How to Price Cavity Wall Insulation: Blown Bead, Mineral Fibre and EPS Bead

Quick Answer: Cavity wall insulation in a typical UK 3-bed semi costs £450–£900 in 2026 — around £20–£35 per m² of external wall area. Blown EPS (expanded polystyrene) bead is the dominant material; mineral fibre and urea-formaldehyde foam are alternatives but less common now. Installation takes a half-day for an average house. The work is notifiable to Building Control under Part L unless installed by a CIGA / NIA accredited installer self-certifying. Government grants (ECO4, Great British Insulation Scheme) frequently cover all or most of the cost for eligible homeowners. Suitability survey is mandatory — not all cavities are suitable.

Summary

Cavity wall insulation is one of the highest-return energy efficiency upgrades for any UK property with an unfilled cavity. Typical savings are £150–£300 per year on heating bills for a 3-bed semi, with payback in 3–6 years even at full retail cost — and frequently free under government schemes. The volume installer market has been dominated by ECO/grant-funded work for over a decade.

For a contractor pricing the work, the key variables are the wall fabric type, cavity width, exposure zone, and customer's grant eligibility. The work is highly process-driven — it's not artisanal craft, it's a survey, drill, blow operation that takes a competent two-person crew a half-day on a typical house. The margin is volume-driven; specialist installers handle 800+ properties per year per crew.

The single biggest risk is unsuitable cavities. Damp ingress to insulated cavities causes serious mould and damp problems that can take 5+ years to manifest. The CIGA (Cavity Insulation Guarantee Agency) scheme provides 25-year guarantees to the homeowner, but installation MUST be preceded by a suitability survey: cavity width, wall condition, exposure zone, render and pointing condition, and presence of existing fill. Skip the survey at your peril — and the customer's.

Key Facts

Quick Reference Table

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Property type External wall area Typical retail cost Programme Annual savings
Mid-terrace 2-bed 30–45 m² £350–£700 2–4 hours £100–£200
End-terrace 2-bed 50–65 m² £450–£900 3–5 hours £150–£250
Semi-detached 3-bed 65–85 m² £450–£900 4–6 hours £150–£300
Detached 3-bed 90–120 m² £700–£1,400 5–7 hours £200–£400
Detached 4-bed 110–150 m² £900–£1,800 6–8 hours £250–£500
Bungalow (3-bed) 60–80 m² £450–£800 4–6 hours £150–£250
Material Cost per m² Pros Cons
EPS bead (blown) £18–£28 Cheap, robust, drains well Slight settling over decades
Mineral fibre (blown) £22–£32 Fits irregular cavities Compact under prolonged damp
Polyurethane foam £35–£55 Highest insulation per mm Expensive, fewer installers
Urea-formaldehyde foam (legacy) n/a Was cheap Often must be removed before re-fill

Detailed Guidance

Eligibility: When CWI is Suitable

A cavity is suitable for fill if:

  1. Cavity width 25–100 mm — confirmed by boroscope inspection, not just by drilling
  2. Cavity is clear — no debris, mortar droppings, slate tiers from previous attempts
  3. Brickwork in good condition — no missing or perished pointing, no rendered cracks letting water in
  4. Exposure zone permits — high-exposure areas (coastal, exposed upland) may require partial fill or specific products
  5. No existing fill — except where confirmed clear by boroscope
  6. DPC at floor level — and at window/door reveals; insulation must not bridge the DPC
  7. Wall tie condition — corroded ties may need replacement before fill

Not suitable:

The Survey Process

A proper survey takes 30–60 minutes:

  1. External condition inspection — pointing, render, sealants
  2. Internal condition — any signs of existing damp, cold spots, mould
  3. Boroscope inspection — drill 25 mm pilot hole, insert camera, inspect cavity at 4–6 points (top, middle, bottom of each elevation)
  4. Cavity width measurement — at multiple points (cavities aren't always uniform)
  5. Wall tie inspection — note type, spacing, corrosion
  6. Exposure zone — postcode lookup or boundary survey
  7. Suitability report — written confirmation of suitability and chosen product

A survey costing £100–£200 is normal where the work isn't grant-funded; under grant schemes it's free with the install.

Drilling and Filling

Standard process:

  1. Set up scaffold or ladder access (most jobs use long ladders for one-day works; scaffold for storey-and-a-half or higher)
  2. Mark drilling pattern: typically 600 mm grid, every other course
  3. Drill 25 mm holes through outer leaf only (not through inner leaf)
  4. Connect blown delivery hose to first row of holes
  5. Inject fill material through bottom row, working upwards
  6. Confirm filling by listening (audible flow stop) and tapping (sound change as cavity fills)
  7. Plug holes with mortar (closely matching existing pointing)
  8. Clean up exterior

Programme: 4–6 hours for typical 3-bed.

Material Selection in Practice

EPS bead (Eurobead, Whitefield, Springvale) — small (3–6 mm) polystyrene beads bonded with a binder. Pros: good thermal performance, water-shedding (dries out if damp). Cons: slight long-term settling, can leak through gaps if outer leaf is breached.

Mineral fibre (Knauf Supafil 34, Rockwool Energy Saver) — chopped mineral fibre blown into cavity. Pros: fits irregular cavities, BBA-certified for varying widths. Cons: under prolonged damp can slump and leave uninsulated voids.

Polyurethane (PUR) foam — expanding rigid foam. Pros: highest thermal performance, fills irregular voids. Cons: expensive (50% premium), few installers, not the right product for most domestic cavities.

Urea-formaldehyde (UFFI) — legacy product banned for new fills since the 1980s due to formaldehyde off-gassing. Where present in older cavities, often must be removed before new fill.

For most modern jobs, EPS bead is the default.

Government Grants and Funding

Multiple schemes have funded cavity wall insulation since 2008. The current landscape (2025):

Eligible homeowners frequently get the work done at no cost. Quote always asks if the customer has checked grant eligibility — and signposts to the relevant scheme. Many customers are eligible without realising.

Re-pointing the Drill Holes

After the cavity is filled, drill holes are plugged with mortar:

Customers occasionally complain that the drill holes are visible. Match-pointing minimises this; on heavily weathered brickwork, drill holes are nearly invisible after a few months of weathering.

Pre- and Post-Install Checks

Before:

After:

Common Failure Modes

The reasons CWI installations fail:

  1. Existing damp problems not addressed — cavity becomes wet from rain ingress through poor pointing or render
  2. Wall ties corroded — debris in cavity prevents complete fill
  3. DPC bridged — fill placed across DPC at floor level allows damp rise
  4. Window/door reveals not closed — cavity barriers missing at openings, allows fill to escape into door/window frames
  5. High exposure not respected — wrong product for the exposure zone
  6. Settlement — bead or fibre settles over time, leaving cold spots at the top of walls

The CIGA guarantee covers most of these failure modes, provided the original install was certified.

Notification and Sign-Off

Cavity wall insulation falls under Part L (energy efficiency). Notification routes:

Most customers expect the installer to handle all notification. Quote must specify CIGA registration and lifetime guarantee as standard inclusions.

Frequently Asked Questions

Will my house get damp after CWI?

Properly installed in a suitable cavity, no. Improperly installed in an unsuitable cavity, yes — and the resulting damp is hard to fix without removing the fill (an extra £30–£60 per m²). The pre-install survey is the protection against this.

What if the property already has cavity wall insulation but it's failed?

Removal and re-fill is possible — typically £15–£25 per m² for removal, then standard re-fill. Boroscope survey confirms condition before deciding. UFFI removal is more expensive due to disposal as hazardous waste.

Can old houses be cavity-filled?

Houses with cavity walls — typically built post-1920 — are candidates. Solid wall houses (most pre-1920) require external wall insulation or internal wall insulation instead. Stone-walled cavity construction (e.g. some Edwardian) needs careful survey before quoting.

Is mineral fibre or EPS bead better?

For most jobs, both perform equally well. EPS bead is slightly cheaper and more drain-tolerant. Mineral fibre is better for irregular cavities. Most volume installers default to EPS.

What about partial fill?

In high-exposure zones, full cavity fill may not be acceptable — the BBA certificate for the product specifies acceptable exposure zones. Where partial fill is needed, insulation is fixed to the inner leaf during construction (only possible during new build, not retrofit). Retrofit options for high-exposure cavities are limited to specific products.

Regulations & Standards