How Thick Should Cavity Wall Insulation Be? UK Building Regs Explained
Cavity wall insulation must be at least 150mm for full-fill new builds under Part L. Full-fill vs partial-fill, U-value targets, and what to check on site.
How thick should cavity wall insulation be?
Cavity wall insulation must be at least 150mm thick for full-fill installations in new builds, as required by the Approved Document L uplift from June 2022. For partial-fill rigid board, 75-100mm with a 50mm residual cavity is typical. The target wall U-value is 0.18 W/m²K for new builds and 0.26 W/m²K for renovations.
What this means for your project
If you're building an extension or a new-build, the insulation thickness your builder specifies isn't a suggestion - it's driven by the U-value targets in the current Building Regulations. Get this wrong and building control will flag it. Get it very wrong and you'll be stripping off plasterboard to fix it.
The two main approaches are full-fill and partial-fill. Full-fill means the entire cavity is packed with insulation - usually mineral wool batts like Knauf DriTherm or Rockwool. The cavity needs to be wide enough to accommodate the required thickness, which is why modern cavity walls are typically 150mm wide rather than the old 50-75mm standard.
Partial-fill uses rigid insulation boards (PIR or PUR - brands like Celotex or Kingspan) fixed to the inner leaf of the wall. Because these boards have a much better thermal conductivity (around 0.022 W/mK compared to 0.032-0.037 for mineral wool), you need less thickness to hit the same U-value. A 100mm PIR board plus a 50mm residual cavity gives you a 150mm total cavity width but with better thermal performance than a 150mm full-fill.
The residual cavity in partial-fill construction isn't wasted space - it provides a drainage path for any moisture that gets past the outer leaf. This is why partial-fill is often preferred in exposed locations where driving rain is a concern.
For renovations where you're upgrading a thermal element (replacing the external wall construction), the U-value target is more relaxed at 0.26 W/m²K. But if your builder is quoting insulation for an extension, it's the new-build target of 0.18 that applies, because extensions are treated as new construction under Part L.
External walls in new dwellings must achieve a U-value of no worse than 0.18 W/m²K. For renovations upgrading a thermal element, the target is 0.26 W/m²K. The actual insulation thickness depends on the product's thermal conductivity (lambda value) and the overall wall construction.
Ask your builder what insulation product they're using and what thickness. Check the product's lambda value on the manufacturer's data sheet. For full-fill mineral wool, expect 150mm minimum. For partial-fill PIR/PUR, expect 75-100mm plus a 50mm residual cavity.
Common mistakes
Gaps and voids in the insulation. This is the number one problem. Insulation only works when it's continuous. A 5% gap in coverage can reduce thermal performance by 25% or more due to thermal bridging. Batts should be cut neatly to fit around services and wall ties - not ripped, compressed, or left with daylight showing through.
Mortar droppings bridging the cavity. In partial-fill construction, mortar droppings that accumulate on top of the insulation board create a bridge between the outer and inner leaf. Moisture tracks across the bridge and into your internal wall. A cavity tray or mortar deflector should prevent this, but sloppy bricklaying makes it worse.
Using the wrong product for the exposure zone. Full-fill cavity insulation shouldn't be used in severely exposed locations (wind-driven rain zones 3 and 4) without careful detailing. If your site is on the west coast or on high ground, partial-fill with a clear cavity may be the safer choice. Your builder should know this - if they don't, that's a concern.
Cavity Wall Insulation Checklist
A printable checklist covering insulation thickness, U-value targets, and what to photograph before plasterboard goes up.