FAQs - Can I build with 100mm cavities still?
The answer is yes – but in reality, it will become increasingly difficult!
Whilst it is fairly easy to get a SAP Calculation to pass with a 100mm cavity currently, the update to Part L will make it much more difficult.
The ‘notional U-Value’ which SAP uses to generate the target is set to become 0.18 W/m²K. An average U-Value with a 100mm cavity and e.g. Dritherm 32, achieves around 0.25 W/m²K.
Whilst the maximum U-Value allowed for a wall is set to be set at 0.25 W/m²K – achieving anything over the notional value will mean going above and beyond elsewhere.
With the standards becoming stricter across the board, this will be very difficult to do. Meaning that 125/150mm cavities will become widespread.
The main alternative to this would be to install high-performance PIR insulation such as Celotex or Kingspan. This will achieve a higher U-Value with a smaller cavity width than the traditional roll insulations.
In summary – 100mm cavities are not being ‘banned’ as such. However, U-Values of external walls will have to improve, whether via wider cavities or via higher performing insulation instead.