VIEWPOINT
YOUR CUSTOMERS ARE CHANGING THE WAY THEY
BUILD CAVITY WALLS Matt Neary, sales director Knauf Insulation
FOR AT LEAST two decades, the standard width for masonry cavity walls has been 100mm. But with the 2022 updates to Part L, and the long-anticipated introduction of the Future Homes Standard (FHS), best practice is changing. Wider cavities are increasingly becoming the new normal, and merchants have a key role to play in helping customers make that transition smoothly.
The shift to 150mm Regulatory change has accelerated the industry’s shift to improved fabric performance. In England, the 2022 uplift to Building Regulations through Part L and Part F was designed as a stepping stone towards the Future Homes Standard. These changes tightened limiting U-values and raised expectations on ventilation. Part O was also introduced, making overheating mitigation a new consideration for residential design. Part L (Conservation of fuel and power) now requires all walls to have a maximum U-value of 0.26 W/m²K. Meeting these targets in practice means builders are increasingly looking at the full wall build-up, and cavity width is an obvious place to start. A survey by Professional Builder in association with Knauf Insulation in 2023 found that 41% of housebuilders were already building with masonry cavity walls. To give a more up- to-date insight into changing practices, a new survey was conducted in late 2025. It found that 66% of respondents who build with masonry cavity walls will start building with 150mm cavities when the FHS comes into effect, with a further 23% considering it.
This points to a clear trend: cavity widths are increasing, and many builders are already
Left: Fabric performance
becomes even more important as homes transition towards low and zero carbon technologies
Below: 54% of respondents identified mineral wool, either rock or glass, as their preferred material for insulating cavity walls
thermal requirements of Part L and the Future Homes Standard. The same survey found that 54% of respondents identified mineral wool, either rock or glass, as their preferred material for insulating cavity walls, compared with 38% for rigid boards such as PIR and EPS. Mineral wool’s ability to accommodate minor
imperfections in the substrate and knit together at abutting edges in the cavity helps reduce the risk of gaps and thermal bypasses. In a 150mm cavity, it can support builders in meeting performance targets both on paper and in practice.
For merchants, this matters. Product selection, stock planning and customer support increasingly need to reflect what is happening on site. Wider cavities also influence what builders ask for in terms of insulation thicknesses, wall tie specification, cavity closers, cavity trays and associated accessories.
Becoming the new baseline
preparing for 150mm as standard practice. How can merchants ensure they’re supplying the right stock to support this transition?
Buildability is crucial Thermal performance is a key driver behind wider cavities, but buildability is just as important. Bricklaying and installing cavity wall insulation is a technically challenging job, and even small gaps can have a major impact on overall performance.
In the late 2025 survey, 38% of respondents who build masonry cavity walls said that ensuring a continuous layer of insulation is one of the most difficult aspects of
April 2026
www.buildersmerchantsjournal.net
insulating a cavity wall. Air gaps can reduce the thermal performance of the finished build-up, creating cold spots and localised condensation on the inner wall.
The next most common challenges were ensuring boards sit tight to the inner leaf (17%) and making cuts (12%). These are familiar issues across the sector, and they underline why the choice of insulation material is often about what can be installed well, not simply what performs best on paper.
One of the benefits of building with 150mm cavities is that it allows builders to use more forgiving materials to meet the
Wider cavities are becoming the new normal. They allow builders to use rock and glass mineral wool to meet the updated standards in Part L and Part F, with materials that are more forgiving to install and offer added benefits such as acoustic and fire performance that are increasingly desirable in homes fit for the future.
For merchants, the message is simple: understanding this shift and preparing for it now will help ensure product ranges and technical support remain aligned with modern building practices. As cavity widths increase, merchants have an opportunity to support customers with insulation solutions that help deliver compliance, buildability and performance in one package. BMJ
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