Thermal Performance in New Build Walls: Cavity Width, Insulation, and Part L
Approved Document Part L sets the thermal performance target. The cavity width, insulation type, and blockwork specification determine whether you hit it.
A new build external wall in 2026 must achieve a U-value of 0.26 W/m2K or better under Approved Document Part L (Conservation of Fuel and Power). That figure determines the combination of insulation thickness, block conductivity, and cavity width that makes up the wall build-up.
Typical Wall Build-Up
The standard specification for a new build in Kent in 2026 is:
- 102.5mm facing brick outer leaf.
- 100mm full-fill cavity insulation (Kingspan Kooltherm K106 at 0.018 W/mK, or Celotex CW4000 at 0.022 W/mK).
- 100mm Thermalite Shield aircrete block inner leaf (0.11 W/mK).
- 12.5mm plasterboard on dabs.
This achieves a U-value of approximately 0.20 W/m2K, comfortably under the Part L target. Using a denser block for the inner leaf (say, a 7 N/mm2 aggregate block at 1.3 W/mK) would push the U-value above the target unless the cavity insulation thickness is increased to 125mm or 150mm.
Why Clean Cavities Matter
Mortar droppings on wall ties and insulation boards create thermal bridges, cold spots where heat escapes and condensation forms. A clean cavity is not just an NHBC inspection requirement; it directly affects the thermal performance of the finished wall. We use cavity battens (boards that sit on the wall ties during construction, catching droppings and lifted out as the wall rises) to keep the cavity clean.
See our article on new build cavity wall construction for the full specification. View our new build services.