Cavity Wall Insulation: Blown Fibre, Foam & Bead — When It Goes Wrong

Summary

Cavity wall insulation is the second most cost-effective thermal upgrade for UK homes after loft insulation. About 8 million UK homes with unfilled cavity walls could benefit from it. When installed correctly in a suitable wall, it is largely invisible, requires no internal or external disruption, and can save £200–£400 per year in heating costs.

The problem is the word "suitable." CWI has been responsible for widespread damp problems in UK homes, primarily through the misuse of fibre insulation in exposed or defective walls. The cavity wall construction that became standard from the 1930s onwards relies on the cavity itself as a moisture barrier — rain penetrating the outer leaf has nowhere to go except down and out at the base of the cavity. Fill the cavity with fibre, and any water penetrating the outer leaf now has a direct path to the inner leaf.

The CIGA (Cavity Insulation Guarantee Agency) guarantee scheme was introduced to provide long-term protection, but thousands of poorly installed jobs from the 1990s and 2000s have resulted in costly remediation. Understanding suitability — and being willing to say no to a job that will fail — is essential for any contractor working in this area.

Key Facts

• Cavity width requirement — minimum 50mm unfilled cavity for blown fibre and bead; 25mm minimum for foam
• Wall construction — must be cavity masonry construction; not suitable for solid masonry, no-fines concrete, or rain-screen cladding
• Exposure assessment — BS 8415 Annex A and the CIGA surveyor guidance require exposure assessment; heavily exposed elevations may not be suitable for mineral fibre
• BBA certificate — all CWI products must have a British Board of Agrément (BBA) certificate confirming suitability
• CIGA guarantee — 25-year guarantee available from Cavity Insulation Guarantee Agency for qualifying installations
• Signs of existing problems — check for signs of damp penetration before installation; existing defects will be exacerbated
• Drill pattern — holes drilled from outside at approximately 1,000mm centres horizontally, 450mm centres vertically (staggered); 22mm diameter typically
• Pointing — all drilled holes must be pointed after installation to maintain weathertightness
• Timber frames — homes with timber frames and cavity between the frame and outer leaf (masonry veneer) are NOT suitable for standard CWI; different systems required
• Wider cavities (75mm+) — more suitable than narrow cavities; less risk of bridging
• ECO4 scheme — many installs are partly or fully funded through Energy Company Obligation; check eligibility
• Part L compliance — CWI installation is a relevant energy efficiency measure; may trigger further requirements if done as part of a larger renovation

Quick Reference Table

Detailed Guidance

Suitability Survey

Before any installation, a thorough suitability survey is mandatory under BS 8415 and BBA certificate requirements. A competent CWI installer should:

1. Check wall construction — verify there is a continuous cavity using a borescope (drain down) through a test drill. Confirm cavity width (minimum 50mm for fibre/bead). Look for wall ties, obstructions, and signs of partial fill.

2. Exposure assessment — use the BREDEM exposure zone map or BS 8104 methodology to categorise the site. Zone 3 and 4 (very exposed) sites have restricted CWI options.

3. Check for defects — inspect external pointing; look for cracks, damaged or spalling brickwork; check for efflorescence (white salt deposits indicating moisture movement); check render integrity.

4. Identify obstacles — flues, ducts, beam ends, lintels, and window reveals that interrupt the cavity. These areas will require special attention or local exclusion from filling.

5. Assess cavity cleanliness — mortar droppings on wall ties are common in older buildings; bridging from mortar snots in the cavity can cause damp even before CWI.

The Exposed Site Problem

The single biggest cause of CWI failure is installing mineral fibre insulation in a wall that is too exposed. In exposed conditions, wind-driven rain penetrates the outer leaf and saturates mineral fibre. The water then migrates to the inner leaf, causing damp, mould growth, and damage.

The exposure zones:

• Zone 1 (sheltered): Most inland areas; mineral fibre or EPS bead suitable
• Zone 2 (moderate): Semi-exposed; mineral fibre with caution; EPS bead preferred
• Zone 3 (severe): Coastal, hillside; EPS bead with bonding only; fibre not appropriate
• Zone 4 (very severe): Exposed coasts, highlands; CWI often not appropriate; consider external wall insulation

EPS bead advantage in exposed sites: Polystyrene beads are inherently water-resistant. Each bead has a closed-cell structure that does not absorb moisture. With an adhesive coating ensuring the beads bond together, even wind-driven rain penetrating the outer leaf drains down the inner face of the outer leaf rather than migrating through the fill. This makes EPS bead the preferred choice for moderately exposed sites.

When CWI Fails: Signs and Remediation

Despite the 25-year CIGA guarantee, the remediation process when CWI causes damp is often protracted and contentious. Signs of CWI failure:

• Damp patches on internal walls — particularly on external walls, lower floors, and north-facing walls
• Mould growth — on cold corners, behind furniture on external walls
• Increased heating bills — counterintuitively, wet insulation has reduced thermal performance and may increase heat loss vs an empty cavity
• Efflorescence — white salt deposits on internal plaster indicating water movement through the wall

Remediation options:

1. Investigation — borescope survey to confirm whether CWI is the cause; may also reveal pre-existing wall defects
2. Extraction — CWI can sometimes be extracted (blown mineral fibre and loose beads can be vacuumed out; foam cannot be removed without demolition)
3. External wall treatment — apply silicone masonry cream or render to reduce rain penetration of the outer leaf; this addresses the symptom (water entry) rather than the cause
4. External wall insulation — fit external cladding with insulation, effectively creating a waterproof layer over the existing wall; the most comprehensive solution but expensive

For foam-injected properties, extraction is not possible. The only options are external treatment to reduce water ingress, or waiting for the foam to naturally dry out (may take months in wet conditions).

Polyurethane Foam Injection: The Controversial Product

PUR foam CWI was sold heavily in the 2000s and 2010s as a premium product with claimed R-values superior to mineral fibre. It has since become extremely controversial because:

1. Mortgage issues — many UK mortgage lenders refuse to lend on properties with foam CWI because the foam's long-term structural effects on wall ties and masonry are uncertain. The wall cannot be inspected for wall tie corrosion without destructive investigation.

2. Cannot be removed — unlike blown fibre or bead, foam cannot be extracted. It is permanent.

3. Wall tie interaction — foam-encased wall ties may corrode faster due to moisture retention; the foam prevents inspection and replacement of failing ties.

4. Survey reports — RICS surveyors often flag foam CWI in valuation reports, recommending specialist investigation, which deters buyers and lenders.

If a customer asks about foam CWI: be cautious. The product is technically capable of performing well, but the mortgage and resale implications are severe. EPS bead or mineral fibre are almost always the better recommendation.

Frequently Asked Questions

My house has partial CWI — just the top floor. Should I fill the rest?

This is common in properties where CWI was partially installed under a grant scheme. It is worth investigating, but be aware that the lower floors may have a different exposure level to the upper floors (less sheltered from ground vegetation and trees) and should be assessed separately. Get a borescope survey of a lower-floor cavity before proceeding. If the cavity at lower levels is wet or has mortar bridging, remediate those issues before filling.

Can CWI be installed in a house with a flat roof extension?

The flat roof area itself is a separate insulation challenge, but the cavity walls below it can be filled normally if they meet the suitability criteria. However, take care at the junction between the flat roof and the cavity wall — this is a common moisture entry point that CWI can exacerbate if the junction detail is poor. Inspect and seal the flat roof/wall junction before installation.

How do I tell if my walls are suitable without a professional survey?

Do this simple assessment:

1. Check if you have a cavity: if your house was built between 1935 and the present, and the exterior brick is in running bond (stretchers only visible), it almost certainly has a cavity. Pre-1935 houses are more likely to be solid masonry.
2. Check the pointing: run your finger along a mortar joint. If it's crumbling, powdery, or missing, it needs repair before CWI.
3. Look for damp: check internal walls on cold, rainy days. Any signs of damp on external walls means CWI should wait until the cause is investigated.
4. Check the site: are you on a west- or south-west-facing elevation near the coast or on a hillside? If so, consider EPS bead.

What should a CWI certificate include?

A CIGA-guaranteed installation should provide: the CIGA guarantee certificate (25 years) with the property address, installation date, contractor details, and product used; a BBA certificate number for the product; a record of the installation drill pattern; and confirmation of pointing completion.

Regulations & Standards

• BS 8415 — Specifications for materials for filling of masonry cavity walls with blown man-made mineral fibre; covers suitability assessment

• BBA Certificate — required for all CWI products installed in the UK; confirms performance and suitability criteria

• CIGA (Cavity Insulation Guarantee Agency) — 25-year guarantee scheme; installer registration requirement

• Building Regulations Approved Document L — energy conservation; CWI as a relevant measure

• BREDEM exposure zone assessment — methodology for determining site exposure for CWI suitability

• CIGA: What You Need to Know — guarantee scheme information and finding approved installers

• Energy Saving Trust: Cavity Wall Insulation — consumer guidance

• Which? CWI Guide — independent consumer assessment including failure cases

• BBA: Approved Products Database — verify BBA certificates for CWI products

• solid wall — alternative for solid masonry walls or where CWI is not suitable

• thermal bridging — cold bridging around CWI-filled walls

• epc ratings — how CWI affects EPC score

• loft insulation — complementary measure to CWI for whole-house energy efficiency