How the cavity wall calculation works
Many British houses have cavity walls: two leaves of brick or block with a gap between them. Filling that gap with insulation is a quick job for an installer. The calculator works out the U-value once the cavity is filled and gives a rough installed cost.
The whole cavity gets filled
Cavity fill can't be installed part-way, because the installer fills the whole gap. So the calculator doesn't size the insulation to the target. It fills the cavity you select (50, 65, 75, 85 or 100 mm) and reports the U-value you end up with. You'll find the cavity width under More options.
The formula
The calculator uses the BS EN ISO 6946 method:
U = 1 ÷ (Rsi + Σ thickness/λ + Rse)
For a wall, the inside surface (Rsi) is 0.13 and the outside surface (Rse) is 0.04 m²K/W. The rest of the wall (its two leaves, plaster and air gap) adds a calibrated 0.51, which gives an unfilled wall about U-value 1.47 W/m²K. Filling the cavity with blown mineral wool at λ 0.040 (Knauf Supafil 40) adds an R-value of cavity width ÷ λ. A 65 mm cavity adds 0.065 ÷ 0.040 = R 1.63, and the wall drops to U-value 0.43, which matches Knauf's published tables for a filled 65 mm cavity.
The target
Approved Document L, Table 4.3, gives U-value 0.55 W/m²K for a wall upgraded with cavity insulation. Note 5 to that table says the value applies only to walls suitable for cavity fill. Otherwise, the wall is treated like a solid wall. There's no separate new-build value here, because you can only ever fill a cavity, not make it wider.
Worked example: 90 m² of external wall
Say your house has 90 m² of external wall once windows and doors are taken off. If you measure the walls in the calculator, it deducts 1.9 m² per door and 1.5 m² per window.
| Cavity width | U-value after filling | Meets Part L 0.55? | Estimated installed cost |
|---|---|---|---|
| Unfilled | 1.47 | No | – |
| 50 mm | 0.52 | Yes | £1,800–£2,475 |
| 65 mm | 0.43 | Yes | £1,800–£2,475 |
| 75 mm | 0.39 | Yes | £1,800–£2,475 |
| 85 mm | 0.36 | Yes | £1,800–£2,475 |
| 100 mm | 0.31 | Yes | £1,800–£2,475 |
The cost is the same whatever the cavity width, because it's an estimate per square metre of wall. It comes from the Energy Saving Trust's figure of about £2,200 for a typical home: assuming 80–110 m² of external wall, that's £20–£27.50 per m². No waste allowance is added to installed prices. The Energy Saving Trust also says the job typically takes about two hours and often pays for itself within five years.
Is your wall suitable?
Cavity fill works well in the right wall and can cause damp in the wrong one. A good installer will survey first and check:
- Cavity width. The Energy Saving Trust says the cavity should be at least 50 mm wide. Narrower cavities, common in older houses, are a risk.
- A clear cavity. Rubble, mortar snots or debris in the cavity can carry water across to the inner leaf once it's filled.
- Exposure to driving rain. Walls that face the prevailing weather in exposed places need careful assessment, and some aren't suitable.
- The condition of the wall. Cracked render, failed pointing, damaged bricks or existing damp should be put right first.
- Existing insulation. Some cavities were partly filled years ago. The installer should check before adding more.
- How the house is built. Timber-frame, steel-frame and other non-traditional houses need a specialist assessment.
Why you can't do it yourself
The installer drills a pattern of small holes through the mortar joints of the outer leaf, blows the insulation in with a machine until the cavity is full, then fills the holes to match. It needs specialist equipment and training, and the survey is as important as the installation. Use a registered installer, ask what guarantee comes with the work (the Energy Saving Trust advises 25 years, through CIGA or an insurance-backed scheme), and keep the paperwork.
Providing cavity wall insulation counts as renovating the wall under Approved Document L, so the work has to meet the Building Regulations. Ask your installer how it will be signed off.
What next
If your walls turn out to be solid, use the solid wall insulation calculator. The loft is usually the cheapest place to start: see the loft insulation calculator. All the targets are listed on Part L U-values, and you can check the sums with the U-value calculator.
Sources
- Approved Document L, Volume 1: Dwellings (2021 edition incorporating 2023 amendments)
- Energy Saving Trust: Cavity wall insulation
- Knauf: Supafil 40 cavity wall insulation
- Knauf Insulation: UK & Ireland Insulation Solutions Guide (June 2026)
- BRE BR 443: Conventions for U-value calculations (2019)
- RdSAP10 Specification (BRE, February 2024)