If you are planning to insulate the outside of your home, you will quickly discover that there is more than one type of external wall insulation available. EPS, Mineral Wool, phenolic insulation and Wood Fibre can all be used within suitable external wall insulation systems, so which is the best type of external wall insulation?
There is no single insulation material that is best for every home.
For many standard residential projects, EPS is often the practical starting point because it combines good thermal performance with a relatively lightweight and cost-effective board. Mineral Wool may be a better choice where fire performance, breathability or acoustic performance are important. Phenolic insulation can be useful where space is limited, while Wood Fibre may suit projects where a more natural, vapour-permeable build-up is required.
The best external wall insulation is therefore the one that suits your property, your priorities and the complete system being installed.
What Is External Wall Insulation?
External wall insulation, or EWI, involves fitting insulation to the outside of a property before covering it with a reinforced protective and decorative finish.
Rather than taking up space inside your rooms, the insulation creates an additional thermal layer around the outside of the building.
A typical rendered EWI system includes insulation boards alongside adhesives, mechanical fixings, reinforcement, primer and a final render finish.
External wall insulation can help reduce heat loss through the walls, improve comfort inside the home and renew the appearance of the exterior at the same time.
But the insulation board itself is only one part of the system, which is why choosing the right material matters.
Which Is the Best External Wall Insulation for Most Homes?
For many conventional residential external wall insulation projects, graphite EPS is often a good all-round option.
EPS stands for expanded polystyrene. The graphite-enhanced EPS used for external wall insulation is lightweight, thermally efficient and comparatively easy to handle and cut.
It is also generally one of the more cost-effective external wall insulation options, which has helped make it a popular choice for residential EWI projects.
EPS can therefore be a strong option if you are looking for a balance between:
- Thermal performance
- Cost
- Ease of installation
- Weight
- Availability
- A relatively straightforward residential EWI build-up
However, being the most practical option for many homes does not make EPS automatically suitable for every property.
If factors such as fire performance, breathability, acoustic performance or limited space are particularly important, another insulation may be a better choice.
Is EPS the Best Type of External Wall Insulation?
EPS can be one of the best external wall insulation options for a typical residential project where cost and thermal performance are the main priorities.
Graphite EPS is designed to reduce heat transfer through external walls while remaining lightweight enough to handle easily during installation.
It can also be cut relatively easily around building details, making it practical for many houses.
This makes EPS particularly worth considering for:
Standard houses being upgraded with EWI, homeowners working to a more budget-conscious specification, projects requiring a lightweight insulation board and properties where there are no specific requirements that point towards a different insulation material.
The main thing to remember is that EPS still needs to form part of the correct external wall insulation system.
The board should not simply be chosen based on price and then combined with unrelated adhesives, fixings or renders.
Is Mineral Wool the Best External Wall Insulation?
For some properties, Mineral Wool may be the better option.
Mineral Wool external wall insulation boards are made from stone wool and offer a different combination of properties to EPS.
One of their major advantages is fire performance. The Rockwool External Wall Dual Density Slabs currently supplied by Insulation shop EWI Store have a Euroclass A1 reaction-to-fire classification, offering a strong option for projects where fire performance is an important consideration. They are also vapour permeable and provide acoustic benefits.
This makes Mineral Wool particularly useful where:
Fire performance is a major consideration, a more vapour-permeable wall build-up is required, reducing external noise is important, or the project specification specifically calls for Mineral Wool.
The material is denser and heavier than EPS, so installation is different. It requires suitable adhesives, mechanical fixings and a system designed specifically around Mineral Wool.
So is Mineral Wool better than EPS?
Not automatically.
It provides advantages in certain areas, but those advantages need to matter to the project. If you are insulating a straightforward residential property where cost and ease of installation are the main concerns, EPS may still be the more appropriate option.
Is Mineral Wool Better for Older Homes?
Mineral Wool can be a good option where breathability and moisture movement are important considerations.
Its open structure allows water vapour to move through the material more readily than some more closed-cell insulation products.
This can make it useful within appropriately designed vapour-permeable systems.
However, simply choosing a breathable insulation does not automatically make an EWI system suitable for an old property.
Older buildings can contain different masonry, lime mortars and other traditional materials, so the complete wall construction needs to be understood before insulation is added.
Where a building is particularly old or moisture-sensitive, Wood Fibre may also be worth considering.
Is Wood Fibre the Best External Wall Insulation for Older Properties?
Wood Fibre insulation can be particularly well suited to older and traditional buildings where a more vapour-permeable build-up is required.
Wood Fibre boards are manufactured from wood fibres and are commonly associated with natural, breathable insulation systems.
Rather than simply trying to stop moisture from moving through the wall, a compatible Wood Fibre system can allow water vapour to move through the construction.
This can make Wood Fibre worth considering for:
Traditional properties, renovation projects, more moisture-sensitive wall constructions and projects where using a natural insulation material is an important priority.
EWI Store’s existing insulation guidance identifies Wood Fibre as a particularly useful option for projects where breathability and a more natural system are key considerations.
Wood Fibre does not necessarily provide the highest thermal performance for the thinnest possible board, so it is normally chosen because its wider characteristics suit the building.
In other words, it is a good example of why the best external wall insulation is not always simply the material with the best thermal figure.
What Is the Best External Wall Insulation When Space Is Limited?
If space is a major restriction, phenolic insulation such as Kingspan Kooltherm K5 can be particularly useful.
K5 is a rigid phenolic insulation board designed to provide strong thermal performance without requiring as much thickness as some alternative insulation materials.
We currently list a thermal conductivity of 0.021 W/mK for K5 boards 45mm thick and above, which helps it provide substantial thermal resistance within a relatively slim build-up.
This can be useful where thicker external insulation would create difficulties around areas such as:
Narrow side passages, window sills, rooflines, boundaries, door openings or other locations where there is limited room for the EWI system.
That does not mean everyone should use K5 simply because it can achieve good thermal performance with a thinner board.
It is a premium insulation product, so budget and complete system compatibility also need to be considered.
What About XPS Insulation?
You may also see XPS, or extruded polystyrene, when researching external wall insulation.
XPS is a dense, moisture-resistant insulation board, but it is often used for particular areas of an EWI system rather than as the main insulation across the entire façade.
For example, XPS can be useful around plinths and below the damp-proof course, where insulation is closer to ground level and exposed to different moisture conditions.
It can also be used for specialist details such as extending certain window sill arrangements or where a denser insulation material is required.
So while XPS has an important role within external wall insulation, it should not necessarily be compared directly with EPS, Mineral Wool, Wood Fibre and K5 as though they all perform exactly the same job.
Which External Wall Insulation Has the Best Thermal Performance?
If your only priority is achieving strong thermal performance with the smallest possible thickness, phenolic insulation such as K5 is one of the strongest options within the EWI Store range. Its lower thermal conductivity means less thickness can be required to provide a similar level of thermal resistance.
However, thermal performance is only one part of choosing insulation.
For example, Mineral Wool may require a thicker build-up but provides benefits in fire performance, breathability and acoustics. EPS may provide a better balance between thermal performance and cost. Wood Fibre may offer characteristics that better suit a traditional building.
This is why comparing insulation purely by how thin the board can be is rarely the best approach.
Which External Wall Insulation Is Best for Fire Performance?
Where non-combustibility is a key consideration, Mineral Wool has a clear advantage.
Rockwool External Wall Dual Density Slabs supplied by EWI Store achieve a Euroclass A1 reaction-to-fire classification, making them a strong option for projects where fire performance is an important consideration.
Fire requirements can also depend on the type, height and use of the building, so insulation should always be selected as part of the appropriate system specification rather than relying on one product characteristic alone.
For homeowners working on a conventional low-rise residential property, your installer or system designer can advise what requirements apply.
Which External Wall Insulation Is Best for Sound?
Mineral Wool is also a strong option where acoustic performance is particularly important.
Its fibrous, open structure can help absorb sound, which can make it useful for properties in busy or noisy locations.
This might be worth considering if your home is close to:
A busy road, railway, commercial area or another regular source of external noise.
This does not mean external wall insulation should be installed solely as soundproofing, but acoustic performance can be an additional consideration when choosing between insulation materials.
Which External Wall Insulation Is Best for Breathability?
If breathability is a major part of the project, Mineral Wool and Wood Fibre are two of the main options worth considering.
Both can form part of vapour-permeable EWI systems when combined with compatible layers and finishes. EWI Store’s existing guidance highlights breathability as one of the main reasons these materials may be chosen instead of EPS.
This can be especially important with some older buildings.
However, breathability applies to the whole wall system, not just the insulation board.
Putting a highly vapour-permeable insulation behind incompatible layers can undermine the reason it was selected in the first place.
Which External Wall Insulation Is the Most Cost-Effective?
For many standard residential projects, EPS is generally the most budget-friendly starting point.
EWI Store’s existing comparison guidance identifies EPS as a popular choice because it combines good thermal performance with value and straightforward installation.
But looking at the board price alone does not tell you the complete project cost.
Different insulation materials can require different:
Adhesives, fixing types, fixing quantities, board thicknesses, accessories and installation methods.
It therefore makes more sense to compare the cost of the complete external wall insulation system rather than simply comparing the price of individual boards.
Is the Best External Wall Insulation Always the Thickest?
No.
Board thickness and insulation type work together.
A thinner board made from a material with stronger thermal performance could provide similar thermal resistance to a thicker board made from another material.
But thickness also needs to be chosen according to the thermal performance required from the wall.
This means 50mm, 90mm or 100mm cannot automatically be described as the right thickness without knowing the property and insulation material.
If you are trying to decide how much insulation your property actually needs, our guide “What Thickness of Insulation Board Do I Need?” explains this in more detail.
Does the Existing Wall Affect Which External Wall Insulation Is Best?
Yes.
You should always start with the building rather than the insulation board.
The right insulation can depend on:
The wall construction, age of the property, condition of the substrate, existing moisture issues, thermal performance required, available space, fire requirements and the final finish being installed.
External Wall Insulation Is a Complete System
One of the most important things to remember when comparing insulation is that the insulation board does not work alone.
A rendered external wall insulation system normally combines several compatible components.
The insulation affects which adhesive, fixings and basecoat are required, while the reinforcement, primer and final finish also need to work with the chosen build-up.
EWI Store’s current insulation guidance therefore recommends selecting the board with the complete system in mind rather than treating insulation as a standalone purchase.
EWI Pro also recommends obtaining a full system specification where warranties or compliance are required.
Can External Wall Insulation Really Make a Difference?
For suitable homes, yes.
External wall insulation adds thermal resistance to the outside of the existing walls, helping reduce heat loss and keep more warmth inside.
Energy Saving Trust estimates that around one-third of heat lost from an uninsulated home can escape through its walls, so insulating suitable solid walls can address a significant source of heat loss.
Where Can You Buy External Wall Insulation?
EWI Store supplies a range of external wall insulation materials for different property types and system requirements, including EPS, Mineral Wool, phenolic insulation and specialist insulation boards.
You can also purchase the other components needed to create a complete system, including adhesives, mechanical fixings, fibreglass mesh, basecoats, primers and decorative renders.
Before ordering, make sure the insulation is appropriate for the property and that the complete build-up has been considered.
For projects where the correct specification is unclear, speak to the EWI Store team before choosing the insulation material or thickness.
So, Which Is the Best Type of External Wall Insulation?
There isn’t one best type of external wall insulation for every property.
For many typical residential EWI projects, graphite EPS is a strong all-round choice because it combines thermal performance, low weight and affordability.
But other materials may be better suited to particular requirements.
Mineral Wool is worth considering where fire performance, breathability or acoustic performance are priorities.
Kingspan Kooltherm K5 can be particularly useful when you need strong thermal performance but have limited space for the insulation.
Wood Fibre can be appropriate for older, traditional or more moisture-sensitive buildings where a natural, vapour-permeable system is preferred.
And XPS has an important role in specific areas such as ground-level and below-DPC detailing.
The best approach is therefore not to ask:
“Which insulation board has the best specification?”
Instead, ask:
“Which insulation system best suits my home?”
Once you understand the property, required thermal performance and practical constraints, choosing the right external wall insulation becomes much easier.
