Cavity Wall Analysis

Cavity Wall U-Value Calculator

Calculate U-values for full-fill and partial-fill cavity wall constructions. Optimise insulation thickness for Part L compliance.

Typical Cavity Wall Constructions

Full-Fill Cavity (100mm)

  • 102mm Brick outer leaf
  • 100mm Mineral wool (full-fill)
  • 100mm Concrete block
  • 13mm Plasterboard
U = 0.28 W/m²K

Partial-Fill PIR (75mm)

  • 102mm Brick outer leaf
  • 50mm Clear cavity
  • 75mm PIR board
  • 100mm Concrete block
  • 13mm Plasterboard
U = 0.22 W/m²K

Frequently Asked Questions

What is a good U-value for a cavity wall?

For new builds under Part L 2021, cavity walls should achieve 0.18 W/m²K. With full-fill mineral wool (100mm), expect around 0.28-0.32 W/m²K. With PIR insulation, you can achieve below 0.20 W/m²K.

What is the difference between full-fill and partial-fill?

Full-fill insulation completely fills the cavity with no air gap. Partial-fill leaves a 25-50mm air gap on the outer side. Full-fill gives better thermal performance but requires careful detailing to prevent moisture bridging.

Does the residual cavity affect U-value?

Yes, a clear air cavity (25mm+) adds approximately 0.18 m²K/W of thermal resistance. However, this is much less than the same thickness of insulation, so maximising insulation thickness is usually preferable.

Calculate Your Cavity Wall U-Value

Enter your specific construction details and get accurate results instantly.