Matthew Prowse, Specification and Housing Director at Knauf Insulation, discusses the benefits of designing with wider wall cavities to retain design flexibility and prepare for stricter future regulations.

The latest updates to Approved Document L have increased the thermal performance standards for new homes, and this is due to tighten further in 2025. The Building Regulations England Part L (BREL) report now mandates photographic proof of thermal continuity and quality insulation. Additionally, the Building Safety Act has increased scrutiny of new builds by introducing Gateway ‘checkpoints’ for the Health and Safety Executive to ensure Building Regulation compliance. As regulations continually evolve, architects are pressed to find reliable solutions that accommodate both current and future requirements without compromising on flexibility. So, how can wider cavities help?

Optimal thermal performance
Optimising a home’s thermal performance is a clear priority for architects. The lambda value of insulation is a good place to start, as it gives an indication of thermal efficiency. But achieving that expected performance also depends heavily on correct installation.

Some materials are more prone to creating unintentional air gaps than others. Rigid board insulation for instance, won’t sit flush against a cavity wall unless it is perfectly uniform and flat. A study into thermal looping in cavity walls, partially filled with insulation boards, found that a mere 6mm gap could increase heat transfer by 158%¹. Although 100mm of rigid board might seem sufficient on paper, its performance in practice can be quite different.

Designing with narrower cavities limits your insulation options. By contrast, 150mm cavities allow you to use flexible materials like mineral wool insulation, which adapts to slight imperfections in the substrate and fully fills the cavity. Mineral wool also ‘knits’ together where two edges meet, minimising gaps and maximising thermal performance.

Safety and environmental considerations
Thermal performance isn’t the only area where standards are becoming more stringent. Awareness is also increasing around fire safety and sustainability in residential construction. The updated Approved Document B features tighter guidance on using combustible materials and London Plan guidance now states that major development proposals must be submitted with a Fire Statement confirming that no combustible materials will be incorporated in the development’s external walls. Wider cavities provide the freedom to use non-combustible products like Knauf Insulation’s DriTherm® Cavity Slab 32, which has the best possible Euroclass A1 reaction to fire classification.

The Future Homes Standard is set to lower the operational carbon footprint of new homes and attention is now turning towards embodied carbon. That’s the total greenhouse gas emissions released in producing a build asset, from things like raw materials and transportation. There is already industry support for a proposed ‘Part Z’ Building Regulation, which would assess embodied carbon on all building projects. Again, designing with 150mm cavities allows you to specify products like glass mineral wool insulation, which has the lowest levels of embodied carbon of any mainstream insulation material.

A recent survey of housebuilders revealed that 41% of participants had already made the switch to 150mm for new homes and just over a quarter were actively considering it,² suggesting that wider cavities are an emerging standard.

A practical example
Home energy efficiency is measured as a whole, so increasing the thermal performance of your fabric provides flexibility to specify other elements, like photovoltaic panels or heat source, based on the specific needs of your project.

For instance, a four-bedroom detached house could use 150mm of DriTherm® Cavity Slab 32 insulation in the walls and 500mm of Loft Roll 44 insulation in the loft. This combination would support other features like double-glazed windows, an air source heat pump, and heating controls that use time and temperature zone control. (See table for full example recipe.)

By opting for wider cavities, you can maintain the flexibility you need as regulatory landscapes evolve. With 150mm cavities, you can choose the best materials and wider whole-house ‘recipe’ for your project, ensuring compliance today and readiness for future changes.

Contact Details
For more example recipes, download the free Part L Guide at Knauf Insulation’s Housebuilders Hub.