Cavity Wall Insulation Cost UK: Blown Wool & EPS Bead Guide

Quick Answer: Cavity wall insulation (CWI) for a typical 3-bedroom semi prices at roughly £700-£2,000 installed, depending on wall area, fill material, and access. Blown mineral wool and EPS (polystyrene) bead are the common fills, injected through ~22-25mm holes drilled in the mortar joints on a grid, then made good. Pricing is driven by external wall area, scaffolding/access, and whether a survey confirms the cavity is suitable (clear, ≥50mm, no existing fill, exposure zone acceptable). Installation must follow BS EN 14064 / BS 8208 suitability assessment and is normally done by a CIGA-registered, PAS 2030/2035-compliant installer to attract guarantees and grant funding (ECO4, GBIS).

Summary

Cavity wall insulation is one of the cheapest fabric upgrades per kWh saved, which is why it's been heavily grant-funded for years. The basic operation is simple: drill a grid of small holes through the outer leaf, inject insulation to fill the cavity, and make good. But the pricing — and the suitability — hinges on factors that aren't visible from the kerb: the cavity width, whether it's already (partially) filled, the condition of the wall ties and mortar, and crucially the exposure to wind-driven rain, because filling the cavity of a severely exposed wall can bridge it and cause penetrating damp.

The most important thing a tradesperson can do on a CWI job is the suitability survey first. A borescope inspection of the cavity (through a drilled hole) confirms the cavity width, that it's clear and unfilled, and the condition inside. The exposure zone (to BS 8104 / the published exposure maps) determines whether full-fill is even appropriate — in the most severe exposure zones, full-fill CWI is often not recommended. Pricing a job and starting to drill without this assessment is how installers end up causing the damp problems that gave CWI a bad name.

This guide covers the two main fill materials (blown mineral wool and EPS bead), realistic pricing by property size, the survey and suitability requirements, the grant/guarantee framework (CIGA, ECO4, GBIS, PAS 2030/2035), and the damp risks that make a careful survey non-negotiable. For the insulation principles see cavity wall; for the alternative on solid or unsuitable walls see external wall insulation pricing guide.

Key Facts

• Cavity wall insulation (CWI) — insulation injected into the gap between the two leaves of a cavity wall
• Blown mineral wool — fibre fill (glass/rock wool); long-established; water-shedding fibres
• EPS bead — expanded polystyrene beads, often bonded with adhesive; flows well, good for irregular cavities
• Drill pattern — ~22-25mm holes drilled in mortar joints on a grid (typically ~1m horizontal, staggered), then filled/made good
• Minimum cavity width — typically ≥50mm clear cavity needed for effective fill
• Survey first — borescope inspection to confirm cavity width, clear/unfilled, tie condition
• Exposure zone — to BS 8104; severe exposure (Zone 3/4 / very severe) may make full-fill unsuitable (penetrating damp risk)
• U-value improvement — an unfilled cavity wall (~1.5 W/m²K) can improve to ~0.5-0.6 W/m²K filled
• CIGA — Cavity Insulation Guarantee Agency; provides 25-year guarantees via registered installers
• PAS 2030 / PAS 2035 — installation and retrofit coordination standards required for grant-funded work
• ECO4 / GBIS (Great British Insulation Scheme) — grant funding schemes that can fully or partly fund CWI for eligible households
• BS EN 14064 — blown mineral wool loose-fill product standard; BS 8208 — assessment of suitability of existing walls for filling
• Making good — drilled holes filled with colour-matched mortar; quality of make-good is visible and matters
• Not for — solid walls (no cavity), already-filled cavities, severely exposed walls, walls with significant defects/damp until repaired

Quick Reference Table

Detailed Guidance

Step 1 — The Suitability Survey (Never Skip)

Before any pricing is final, the cavity must be assessed. A competent installer:

1. Drills a small inspection hole and uses a borescope to look inside the cavity — confirming it's a genuine cavity, its width (≥50mm clear), that it's empty (not already filled), and the condition of the wall ties and mortar droppings.
2. Assesses exposure to wind-driven rain using BS 8104 / the published exposure zone maps. In severe/very severe exposure zones, full-fill CWI can bridge the cavity and transmit rain to the inner leaf — so full-fill may be ruled out and partial-fill or a different approach recommended.
3. Checks for defects — cracked render, defective pointing, failed wall ties, existing damp. These must be repaired before filling; insulating a wall with active water ingress traps the problem.
4. Confirms no existing fill — re-filling an already-filled or partially-filled cavity is a known cause of damp bridging.

This survey is what separates a durable job from a damp-causing disaster, and PAS 2035 retrofit coordination formalises it for grant work.

Step 2 — Choosing the Fill

Blown mineral wool (glass or rock wool fibres): the long-established fill. The fibres shed water and resist moisture transfer across the cavity reasonably well. Good thermal performance, non-combustible (rock wool especially). Suits standard, clear cavities of consistent width.

EPS bead (expanded polystyrene, usually bonded with an adhesive as it's injected): the beads flow and pack well, making them good for narrow or irregular cavities and harder-to-fill geometries. The bonding adhesive helps the beads stay put and reduces slumping. Widely used and well-proven.

Polyurethane foam: expands and bonds, sometimes used where the wall structure needs some stabilisation, but care is needed because cured foam is hard to remove later and can be harder to assess. More specialist.

The installer matches the fill to the cavity condition and exposure; the householder rarely specifies it. Material cost differences are modest relative to total job cost.

Step 3 — The Installation

1. Set out the drill grid in the mortar joints (joints, not bricks, so make-good is invisible) — typically around 1m horizontal spacing, staggered vertically, plus extra holes at corners, around openings, and at cavity barriers.
2. Drill ~22-25mm holes through the outer leaf.
3. Inject the fill, working systematically (usually bottom-up for bead, methodically for wool) to ensure complete fill with no voids.
4. Plug and make good each hole with colour-matched mortar — the visible quality of the make-good is what the customer judges.
5. Seal cavity barriers / stop ends at openings and the wall top to contain the fill.

A typical semi is a 1-day job for a 2-person crew once scaffolding/access is sorted; access often dictates the timeline more than the filling.

Pricing Example (3-bed semi, EPS bead, regional)

Grant-funded jobs (ECO4/GBIS) can reduce or eliminate the customer's cost where they qualify, but the installer must be PAS 2030 certified and the work PAS 2035 coordinated.

Grants, Guarantees and Standards

• CIGA (Cavity Insulation Guarantee Agency) — registered installers provide a 25-year guarantee covering material and workmanship; a major selling point and protects the homeowner.
• PAS 2030 — the installation standard the installer must hold for grant-funded work.
• PAS 2035 — the retrofit coordination/whole-house standard governing assessment, design, and risk for grant-funded retrofit (including CWI).
• ECO4 — the Energy Company Obligation scheme; funds CWI for eligible (low-income/vulnerable) households via energy suppliers.
• GBIS (Great British Insulation Scheme) — broader-eligibility grant scheme funding insulation including CWI.

For commercial (non-grant) work CIGA registration and a sound survey are still the marks of a credible installer.

The Damp Risk — Why CWI Got a Bad Name

Badly surveyed CWI caused damp problems in past decades, and it's worth understanding so you don't repeat it:

• Exposure bridging — filling a severely exposed wall lets wind-driven rain track across the (now filled) cavity to the inner leaf, causing penetrating damp. Hence the exposure assessment.
• Slumping/voids — incomplete fill or settled fill leaves cold spots and can channel water.
• Pre-existing defects — filling over cracked render, failed pointing, or broken wall ties seals in the water path.
• Wrong material for the cavity — wool in a damp/irregular cavity, bead without proper containment.

A correct survey, defect repair first, and the right material for the exposure and cavity eliminate these. When a wall isn't suitable for CWI, the honest answer is to recommend external or internal wall insulation instead, not to fill it anyway. See penetrating damp and cavity wall tie failure.

Frequently Asked Questions

How much does cavity wall insulation cost?

For a typical 3-bed semi, roughly £700-£2,000 installed (less for a mid-terrace with fewer exposed walls, more for a detached house). Cost is driven by external wall area, access/scaffolding, and the fill material. Many eligible households pay little or nothing through grant schemes (ECO4, GBIS), where the installer must be PAS 2030 certified.

How do I know if my cavity is suitable for filling?

A borescope survey through a small drilled hole confirms the cavity is a genuine clear cavity at least ~50mm wide, currently empty (not already filled), and free of defects. The installer also assesses the wall's exposure to wind-driven rain (BS 8104) — in severe/very severe exposure zones, full-fill is often not recommended because it can bridge the cavity and cause penetrating damp. Never fill without this survey.

Blown mineral wool or EPS bead — which is better?

Both are well-proven; the installer matches the fill to the cavity. Mineral wool is non-combustible and water-shedding, good for standard clear cavities. EPS bead (bonded as it's injected) flows and packs well, making it better for narrow or irregular cavities and reducing slumping. Material cost differences are small relative to the total job; suitability and installation quality matter far more than which of the two is chosen.

Can cavity wall insulation cause damp?

It can if installed badly — filling a severely exposed wall (bridging the cavity), incomplete fill leaving cold spots, or filling over existing defects like cracked render or failed wall ties. A correct exposure assessment, repairing defects first, and the right material for the cavity prevent this. Where a wall genuinely isn't suitable, the right answer is external or internal wall insulation, not filling anyway.

Is there a guarantee with cavity wall insulation?

Yes — registered installers provide a CIGA (Cavity Insulation Guarantee Agency) 25-year guarantee covering materials and workmanship. Always use a CIGA-registered installer so the guarantee is valid; it protects the homeowner if problems arise and is a sign the installer follows the proper survey and installation standards (BS EN 14064, BS 8208, and PAS 2030 for grant work).

Regulations & Standards

• BS 8208-1 — Guide to assessment of suitability of existing walls for filling with thermal insulation (cavity fill)

• BS EN 14064-1/2 — Thermal insulation products — loose-fill mineral wool

• BS EN 13166 / BS EN 13163 — EPS and other rigid/bead insulation product standards

• BS 8104 — Method for assessing exposure to wind-driven rain (exposure zones)

• PAS 2030:2019 — Installation of energy efficiency measures (grant-funded work)

• PAS 2035:2023 — Retrofitting dwellings for improved energy efficiency (coordination, risk)

• Building Regulations Part L — conservation of fuel and power (U-value targets)

• Building Regulations Part C — resistance to moisture (damp risk)

• CIGA guarantee scheme — 25-year cavity insulation guarantee

• CIGA — Cavity Insulation Guarantee Agency — guarantees and registered installers

• GOV.UK — Great British Insulation Scheme — grant funding

• Ofgem — Energy Company Obligation (ECO) — ECO4 funding

• GOV.UK — Approved Document L — thermal targets

• TrustMark — PAS 2030/2035 registered businesses

• cavity wall — cavity wall insulation principles

• external wall insulation pricing guide — alternative for solid/unsuitable walls

• epc ratings — how CWI affects EPC bands

• cavity wall tie failure — wall tie condition affects suitability

• penetrating damp — the damp risk a survey prevents