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How to Insulate Masonry Walls

Masonry walls, commonly found in older homes and buildings, are known for their durability and aesthetic appeal. However, they often fall short regarding energy efficiency, particularly in insulation. Learning how to insulate masonry walls can significantly reduce energy bills, enhance comfort, and improve a property’s overall environmental footprint.

Assess the existing masonry walls

Before beginning any insulation project, it’s crucial to assess the type and condition of the masonry walls. Look for any signs of dampness, cracks, or structural issues. If not properly addressed, masonry walls are prone to moisture, leading to insulation degradation.

Most porous building materials have a persistent damp problem. A moisture meter can detect the saturation level of an existing masonry wall and dictate the choice of insulating material. Moisture must not be trapped behind the insulation; therefore, breathable solutions are often necessary. Trapped moisture degrades materials and leads to mould.

Cracks vary in size, impact, and solution. Minor cracks can be patched more easily; however, larger cracks impact structural integrity.

Types of masonry walls

Understanding the specific type of masonry wall you are dealing with is essential for choosing the correct approach to insulation.

  • Load-bearing Masonry Wall: This traditional type of wall supports the weight of the floors and roof above. It is often made of solid bricks or stones and is particularly common in older buildings. Insulating these walls requires careful consideration to avoid trapping moisture and compromising the wall’s structural integrity.
  • Reinforced Masonry Wall: Reinforcement bars embedded within the masonry units strengthen these walls, providing additional structural stability. Insulation methods must account for these reinforcements to ensure they do not adversely affect the wall’s performance.
  • Hollow/Cavity Masonry Wall: This is a more modern construction style. These walls have a gap (cavity) between two layers of brick or block. This cavity can be filled with insulation material, making it an efficient way to improve thermal performance without significantly altering the wall structure.
  • Composite Masonry Wall: These use different materials, such as hollow blocks with concrete or bricks combined with stones. The varied structure can affect the insulation choice, as each material may have different thermal properties and moisture-handling capabilities.
  • Post-tensioned Masonry Wall: Designers incorporate tensioned steel bars after constructing the masonry to make these walls withstand additional loads. Special care is necessary to insulate these walls to ensure the tensioning system remains undisturbed.

Types of materials used to build masonry walls

  1. Concrete Blocks: Known for their strength and durability. Concrete blocks can be solid or hollow, with hollow blocks often used to improve insulation. These blocks are ideal for load-bearing walls and are frequently used in residential and commercial construction.
  2. Aerated Concrete Blocks (AAC): Lightweight and easy to work with, AAC blocks contain millions of tiny air pockets, giving them excellent thermal and sound insulation properties. They are ideal for interior partitions and exterior walls where weight reduction is a priority.
  3. Cinder Blocks: Made from coal cinders and cement, these blocks are lighter than standard concrete blocks. While not as strong as other types, they provide good insulation and are often used for non-load-bearing walls in basements and garages.
  4. Clay and Terracotta Blocks: These blocks offer a more traditional look and have good thermal properties. They are not as widespread as concrete blocks, but their aesthetic appearance and breathability are popular.
  1. Sandstone: Known for its distinctive warm colours and natural textures, sandstone is a popular choice for decorative walls. It provides moderate strength and is often used in the outer facades of buildings.
  2. Limestone: A softer stone that is easier to work with, limestone has been used in buildings for centuries. It offers a refined aesthetic but can be susceptible to weathering, so it often requires protective treatments when used externally.
  3. Granite: Granite is one of the hardest stones used in construction. Its high density makes it less porous and weather-resistant, making it ideal for load-bearing and exterior walls.
  4. Slate: This fine-grained stone is commonly used for roofing but also finds application in masonry walls for its aesthetic appeal and durability. It offers a unique look but is more challenging to work with due to its layered structure.
  1. Clay Bricks: The most traditional and widespread type, clay bricks are known for their durability and strength.
  2. Concrete Bricks: Concrete bricks are a more modern alternative to clay bricks. They are versatile in colour and finish and suitable for specific architectural aesthetics.
  3. Sand-Lime Bricks: Made from sand, lime, and water with a smooth finish and uniform colour. They offer good load-bearing capacity and are used in structural and decorative applications.
  4. Engineering Bricks: Designed for strength and durability, engineering bricks are less porous and highly resistant to moisture and frost, therefore suitable for sewage projects.

Should you insulate masonry walls internally or externally?

Both are valid options, depending on the type of walls. Internal insulation involves fixing insulation boards or applying spray foam directly to the interior side of the walls. It’s less disruptive to the building’s exterior appearance but slightly reduces the internal living space. Applying insulation (external wall insulation or EWI) on the outside of the walls improves the thermal envelope without affecting the interior space. However, it may alter the building’s external appearance and require weatherproof finishes. Older homes often have desirable heritage features. In some cases, they may even have a listed status. Therefore, consulting with an expert regarding the UK Building Regulations is imperative. Installing certain forms of insulation on protected properties is also fraught with issues; the removal cost can outweigh the initial installation.

What insulation material should you choose?

Rigid foam boards are popular for their ease of installation and excellent thermal resistance. These boards, made from polystyrene, polyisocyanurate, and polyurethane, offer high R-values (a measure of thermal resistance) per inch of thickness. This makes them particularly suited for spaces where maintaining interior square footage is important. They are applied directly to the masonry surface, inside or outside. Insulation prevents thermal bridging, where heat bypasses insulation through more conductive materials. However, when using these boards, it’s crucial to ensure a proper vapour barrier is in place to prevent moisture from penetrating and degrading the insulation.

Spray foam insulation offers the dual benefits of high thermal resistance and air-sealing properties. This type of insulation expands upon application, filling cracks, crevices, and irregular gaps, making it ideal for uneven masonry surfaces or areas with difficult access. Open-cell spray foam is lighter and more flexible, allowing some moisture permeability, which can be beneficial in allowing walls to ‘breathe.’ On the other hand, closed-cell spray foam provides a denser barrier, offering higher R-values and moisture resistance. This makes it suitable for external applications or areas with high moisture exposure. However, spray foam requires professional application and can be more costly than other options.

Mineral wool is fire-resistant and offers acoustic insulation and excellent thermal efficiency. This material is particularly effective for the internal insulation of masonry walls as it allows moisture to move through, reducing the risk of trapped dampness. Mineral wool comes in flexible batts or rigid boards, making it adaptable to various installation needs. Its natural breathability makes it an excellent choice for maintaining the health of masonry walls by allowing for necessary air circulation while still improving energy efficiency.

Insulating with Rockwool and Lime Basecoat

How do you insulate masonry walls internally and externally?

  1. Prepare the walls: Ensure the walls are clean and dry. Repair any cracks or damages.
  2. Install a vapour barrier: This step is crucial to prevent moisture from penetrating the insulation.
  3. Fit the insulation boards: adhesive or mechanical fixings.
  4. Seal the joints: Use tape or sealant to cover gaps between the boards to ensure a continuous thermal barrier.
  5. Finish the walls: Apply plasterboard over the insulation for a neat finish ready for painting or wallpapering.
  1. Prepare the exterior walls: Clean the walls and repair any structural issues.
  2. Attach the insulation boards: Mechanical fixings attach the boards to the exterior walls.
  3. Apply a weatherproof layer: This could be a render or a cladding system to protect the insulation from weather elements.
  4. Finish according to taste: Choose finishes matching the building’s original style or modern look.

Regulatory compliance

Building regulations critically guide insulation projects. Always consult local authorities to determine if your project needs planning permission, especially for properties listed or in conservation areas. These regulations ensure that modifications maintain the area’s character and meet safety standards.

Ensure you comply with building codes that dictate the installation of insulation. These codes often specify minimum R-values, acceptable materials, and installation methods to guarantee the insulation’s safe and effective performance.


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