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Full Fill vs Partial Fill Cavity Wall Insulation

Last modified: 11.08.2024

When it comes to insulating cavity walls, homeowners and builders often face the choice between full-fill and partial-fill insulation. Both methods have advantages and disadvantages, impacting a building's energy efficiency, cost, and overall performance. Cavity wall insulation is a method used to improve the thermal efficiency of buildings with cavity walls, which are constructed with two layers of masonry separated by a gap (or cavity). This cavity can lead to significant heat loss if not properly insulated. By filling this gap with insulating material, you can drastically reduce heat loss, lower energy bills, and improve the overall comfort of your home. The materials used for cavity wall insulation include mineral wool, polystyrene beads, or polyurethane foam, each with its unique properties and benefits. These materials are injected or blown into the cavity through small holes drilled into the exterior wall. The holes are sealed once the cavity is filled, and the insulation provides an effective barrier against heat loss. Properly installed cavity wall insulation can reduce heat loss by up to 35%, making it one of the most cost-effective ways to improve a building's energy efficiency. It also helps to maintain a more consistent indoor temperature. This reduces the need for heating in winter and cooling in summer. Additionally, cavity wall insulation can help prevent moisture from penetrating the walls, reducing the risk of damp and mould. However, assessing whether your home is suitable for cavity wall insulation is important. Buildings constructed before the 1920s typically have solid walls rather than cavity walls. Some newer buildings may already have insulation installed.

Full-fill cavity wall insulation involves completely filling the gap between the inner and outer leaves of a cavity wall with insulation material. This method uses materials such as mineral wool, expanded polystyrene (EPS) beads, or polyurethane foam, ensuring that the entire cavity is occupied and leaving no gaps. In the case of spray beads, small holes are drilled into the outer leaf of the cavity wall at regular intervals to allow for the injection of the insulation material. The insulation material is then injected into the cavity under pressure, ensuring it fills the entire space evenly. This process is carefully monitored to avoid any voids or gaps.

  1. Superior Thermal Performance: By filling the entire cavity, full-fill insulation eliminates air gaps, significantly reducing heat loss and enhancing the building’s energy efficiency. This results in lower heating bills and a more comfortable living environment.
  2. Enhanced Moisture Control: Modern full-fill insulation materials are designed to be water-resistant. They prevent moisture from penetrating the cavity and reaching the interior walls, which helps reduce the risk of damp and mould growth.
  3. Cost-Effective: Full-fill insulation can be more economical in the long run. It maximises the use of the cavity space and provides better thermal performance. Ultimately, this leads to lower energy costs over time.
  4. Sound Insulation: Filling the cavity completely can also improve the wall's sound insulation, reducing noise transmission from outside.
  1. Installation Complexity: Proper installation of full-fill insulation requires precision to ensure the material fills the entire cavity without leaving voids. Incorrect installation can lead to thermal bridging, where heat escapes through gaps in the insulation.
  2. Potential for Thermal Bridging: Although full-fill insulation aims to eliminate gaps, any inconsistency during installation can create thermal bridges. These are areas where heat can escape, reducing the overall effectiveness of the insulation.
  3. Material Suitability: Not all insulation materials are suitable for all types of buildings. To perform effectively, the chosen material must be compatible with the building structure and local climate conditions.

Partial fill cavity wall insulation involves placing a layer of insulation material against the inner leaf of the cavity wall, leaving a gap between the insulation and the outer leaf. This method typically employs rigid foam boards or other rigid insulation materials that provide effective thermal resistance while maintaining an air gap for additional protection against moisture. This gap typically ranges from 25mm to 50mm, depending on the specific requirements of the building and local building regulations.

  1. Moisture Control: One of the primary benefits of partial fill insulation is its ability to control moisture. The air gap is an additional barrier that helps prevent water from penetrating the outer leaf and reaching the inner wall, reducing the risk of dampness and mould growth.
  2. Easier Installation: Partial-fill insulation is generally easier to install correctly than full-fill insulation. The rigid foam boards are cut to size and fitted snugly against the inner leaf, ensuring a precise and secure installation without the risk of gaps or voids.
  3. Reduced Thermal Bridging: The air gap in partial fill insulation can help to minimise thermal bridging. Thermal bridging occurs when heat bypasses the insulation through more conductive elements of the wall. By maintaining a consistent air gap, partial fill insulation helps to interrupt these thermal bridges.
  4. Flexibility in Material Choice: Partial fill insulation allows for the use of various materials, including rigid foam boards like polyurethane (PUR) or polyisocyanurate (PIR). These materials offer excellent thermal performance and are suitable for various building types.
  1. Lower Thermal Performance: Compared to full-fill insulation, partial-fill methods may have slightly lower overall thermal efficiency. While beneficial for moisture control, the air gap can also allow for some heat loss, reducing the insulation's effectiveness.
  2. Increased Material Costs: The need for high-quality rigid insulation boards and the precise fitting required can increase material and labour costs, making partial fill insulation a more expensive option in some cases.

Cavity wall insulation obviously depends on whether the walls of your home have an uninsulated cavity. As aforementioned, a home built before the 1920s in the UK will likely have solid walls. Solid walls are single-leaf, which means they can only be insulated externally or internally. External wall insulation has several advantages over internal wall insulation, the major being that internal wall insulation significantly impacts valuable floor space. However, how does EWI compare with cavity wall insulation?

  • EWI generally offers superior thermal performance due to the continuous insulation layer, which eliminates thermal bridging. CWI, while effective, may have gaps that reduce its overall efficiency.
  • Both methods provide moisture control, but EWI offers additional protection by covering the entire exterior surface, which can be particularly beneficial in wet climates.
  • CWI is more cost-effective and quicker to install, making it an attractive option for newer buildings with cavity walls. EWI, although more expensive, is ideal for older buildings or those with solid walls.
  • EWI offers the opportunity to improve the building’s appearance and provides extra protection against weather. CWI, on the other hand, is less intrusive and maintains the building’s existing exterior look.