How Thick Should Cavity Wall Insulation Be? UK Building Regs Explained
Part L sets a U-value, not a thickness: 0.18 W/m²K on an extension wall. In practice, 150mm full-fill mineral wool or 75–100mm PIR plus a 50mm cavity.
How thick should cavity wall insulation be?
Full-fill cavity insulation is typically 150mm in new construction and partial-fill rigid board 75-100mm with a 50mm residual cavity, but Approved Document L, which applies in England, does not set a thickness. Wales has its own Approved Document L, Scotland works to Technical Handbook Section 6 and Northern Ireland to Technical Booklet F1, so check those if your project is not in England. It sets a U-value, and the thickness follows from the product you use. The wall targets are 0.26 W/m²K for a new dwelling (Table 4.1), 0.18 W/m²K for a new wall in an extension (Table 4.2), and 0.55 W/m²K for filling the cavity of an existing wall (Table 4.3).
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.
Which number applies to you is the thing most quotes get wrong, and it is worth being precise about. A new wall in an extension is a new thermal element in an existing dwelling, so it falls under Table 4.2 at 0.18 W/m²K. That is stricter than the 0.26 W/m²K limiting value for a wall in a brand new house under Table 4.1, which surprises people. Insulating a wall that is already there is a different job again: that is renovating a thermal element under Table 4.3, where filling the cavity has to reach 0.55 W/m²K and adding internal or external insulation has to reach 0.30 W/m²K. If your builder is quoting an extension and working to anything looser than 0.18, ask which table they think they are on.
Limiting wall U-values sit in three separate tables. Table 4.1 covers a new dwelling at 0.26 W/m²K. Table 4.2 covers new elements in an existing dwelling, including extensions, at 0.18 W/m²K. Table 4.3 covers existing elements: 0.55 W/m²K where the cavity is filled and 0.30 W/m²K where internal or external insulation is added. 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: which U-value applies to your wall by table, and what to check and photograph before the plasterboard goes up.
Was this useful?