it meets the roofline. Specialist roofline closure systems can be used in some installations to help protect this junction from water ingress and reduce thermal bridging. 

Getting these details right is just as important as choosing the insulation itself.

Will External Wall Insulation Make My House Warmer?

External wall insulation does not create heat. Instead, it helps the home retain the heat that is already being produced.

By slowing down heat loss through the external walls, rooms can remain at a more stable temperature, and the heating system may not need to work as hard to maintain the same level of comfort.

Homeowners may also notice that the inside surface of previously cold external walls feels less cold after suitable insulation has been installed.

Can External Wall Insulation Reduce Energy Bills?

External wall insulation can reduce the amount of energy needed to maintain a comfortable temperature, which can in turn reduce heating costs.

However, it is important not to promise a particular saving.

How much a household saves will depend on:

• Property size

• Existing insulation

• Heating system

• Energy prices

• Indoor temperature

• Heating habits

• Wall construction

• Performance of the installed EWI system

The most reliable way to describe the benefit is that EWI reduces heat loss and therefore has the potential to reduce heating demand.  

Can External Wall Insulation Cause Damp?

External wall insulation should not be viewed as a treatment for existing damp.

If water is entering through damaged guttering, leaking pipework, defective masonry or another source, that problem should be identified and resolved before insulation is installed.

A correctly specified EWI system can actually help reduce the risk of condensation forming on internal wall surfaces because the insulation helps keep those surfaces warmer. 

However, moisture management needs to be considered as part of the whole building.

This is particularly important with older properties, where the original wall construction may rely on more vapour-permeable materials.

Does External Wall Insulation Work on Older Homes?

Yes, external wall insulation can be suitable for older properties, particularly homes with solid walls that cannot be insulated using conventional cavity wall insulation.

Older homes do require greater consideration.

Before choosing an EWI system, it is worth establishing:

• What the walls are constructed from

• Whether they are solid or cavity walls

• How the existing building manages moisture

• Whether there is existing damp

• Whether the property is listed or in a conservation area

• Which insulation and finish are compatible with the building

For traditional properties, a more vapour-permeable insulation and finish may sometimes be appropriate.

Does External Wall Insulation Protect the Existing Wall?

An EWI system also creates a new external layer over the original façade.

Once correctly installed, the insulation and protective finish help shield the underlying wall from direct exposure to the weather.

This can be particularly useful where a property is already due for external refurbishment.

Rather than carrying out façade repairs and then considering insulation separately at a later date, it may sometimes make sense to coordinate the work as one project.

Does External Wall Insulation Change How a House Looks?

Yes.

Because the insulation is installed on the outside of the property, it needs a new external finish.

This means EWI can provide an opportunity to improve the appearance of the house at the same time as its thermal performance.

Depending on the specified system, finishes can include:

• Silicone render

• Textured render

• Dry dash

• Brick-effect finishes

• Brick slips

• Suitable cladding systems

For homeowners with tired render, inconsistent brickwork or an exterior already requiring renovation, this can be an additional benefit.

Does External Wall Insulation Reduce Room Size?

No.

Because the insulation is installed externally, it does not take usable space away from the rooms inside your home.

The trade-off is that the outside of the building becomes thicker, so areas such as window sills, rooflines and external services need to be considered as part of the installation.

Is External Wall Insulation Worth It?

Whether external wall insulation is worth it ultimately depends on the property.

It may be particularly worth investigating if:

• Your home has uninsulated solid walls

• External walls feel cold during winter

• You are already planning external refurbishment

• Your existing render needs replacing

• You want to improve thermal performance without reducing internal room sizes

• You want to update the appearance of the property

EWI is a more substantial project than measures such as draught-proofing or adding loft insulation, so it should be properly planned.

The cost also needs to be considered alongside the expected thermal improvement, condition of the property and any other work already required.

How Can You Avoid Problems with External Wall Insulation?

The best way to approach EWI is to start with the building, not the products.

Before work begins:

1. Assess the existing property.
   Understand the wall construction and identify any existing defects or moisture problems.

2. Choose a suitable insulation system.
   EPS, Mineral Wool and other insulation materials have different characteristics.

3. Establish the correct insulation thickness.
   Base this on the thermal performance required rather than guessing.

4. Use a complete compatible system.
   Insulation, adhesives, fixings, reinforcement and finishes should all be specified to work together.

5. Plan the difficult details in advance.
   Consider windows, doors, rooflines, and ground-level junctions before installation begins.

6. Use an appropriately trained installer.
   External solid wall insulation should be fitted by a specialist installer trained by an approved system designer. 

Taking this approach can help avoid many of the problems associated with poorly planned EWI projects.

Where Can You Buy External Wall Insulation?

We supply complete external wall insulation materials, including:

• EPS insulation

• Mineral Wool insulation

• Phenolic insulation

• Adhesives and basecoats

• Mechanical fixings

• Fibreglass mesh

• Beads and profiles

• Primers

• Silicone render

• Other decorative finishes

You can explore External Wall Insulation Systems at EWI Store to compare insulation and system components.  

For homeowners or installers who are unsure which materials are required, our buyer's guides also provide complete build-ups for different insulation types and wall substrates. 

So, Does External Wall Insulation Work?

Yes, external wall insulation works when it is suitable for the building, correctly specified and installed properly.

The principle is straightforward: adding insulation to the outside of the building reduces the rate at which heat passes through the external walls. For an uninsulated home, this can address a significant source of heat loss. 

However, the insulation board alone does not determine whether the project will be successful.

The condition of the existing wall, insulation material, thickness, fixings, reinforcement, moisture management, detailing and quality of installation all play a part.

That is why external wall insulation should always be approached as a complete system designed around the property, rather than simply adding insulation boards to the outside of a house.