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What Thickness of the Insulation Board Do I Need?

Last modified: 23.10.2026
What Thickness of the Insulation Board Do I Need?

If you are planning to insulate the outside of your home, one of the first questions you may have is: what thickness of insulation board do I need?

You will find external wall insulation boards available in a wide range of thicknesses, from relatively thin boards through to 100mm and beyond. While it can be tempting to assume that thicker insulation is always better, there is no single thickness that is right for every property.

The right insulation board thickness depends on several things, including what your existing walls are made from, which insulation material you choose and the level of thermal improvement you need to achieve.

For many conventional external wall insulation projects, thicknesses around 90mm to 120mm can give you an idea of the sort of build-up involved. However, this should only be treated as a guide. The correct thickness for your home needs to be worked out from the property itself. 

The purpose of insulation is to slow down the movement of heat through the walls of your home.

Generally, increasing the thickness of the same insulation material increases its thermal resistance. This means less heat can pass through the wall.

However, insulation thickness also affects the physical size of the finished wall.

Adding thicker external wall insulation may have an impact on:

  • Window sills

  • Window and door reveals

  • Roof overhangs

  • Eaves and soffits

  • Guttering and downpipes

  • External pipes and services

  • Vents

  • Boundaries between properties

So, the aim is not simply to fit as much insulation as possible. You need enough insulation to achieve the required thermal performance while still being able to detail the property correctly. 

There is no single standard external wall insulation thickness.

You will commonly see systems using insulation around:

  • 90mm

  • 100mm

  • 110mm

  • 120mm

However, these are not fixed recommendations.

One home might require 90mm of one insulation material, while another could require 120mm to achieve a similar result. A different insulation product may be able to provide comparable thermal performance using a thinner board.

The existing wall makes a difference too.

This is why you should not assume that the insulation thickness used on your neighbour's house will automatically be right for yours.

100mm insulation can be suitable for many external wall insulation projects, but the thickness alone does not tell you whether it is enough.

For example, EWI Pro's certified sample constructions show that 100mm of graphite EPS can achieve a U-value of 0.30 W/m²K on the example 200mm dense blockwork wall.

But on the example 215mm brick wall, the same target is achieved using 90mm of the same EPS insulation. 

The key point is that the insulation has not changed.

The wall underneath it has.

So rather than starting by asking whether 100mm is enough, the better question is:

What thickness does my particular wall need?

In some cases, yes.

The EWI Pro example mentioned above uses 90mm graphite EPS on a 215mm brick wall to achieve a U-value of 0.30 W/m²K.

On another wall construction, a different thickness may be required. 

This is why figures such as 90mm and 100mm are useful examples but should not be treated as universal specifications.

A calculation based on the actual property is a much more reliable way of selecting the insulation.

A 50mm insulation board can improve the thermal performance of a wall, but that does not necessarily mean it will provide the level of insulation required for a full external wall insulation upgrade.

EWI Pro's example EPS and Mineral Wool constructions require more than 50mm of insulation to achieve a U-value of 0.30 W/m²K on the example brick and dense blockwork walls. 

However, 50mm may still be appropriate in certain circumstances.

For example:

  • The wall may already perform relatively well

  • Space may be restricted

  • A higher-performing insulation material may be used

  • The project may have a different thermal target

  • A specific detail may require a thinner board

So, 50mm is not automatically too thin, but neither should it be chosen without knowing what you are trying to achieve.

Thinner insulation boards are available and can be useful for particular parts of a project.

For example, thinner boards may sometimes be required where there is limited room around a window reveal or another restricted detail.

However, if your main aim is to significantly improve the thermal performance of the main external walls, a very thin board is unlikely to provide the same improvement as a substantially thicker board of the same material.

This is why the main wall and smaller details do not necessarily need to use exactly the same thickness everywhere.

Yes, and this is one of the most important things to understand when choosing insulation.

Different insulation materials do not all perform in exactly the same way.

Three common options for external wall insulation are:

Graphite EPS is a popular choice for residential external wall insulation.

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

It is lightweight and offers good thermal performance, which makes it a practical option for many homes.

As an example, EWI Pro's certified sample calculations use:

  • 90mm EPS on example 215mm brickwork

  • 100mm EPS on example 200mm dense blockwork

to achieve the example target U-value of 0.30 W/m²K. 

These should not be treated as automatic specifications for every property.

Mineral Wool has different properties to EPS and may therefore require a different thickness.

EWI Pro's example constructions use:

  • 110mm Mineral Wool on the example 215mm brick wall

  • 120mm Mineral Wool on the example 200mm dense blockwork wall

for the same example target of 0.30 W/m²K. 

Mineral Wool may also be chosen where characteristics such as fire performance, vapour permeability and acoustic performance are particularly important.

So, choosing an insulation material is not simply a competition to find the thinnest board.

Where the available space is limited, a higher-performing insulation board may sometimes provide the required thermal resistance using a thinner build-up.

Kingspan Kooltherm K5 is one example used for external wall insulation. Its stated thermal conductivity is lower than that of graphite EPS and Mineral Wool, meaning it can provide greater thermal resistance at a given thickness. 

This can be useful where a thicker system would cause difficulties around areas such as window sills, rooflines or narrow access.

The required thickness should still be calculated for the particular wall.

Quite a lot.

External wall insulation is added to an existing wall, so the finished thermal performance comes from the complete combination of materials rather than the insulation board alone.

A solid brick wall and a dense blockwork wall, for example, do not necessarily start with the same thermal performance.

That means adding 100mm of insulation to each wall will not automatically give exactly the same result.

When choosing insulation thickness, you therefore need to know what you are starting with.

If you are unsure which insulation material is best for your property in the first place, our guide to choosing the right external wall insulation board for your property looks at the different options in more detail. 

You may come across the term U-value when researching insulation thickness.

A U-value is simply a way of measuring how easily heat passes through part of a building, such as an external wall.

The lower the U-value, the better the wall is at resisting heat loss.

You do not need to calculate this yourself to understand the basic principle.

Your existing wall has a certain level of thermal performance. Adding insulation improves it. The thickness and type of insulation determine how much additional thermal resistance is added.

A U-value calculation brings these factors together and helps establish what insulation specification is required.

This is another reason why it is better to work backwards from the required performance rather than simply deciding on a board thickness.

For relevant renovation work to existing homes in England, current Approved Document L guidance gives an improved U-value of 0.30 W/m²K for existing walls receiving internal or external insulation, subject to the circumstances and requirements of the project. Building Regulations differ across the UK, so the requirements for an individual property need to be checked. 

Importantly, that does not mean there is one insulation thickness that automatically satisfies Building Regulations.

The thickness required to reach a particular U-value depends on the insulation and wall construction.

From a purely thermal point of view, adding more of the same insulation generally improves thermal resistance.

But there is a practical limit to how useful simply making the system thicker becomes.

Imagine adding a significant depth of insulation to the outside of your home.

The existing window sills may no longer project far enough beyond the new façade. Roof overhangs may become more difficult to detail. External pipework may need moving and windows can appear increasingly recessed.

A thicker system may therefore require:

  • Extended window sills

  • Longer mechanical fixings

  • Changes to roofline details

  • Adjustments to external services

  • Deeper window and door reveals

For this reason, EWI should be designed as a complete system rather than selecting the insulation board in isolation. 

Sometimes the property itself limits how thick the EWI system can be.

This might happen where there are:

  • Narrow paths alongside the building

  • Boundaries close to the wall

  • Short roof overhangs

  • Restricted window sill projections

  • Narrow window or door reveals

In these situations, a higher-performance insulation material may be worth considering because it may provide greater thermal resistance using a thinner board.

However, it is important not to simply reduce the insulation thickness until it fits.

The thermal performance still needs to be checked.

Yes.

External wall insulation sits on the outside of the existing wall, so the finished façade moves further out.

As a result, your windows may look slightly more recessed after EWI is installed.

The thicker the system, the greater this change becomes.

Window sills often need to be extended so they continue to project past the finished render and direct rainwater away from the façade.

Window and door reveals also need careful consideration so that the insulation remains as continuous as possible around the opening.

These are normal parts of EWI design, but they should be planned before materials are ordered.

Rooflines are another reason why simply choosing the thickest insulation board is not always the best approach.

Once the finished wall projects further out, you need to make sure the top of the insulation can be correctly protected by the roof or another suitable detail.

Gutters, downpipes and other services may also need to be repositioned.

These are practical issues rather than reasons to avoid EWI, but they demonstrate why the entire property needs to be considered when choosing the insulation thickness.

You can think of the process in five simple steps.

Start with the existing property.

Is it solid brick, blockwork or another construction?

This gives you the starting point for understanding how the wall currently performs.

Are you carrying out a full thermal upgrade? Are Building Regulations involved? Is the project being completed to a particular specification?

The required performance needs to be established before the board thickness.

Decide whether EPS, Mineral Wool, phenolic insulation or another suitable material is appropriate for the property.

Different insulation products provide different levels of thermal resistance for the same thickness.

Once the wall construction, insulation and required performance are known, the appropriate thickness can be established through a U-value calculation.

EWI Pro's more technical guide to how thick external wall insulation should be explains this process and provides example calculations for different wall constructions. 

Finally, make sure the proposed thickness can be properly detailed around windows, rooflines, doors, services and ground level.

This gives you a solution that works both thermally and practically.

If you need to achieve a particular thermal target, a project-specific U-value calculation is the best way to establish how much insulation is required.

It removes much of the guesswork.

Rather than saying:

"I'll use 100mm because that seems to be the normal thickness."

you can establish the actual wall construction and work out what is required to achieve the intended performance.

EWI Pro specifically recommends project-specific calculations rather than treating sample thicknesses as universal specifications. 

Usually, these decisions need to be considered together.

The insulation material affects how much thickness is needed, while practical restrictions around the property may influence which material is most suitable.

For example, EPS might be an ideal choice for one property, while another may benefit from Mineral Wool or a thinner, higher-performing insulation.

If you are still deciding which board to use, read our guide to How to Choose the Right External Wall Insulation Board for Your Property before choosing the thickness. 

Where Can You Buy External Wall Insulation Boards?

EWI Store supplies insulation boards and complete external wall insulation materials for a range of projects.

Depending on the system, options include graphite EPS, Mineral Wool and high-performance insulation boards alongside the adhesives, fixings, reinforcement and render materials required to complete the installation.

If you already know that EPS is appropriate for your project, you can shop EPS Insulation Boards at EWI Store in a range of thicknesses. 

If you are unsure about the material or thickness, it is worth confirming the complete EWI specification before ordering.

There is no single insulation board thickness that is right for every home.

For many conventional EWI projects, you may see insulation around 90mm to 120mm, but your property could require something thinner or thicker depending on the wall, insulation material and required thermal performance. 

The most important thing is not to choose the thickness by guesswork.

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