Cavity Wall Insulation

Cavity Wall Insulation Types and Applications

Cavity wall insulation is one of the most effective types of wall insulation available for UK homes. Fitted in the gap between the inner and outer leaves of a masonry cavity wall, it works by trapping air within the insulation material and slowing the transfer of heat from the warm interior to the cold outside. Without cavity insulation, walls can account for around 35% of a home's total heat loss, making it one of the single biggest areas where energy efficiency improvements can be made.

There are two main types of cavity wall insulation used in UK construction: full fill and partial fill. Both are designed to improve the thermal performance of cavity walls, but they use different materials and methods depending on the build, the location, and the level of exposure to weather.

Full Fill

Full fill cavity wall insulation fills the entire cavity, sitting tight against both the inner and outer leaves to create a continuous thermal barrier with no gaps. It is the most common method used in new builds and extensions across the UK, and is a popular choice for anyone looking to achieve strong thermal performance from a straightforward installation.
Mineral wool cavity batts are one of the most widely used full fill products. They are non-combustible, provide solid acoustic as well as thermal performance, and are flexible enough to fit closely around wall ties and slight deviations in the cavity width. This helps reduce gaps and maintain insulation continuity across the full wall area, which is critical to how effective cavity wall insulation is once installed. Mineral wool cavity wall insulation is also one of the most cost-effective options available, making it a reliable choice where budget is a consideration without compromising on performance.

Rigid PIR cavity wall insulation boards are the higher-performance full fill alternative. PIR has a significantly lower thermal conductivity than mineral wool, which means it delivers better thermal efficiency from a thinner profile. Full fill PIR boards are typically manufactured with tongue-and-groove or rebated edge profiles that interlock at the joints, helping to prevent moisture tracking and thermal bridging. For builds where achieving a lower U-value from a limited cavity width is important, PIR boards are often the preferred choice.

Partial Fill

Partial fill cavity wall insulation uses a rigid insulation board fixed to the inner leaf of the wall, with a clear air gap maintained between the board and the outer masonry. That air gap is not wasted space. It acts as a drainage channel, allowing any moisture that penetrates the outer brickwork to run down harmlessly without reaching the insulation or the inner leaf. It also allows the wall structure to breathe, which is why partial fill is the preferred method in exposed locations where wind-driven rain is a concern.

Partial fill cavity wall insulation boards are typically manufactured from PIR or phenolic foam, both of which offer low thermal conductivity and strong thermal performance without needing to fill the full cavity width. Phenolic boards in particular have some of the lowest thermal conductivity values on the market, making them a strong option where achieving the best possible U-value from a thinner board is a priority.

Where damp problems have been associated with cavity wall insulation, it is almost always down to the wrong type of insulation being used for the exposure conditions, or poor installation practice. Partial fill with a properly maintained air gap is the industry-standard approach to managing moisture in exposed masonry cavity wall construction, and when fitted correctly it is a long-lasting, reliable solution.

Cavity Wall Insulation and Building Regulations

All new builds and extensions in the UK must meet the thermal performance targets set out in Part L of the Building Regulations. For cavity walls, this means achieving a U-value of 0.18 W/m²K or lower in new builds, or around 0.26 W/m²K for renovations and extensions. If you are carrying out any work that involves replacing or upgrading cavity wall insulation, the installation must comply with the current regulations.

The Part L update in 2022 increased the minimum cavity width for full fill insulation in new builds, and as a result, higher-performance PIR and phenolic insulation boards have become increasingly popular for achieving lower U-values without increasing overall wall thickness. For partial fill applications, a residual air cavity of at least 50mm must be maintained between the insulation board and the outer leaf. The right combination of insulation type, thickness, and material will depend on your cavity width, your exposure zone, and the U-value your project needs to achieve.

Choosing the Right Cavity Wall Insulation

Selecting the best cavity wall insulation for your project comes down to three things: how exposed your home is to wind-driven rain and moisture, what your cavity width is, and what U-value your build needs to hit. Once you have those three nailed down, the right product, material, and thickness pretty much picks itself.

Properly installed cavity wall insulation can last the full lifetime of a building with very little maintenance, so it is worth taking the time to get the specification right from the start. The pros of getting it right are significant: lower heating bills, a more comfortable home, better acoustic performance, and compliance with current building regulations.

Available in a variety of materials including PIR insulation and mineral wools, cavity wall insulation can be paired with accessories such as cavity closers to seal the cavity at door and window reveals. All products are supplied by leading manufacturers to ensure a high level of performance

Why Choose Insulation UK for Cavity Wall Insulation?

Competitive Pricing Some of the best prices available online for cavity wall insulation boards, batts and slabs, with no minimum order value.

All the Leading Brands A full range of full fill and partial fill cavity insulation from the UK's most trusted manufacturers, all stocked and available at trade prices.

Fast UK-Wide Delivery Reliable delivery across the UK with free shipping on orders over £500 (ex VAT)

Expert Technical Support Our team has over 50 years of experience in the insulation industry. Whether you need help selecting the right cavity wall insulation for your project or guidance on achieving a specific U-value, get in touch and we will point you in the right direction.