Navigating Traditional Cavity Construction Changes: Understanding the Viability of 100mm Cavities

In anticipation of the forthcoming Part L Regulations update in June 2022, the question arises: Are 100mm cavities still permissible? The answer is affirmative, but the path forward may pose increasing challenges.

As the regulatory landscape evolves, meeting the standards set by SAP Calculations with a 100mm cavity is currently feasible, albeit with difficulty. However, the impending adjustment to Part L introduces a stricter benchmark—a notional U-Value of 0.18 W/m²K. Presently, an average U-Value achieved with a 100mm cavity and materials like Dritherm 32 hovers around 0.25 W/m²K.

While the maximum allowable U-Value for walls remains at 0.25 W/m²K, surpassing the notional value necessitates compensatory measures elsewhere. With standards tightening, achieving this will prove increasingly arduous, likely prompting a shift towards 125/150mm cavities as the new norm.

Alternatively, opting for high-performance PIR insulation, such as Celotex or Kingspan, presents a viable solution – with partial filling cavity walls. These materials offer superior U-Values, enabling compliance with narrower cavities compared to traditional roll or batt insulations.

To clarify, the utilization of 100mm cavities isn’t outright prohibited. However, meeting the requisite U-Values for external walls mandates either wider cavities or adoption of more efficient insulation options. This underscores the imperative for adaptation and innovation in construction practices.

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