Right Insulation Thickness for Solid Wall External Insulation

Insulating solid brickwork walls plays a pivotal role in enhancing energy efficiency and ensuring compliance with building regulations. Understanding the appropriate thickness of insulation required for your project is crucial to achieve the desired thermal performance. Let’s delve into the specifics to guide you through this crucial decision-making process.

Building Regulation Standards

In the UK, meeting the stipulated maximum U-value for external walls, set at 0.30W/m²K by building regulations, is essential. This U-value represents the rate of heat loss through a material; the lower the value, the better the insulation’s thermal performance.

Insulation Thickness Guidelines

Expanded Polystyrene (EPS) Insulation

Achieving the targeted U-value of 0.30W/m²K for solid brickwork walls often entails installing EPS insulation with a thickness of approximately 90mm². This thickness ensures an effective barrier against heat transfer, meeting the required standard and contributing to energy efficiency.

Mineral Wool Insulation

Alternatively, mineral wool insulation presents another viable option. To meet the building regulations’ U-value of 0.30W/m²K, approximately 110mm of mineral wool insulation is necessary. This material offers excellent thermal properties, aiding in temperature regulation within the building.

Phenolic Insulation

When opting for phenolic insulation as part of an external wall insulation system, a thinner option is available. Utilizing a 60mm thickness of phenolic insulation can achieve the sought-after U-value of 0.30W/m²K. This lightweight yet effective solution ensures compliance while minimizing the overall thickness of the insulation layer.

Factors Influencing Choice

Several factors might influence the choice of insulation besides meeting regulatory standards:

  • Space Constraints: In scenarios where space is limited, opting for a thinner yet high-performing insulation material like phenolic might be advantageous.
  • Cost Considerations: While thicker insulation may offer better thermal efficiency, it’s essential to balance performance with cost-effectiveness.
  • Environmental Impact: Some insulation materials have varying environmental impacts. Considering eco-friendly options can align with sustainability goals.

Selecting the right insulation thickness for solid brickwork walls involves a balance between regulatory compliance, performance, and practical considerations. Whether it’s EPS, mineral wool, or phenolic insulation, each material offers distinct advantages in meeting the desired U-value requirements.

Consulting with insulation experts or utilising U-value calculators specific to your project can provide tailored guidance. Ultimately, making an informed decision ensures not just compliance but also optimal energy efficiency and comfort within the built environment.

What is a U-Value?

If you are interested in energy efficiency, you may have heard of the term U-Value. But what does it mean and why is it important? In this blog post, we will explain what a U-Value is, how it is measured, and how it affects the thermal performance of your home.

A U-Value is a measure of how much heat passes through a material or a structure. It is expressed in watts per square meter per degree Kelvin (W/m2K). The lower the U-Value, the better the insulation. For example, a wall with a U-Value of 0.2 W/m2K will lose less heat than a wall with a U-Value of 0.5 W/m2K.

U-values are important for designing energy-efficient buildings, as they indicate how much heat will be lost or gained through different parts of the building envelope. By choosing materials and structures with low U-values, we can reduce the need for heating and cooling, and save money and resources.

How are U-values calculated?

U-values are calculated by dividing the rate of heat transfer through a material or a structure by the temperature difference across it. For example, if a wall has a U-value of 0.3 W/m²K, it means that for every degree of temperature difference between the inside and the outside of the wall, 0.3 watts of heat will flow through each square meter of the wall.

To calculate the U-value of a composite structure, such as a wall with multiple layers, we need to add up the thermal resistances of each layer. Thermal resistance is the inverse of thermal conductivity, which is a property of each material that describes how easily heat can pass through it. The higher the thermal conductivity, the lower the thermal resistance, and vice versa.

For example, to calculate the U-value of a cavity wall with bricks, insulation and plaster, we would do the following:

– Find the thickness and thermal conductivity of each layer
– Calculate the thermal resistance of each layer by dividing the thickness by the thermal conductivity
– Add up the thermal resistances of all layers, including the internal and external surfaces
– Find the inverse of the total thermal resistance to get the U-value

The lower the U-Value, the better the insulation.

The image shows a cross-sectional view of a wall with different layers and their respective thermal resistances labeled. The U-Value of the wall can be calculated by finding the reciprocal of the sum of all the thermal resistances of each layer, including the inner and outer faces.

The formula is:


  • U is the U-Value in W/m²K
  • Rsi is the thermal resistance of the inner face in m²K/W
  • R1, R2, R3, and R4 are the thermal resistances of the first, second, third, and fourth components of the wall in m²K/W
  • Rso is the thermal resistance of the outer face in m²K/W

To find the thermal resistance of each component, you need to know the thickness and the thermal conductivity of the material. The thermal resistance is calculated by dividing the thickness by the thermal conductivity.
For example, if the first component is a brick wall with a thickness of 0.1 m and a thermal conductivity of 0.7 W/mK, then the thermal resistance is:
R1=0.70.1=0.143 m²K/W