For many external wall insulation projects, an insulation thickness of around 90mm to 120mm may be appropriate, but there is no single thickness that works for every property.

The correct thickness depends on several factors, including the existing wall construction, the insulation material, its thermal conductivity and the U-value the completed wall needs to achieve.

For example, our BBA-certified EPS system includes a sample calculation where 90mm of EPS with a declared thermal conductivity of 0.032 W/mK achieves a U-value of 0.30 W/m²K on a 215mm brick wall. On the sample 200mm dense blockwork construction, 100mm is required to achieve the same U-value.

These figures are useful examples, but they should not be treated as universal specifications. A project-specific U-value calculation should be used to determine the appropriate insulation thickness.

What Is the Typical Thickness of External Wall Insulation?

You will frequently see external wall insulation installed at around 90mm, 100mm, 110mm or 120mm, particularly on existing solid-wall properties.

However, the exact thickness required depends on what you are starting with and what you are trying to achieve.

Our certified EPS and Mineral Wool systems provide some useful examples for a target U-value of 0.30 W/m²K:

Example wall construction	Insulation	Example insulation thickness
215mm brickwork	EPS, λ 0.032 W/mK	90mm
215mm brickwork	Mineral Wool, λ 0.036 W/mK	110mm
200mm dense blockwork	EPS, λ 0.032 W/mK	100mm
200mm dense blockwork	Mineral Wool, λ 0.036 W/mK	120mm

These values come from the sample constructions within our BBA Agrément certificates. The final U-value can be influenced by factors including insulation thickness, the substrate, fixing method and the thermal performance of the existing wall and internal finish.

Therefore, 90mm of EPS does not automatically achieve 0.30 W/m²K on every building, and neither does 110mm of Mineral Wool.

The existing wall needs to be considered first.

Is 90mm External Wall Insulation Enough?

In some cases, yes.

Our BBA-certified EPS system includes a sample construction where 90mm of EPS at 0.032 W/mK achieves a U-value of 0.30 W/m²K on 215mm brickwork.

However, changing the wall construction changes the result.

On the sample 200mm dense blockwork construction within the same certificate, 100mm of EPS is required to achieve 0.30 W/m²K.

This demonstrates why insulation thickness should not be selected in isolation.

If somebody asks whether 90mm external wall insulation is enough, the answer is:

It can be, but the existing wall construction and required U-value need to be considered first.

Is 100mm External Wall Insulation Enough?

100mm is a commonly considered external wall insulation thickness, and it may be suitable for many projects.

Again, though, the thickness alone does not tell us what U-value the finished wall will achieve.

Within our BBA sample calculations, 100mm of EPS at 0.032 W/mK achieves 0.30 W/m²K on the example 200mm dense blockwork wall. On the example 215mm brick wall, the same target is achieved using 90mm.

Whether 100mm is appropriate for a particular project will depend on factors such as:

• the existing wall construction
• the existing thermal performance
• the insulation material
• the required U-value
• thermal bridging and detailing
• the complete EWI system specification.

This is why we recommend working backwards from the required thermal performance, rather than simply choosing a 100mm board because it is a commonly used thickness.

How Thick Should Graphite EPS External Wall Insulation Be?

Graphite EPS is one of the main insulation options used within our external wall insulation systems.

The EPS currently supplied through EWI Store contains graphite and has a stated thermal conductivity of 0.032 W/mK.

Thermal conductivity tells us how readily heat passes through a material. A lower figure indicates better thermal resistance for a given thickness.

Our BBA-certified EPS system also uses a declared thermal conductivity of 0.032 W/mK in its sample calculations.

For a target U-value of 0.30 W/m²K, these examples show:

• 90mm of EPS on the sample 215mm brickwork construction
• 100mm of EPS on the sample 200mm dense blockwork construction.

This difference is important because the insulation product has not changed; the wall underneath it has.

That is why we cannot say that every property simply needs 90mm or 100mm of EPS.

How Thick Should Mineral Wool External Wall Insulation Be?

Mineral Wool has different thermal properties to EPS and may therefore require a different thickness to achieve the same U-value.

The Rockwool External Wall Dual Density Slab supplied through EWI Store has a stated thermal conductivity of 0.036 W/mK. It also provides an A1 reaction-to-fire classification, alongside acoustic and vapour-permeable properties.

Our BBA-certified Mineral Wool system uses the same declared thermal conductivity of 0.036 W/mK.

For a target U-value of 0.30 W/m²K, the sample calculations within our certificate show:

• 110mm of Mineral Wool on the sample 215mm brick wall
• 120mm of Mineral Wool on the sample 200mm dense blockwork wall.

These examples are slightly thicker than the equivalent EPS examples because the materials have different thermal conductivities.

However, thickness is only one consideration when choosing an insulation material.

Mineral Wool may be particularly appropriate where factors such as fire performance, vapour permeability and acoustic performance are important to the project.

The best insulation is therefore not automatically the material that achieves the target U-value using the thinnest board.

Can You Use Thinner External Wall Insulation?

Where the available build-up is restricted, a higher-performing insulation material can sometimes provide the required thermal resistance using a thinner board.

Kingspan Kooltherm K5 is one example.

K5 has a phenolic insulation core and a declared thermal conductivity of:

• 0.022 W/mK for boards 25-44mm thick
• 0.021 W/mK for boards 45mm and thicker.

This is lower than the stated thermal conductivity of graphite EPS at 0.032 W/mK and Mineral Wool at 0.036 W/mK.

As a result, K5 can provide greater thermal resistance for a given thickness and can be useful on projects where the overall EWI build-up needs to be reduced.

This may include situations where there are restrictions around:

• window and door reveals
• existing sill projections
• roof overhangs
• narrow passageways
• adjoining boundaries.

However, we would not recommend assuming that a particular thickness of K5 will achieve a specific U-value without carrying out a calculation.

The complete wall construction still needs to be assessed.

Is 50mm External Wall Insulation Enough?

50mm of external wall insulation can improve the thermal resistance of a wall, but improving a wall is not necessarily the same as achieving the required U-value.

For comparison, the sample calculations within our certified EPS and Mineral Wool systems require more than 50mm to achieve 0.30 W/m²K on the brickwork and dense blockwork examples discussed above.

That does not mean 50mm can never be appropriate.

A thinner specification may be considered where:

• the existing wall already performs relatively well
• a higher-performance insulation material is being used
• a different thermal target applies
• physical constraints limit the available build-up
• a project-specific assessment supports a different specification.

The appropriate thickness should be determined through calculation rather than by choosing a board size first.

Does External Wall Insulation Need to Achieve 0.30 W/m²K?

For relevant renovation work to existing dwellings in England, Approved Document L gives an improved U-value of 0.30 W/m²K for existing walls receiving internal or external insulation.

It also gives external insulation with a rendered finish or cladding as an example approach for renovating an external wall to this target.

However, this should not be interpreted as meaning that every EWI project must achieve exactly 0.30 W/m²K in every circumstance.

Current guidance also considers whether achieving the improved value is technically and functionally feasible and whether the work satisfies the relevant payback criteria.

Building Regulations differ across the UK, so the requirements applicable to an individual project should always be checked.

What About the 2026 Approved Document L?

A new 2026 edition of Approved Document L has already been published.

At the time of writing, it has not yet replaced the current guidance for most building work in England. The new edition generally comes into effect from 24 March 2027, subject to the relevant transitional arrangements.

Importantly for external wall insulation, the new edition continues to give an improved U-value of 0.30 W/m²K for an existing wall receiving internal or external insulation.

For a broader explanation of insulation thickness and thermal requirements, see our guide How Much Insulation is Needed to Satisfy Building Regulations?

What Happens When External Wall Insulation Gets Thicker?

Increasing insulation thickness generally increases the thermal resistance of the wall and reduces its U-value.

However, a thicker EWI system also affects the way the insulation integrates with the rest of the building.

Additional consideration may be needed around:

• window and door reveals
• window sill projections
• roof overhangs and eaves
• soffits
• external pipes and services
• vents
• boundaries
• adjoining properties
• mechanical fixing lengths
• junctions with other building elements.

Approved Document L also highlights the need to consider the technical risk and impact of increased wall thickness on adjoining buildings when external insulation is installed.

This is one reason why simply choosing the thickest insulation available is not necessarily the best approach.

The aim is to achieve the required thermal performance with a system that can also be properly detailed and installed on the building.

Can External Wall Insulation Be Too Thick?

There is no single thickness at which external wall insulation suddenly becomes “too thick”.

However, the thermal benefit and practical impact need to be balanced.

Increasing insulation thickness will improve thermal resistance, but as the system becomes deeper, detailing around windows, roofs, services and boundaries can become more complicated.

The specification may also require:

• longer mechanical fixings
• extended window sills
• altered roofline details
• changes around external services
• deeper window and door reveals.

External wall insulation should therefore be considered as a complete system, rather than treating the insulation board as an isolated product.

Our systems include compatible insulation, basecoats, mechanical fixings, fibreglass mesh, primers and finishes, alongside the detailing required to create a complete EWI build-up.

How Do You Work Out the Correct EWI Thickness?

The best approach starts with the existing wall rather than the insulation board.

1. Identify the wall construction

The first step is establishing how the external wall is built.

Solid brickwork, dense blockwork and other masonry constructions do not necessarily have the same thermal performance.

2. Establish the existing thermal performance

The materials and thicknesses within the existing wall influence its starting U-value.

3. Determine the required performance

The target U-value will depend on the project, applicable Building Regulations and any additional specification requirements.

4. Choose the insulation material

Graphite EPS, Mineral Wool and Kingspan K5 have different thermal conductivities and different performance characteristics.

5. Calculate the required thickness

Once the existing wall and insulation material are known, a U-value calculation can be used to determine the insulation thickness required as part of the complete wall build-up.

6. Check the complete system

Thermal performance is only one part of EWI design.

The substrate, moisture behaviour, fire requirements, wind loading, fixing design, detailing and compatibility of the system components also need to be considered.

So, How Thick Should External Wall Insulation Be?

There is no universal external wall insulation thickness.

For many conventional retrofit projects, around 90mm to 120mm may give a useful indication of the sort of thickness involved, but the actual specification can be thinner or thicker depending on the wall construction, insulation material and thermal target.

Our certified sample constructions demonstrate this clearly.

For a target U-value of 0.30 W/m²K:

• EPS at 0.032 W/mK requires 90mm on the sample 215mm brick wall and 100mm on the sample 200mm dense blockwork wall
• Mineral Wool at 0.036 W/mK requires 110mm on the sample 215mm brick wall and 120mm on the sample 200mm dense blockwork wall.

Higher-performance insulation such as Kingspan K5 can provide greater thermal resistance for a given thickness and may therefore be useful where build-up depth is restricted.

The most important point is simple:

Do not choose external wall insulation thickness by guesswork.

Start with the existing wall, establish the required thermal performance and calculate the appropriate insulation specification from there.

Our technical team can support you with external wall insulation system specification, system build-ups and project requirements.

Once the appropriate system has been established, you can also use our Materials Calculator to estimate the quantities required for your project.

External Wall Insulation Thickness FAQs

What is the standard thickness of external wall insulation?

There is no single standard thickness. Around 90–120mm is common in many EWI specifications, but the required thickness depends on the wall construction, insulation material and thermal target.

Is 90mm EPS enough for external wall insulation?

It can be. Our BBA-certified EPS system includes a sample 215mm brick wall where 90mm of EPS at 0.032 W/mK achieves 0.30 W/m²K. On the sample dense blockwork wall, 100mm is required to achieve the same value.

These are example calculations rather than universal specifications.

Is 100mm external wall insulation enough?

Potentially. Whether 100mm is sufficient depends on the insulation material, existing wall construction and required U-value.

A U-value calculation should be used rather than assuming that 100mm will achieve the required thermal performance.

How thick should Mineral Wool external wall insulation be?

Our BBA-certified Mineral Wool system includes sample calculations where 110mm of Mineral Wool at 0.036 W/mK achieves 0.30 W/m²K on 215mm brickwork, while 120mm is required on the sample dense blockwork construction.

The required thickness for an individual property should still be calculated.

Which external wall insulation is best where space is limited?

Kingspan Kooltherm K5 has a thermal conductivity as low as 0.021 W/mK for boards 45mm and thicker, allowing greater thermal resistance to be achieved within a thinner build-up than some other insulation materials.

However, the complete system requirements and wall construction should still be considered before specification.

What U-value should external wall insulation achieve?

For relevant renovation work to existing dwellings in England, current Approved Document L guidance gives an improved U-value of 0.30 W/m²K for an existing wall receiving internal or external insulation.

Project-specific requirements, feasibility and the regulations applying to the work should always be checked.