Cavity wall insulation is one of the most cost-effective fabric upgrades available to UK homes with brick-and-block cavity construction. The three mainstream fill materials — mineral wool batts, blown expanded polystyrene (EPS) bead, and injected polyurethane (PUR) foam — each suit different situations. Choosing the wrong one for your exposure zone or cavity geometry can lead to damp penetration, so the material decision matters as much as the installation itself.
Full-Fill Mineral Wool
Mineral wool cavity batts (typically glass or rock wool, conforming to BS EN 13162) are the traditional choice for new-build cavities in sheltered and moderate exposure zones. Batts are installed by bricklayers as the wall rises, with the wool held against the inner leaf. Standard products are 75 mm, 90 mm or 100 mm thick, matching common cavity widths.
A 100 mm rockwool full-fill delivers a lambda (λ) of approximately 0.033–0.036 W/mK, yielding a U-value around 0.18–0.22 W/m²K for a standard 328 mm cavity wall (102 mm brick / 100 mm fill / 100 mm block / 13 mm plaster), which comfortably meets the Part L 2021 notional value of 0.18 W/m²K for new dwellings.
Full-fill is not suitable for walls in BRE wind-driven rain exposure zones 3–4 (most of Scotland, Wales and the south-west coast), where driving rain can saturate the outer leaf and wick moisture across the fill to the inner leaf. NHBC Standards Chapter 6.1 and BS 8215 both restrict full-fill use to sheltered and moderate zones unless the outer leaf achieves very low water absorption.
Blown EPS Bead
Expanded polystyrene bead is the dominant retrofit product. Technicians drill 22 mm holes at roughly 1 m centres through the outer leaf, inject a bonded-bead mix (EPS beads coated in adhesive), and re-point the holes. The product must comply with PAS 2030 and be installed under the CIGA Cavity Wall Guarantee scheme for ECO-funded work.
Bonded-bead EPS has a lambda of approximately 0.035–0.040 W/mK and achieves similar U-values to mineral wool in comparable cavities. Because the beads bond together and resist water tracking, it is approved for moderate and — with the correct product specification under BS 8208 — severe exposure zones, making it more broadly applicable for retrofit than full-fill wool.
The key retrofit risk is that pre-existing cavity debris or wall ties can create bridges that prevent complete fill. A reputable installer will conduct a borescope survey beforehand.
PUR Foam
Polyurethane foam (two-component, injected through drilled holes) expands to fill the cavity and cures in place. It gives the best thermal performance of the three options — lambda around 0.025–0.030 W/mK — and forms a partial vapour barrier within the cavity. It is also structurally stiffening to some degree.
PUR foam is generally reserved for cavities that are difficult to fill with bead (irregular widths, built-in obstructions) or where maximum thermal gain per millimetre is required. However, hardened foam is irreversible: removal for future wall-tie replacement or remediation is extremely disruptive. PUR installations must comply with BS 5617 (urea-formaldehyde foam is now largely superseded by polyurethane) and the product should carry BBA Agrément certification.
Cost and Performance Comparison
| Material | Lambda (W/mK) | Typical U-value (100 mm cavity) | Installed cost (m²) | Retrofit / New build | Max exposure zone |
|---|---|---|---|---|---|
| Mineral wool full-fill | 0.033–0.036 | 0.18–0.22 | £8–£14 | New build | Sheltered / moderate |
| Bonded EPS bead | 0.035–0.040 | 0.20–0.25 | £7–£12 | Both | Moderate / severe* |
| PUR foam | 0.025–0.030 | 0.16–0.20 | £15–£22 | Retrofit | Moderate / severe* |
*With BBA-certified product and survey confirming cavity suitability. Costs are 2025–2026 UK averages and vary by region, scaffold access and installer.
Damp Risks and Exposure Zones
The BRE exposure map (derived from BS EN ISO 15927-3 wind-driven rain data) divides England, Scotland and Wales into sheltered, moderate, severe and very severe zones. Any full-fill product used in severe or very severe zones without explicit BBA certification for that zone voids the guarantee and risks damp bridging.
For retrofit specifically, a pre-installation survey under PAS 2035 is mandatory for ECO-funded projects and best practice regardless. The survey must check: cavity width (minimum 50 mm is generally required), wall-tie condition, presence of cavity trays at openings, and damp-proof course continuity. A compromised DPC is a contraindication for any cavity fill.
New Build vs Retrofit
In new build, a full-fill mineral wool batt is the simplest and most cost-effective route to compliance in sheltered zones — the material cost is £3–£6/m² for the batts alone, with installation absorbed into bricklaying labour. A 50 mm partial-fill batt (retaining a 25 mm residual cavity) is the preferred strategy in moderate-to-severe zones: it maintains a drainage cavity while contributing meaningful thermal resistance.
For retrofit, EPS bead is the industry default. Over 7 million UK homes have had cavity wall insulation retrofitted since the 1990s, the majority with bead or loosely blown mineral wool. ECO4 funding (the current energy efficiency scheme) covers costs in full for eligible households; owner-occupier costs in the £500–£1,500 range are typical for a mid-terrace after subsidy.
Relevant UK Standards and Guidance
- BS EN 13162 — mineral wool products for buildings
- BS EN 13163 — EPS products for buildings
- BS 8208 — guide to assessment of suitability of external cavity walls for filling with thermal insulants
- PAS 2030 / PAS 2035 — energy efficiency retrofit competency and assessment standards
- BRE Report BR 262 — thermal insulation: avoiding risks
- Building Regulations Part L (2021) — conservation of fuel and power (U-value targets)
- NHBC Standards Chapter 6.1 — external masonry walls
The choice of cavity wall insulation material is ultimately a function of exposure zone, wall geometry, project stage (new build or retrofit), and budget. In sheltered zones either mineral wool or bead performs well at similar cost; in moderate-to-severe zones, a surveyed bonded-bead installation with BBA certification is the lowest-risk retrofit option.
