Cavity Wall Insulation Damp Problems: Causes & Solutions
Cavity wall insulation is the UK's most common insulation type - and the most common cause of damp problems. If you've developed damp, mould, or condensation after cavity wall insulation was installed, here's everything you need to know about why it happens and how to fix it.
Cavity Wall Insulation by the Numbers
How Cavity Wall Insulation Causes Damp
1. Bridging the Cavity Gap
Cavity walls have a gap (usually 50-100mm) designed to prevent moisture from the outer wall reaching the inner wall. Cavity wall insulation fills this protective gap. When installed incorrectly or in unsuitable properties, the insulation acts as a bridge, allowing water to cross from the outer to inner wall.
Common bridging scenarios:
- • Insulation touching both walls (should have 25mm residual gap in exposed areas)
- • Material compaction over time closing the protective gap
- • Oversized beads or fibers filling the entire cavity
- • Slumping insulation creating moisture paths
2. Water Absorption by Insulation Material
Most cavity wall insulation materials (blown mineral wool, polystyrene beads, foam) can absorb water from rain penetration. Once saturated, they hold moisture against the inner wall instead of allowing it to dry out naturally.
Material-specific problems:
- • Mineral wool: Absorbs water like a sponge, takes weeks to dry out
- • Polystyrene beads: Water pools between beads, creating permanent damp reservoir
- • Foam: Less absorbent but channels water to weak points
- • All types: Trapping moisture prevents natural cavity drying
3. Condensation from Reduced Breathability
Filling the cavity reduces the wall's ability to breathe. Moisture vapor from cooking, showering, and breathing has nowhere to go. It condenses on the coldest surface - usually the inner wall behind furniture or in corners - causing black mould.
Condensation hotspots:
- • Behind wardrobes and beds (restricted airflow)
- • External wall corners (thermal bridges)
- • North-facing walls (coldest, least sun exposure)
- • Rooms with high moisture (bedrooms, bathrooms)
4. Installation Defects
Poor installation is the primary cause of CWI damp problems. Rushed installations (often under government schemes like ECO4) miss critical steps, use wrong materials, or ignore property-specific risk factors.
Typical installation failures:
- • No pre-installation survey to assess wall condition or exposure
- • Installing in properties unsuitable for CWI (exposed locations, porous bricks)
- • Overfilling cavities leaving no residual gap
- • Not clearing existing debris from cavity before installation
- • Damaged or inadequate DPC (damp proof course)
- • Wall ties acting as moisture bridges not addressed
Signs Your Cavity Wall Insulation Is Causing Damp
Visual Signs
- Black mould on walls (especially corners)
- Damp patches appearing after rain
- Peeling wallpaper or paint
- Discoloration or tide marks on walls
- Excessive condensation on windows
- White salt deposits (efflorescence) on walls
Physical & Environmental Signs
- Walls feel cold or damp to touch
- Musty, damp smell in rooms
- Problem worse in winter or after heavy rain
- Damp only on external walls (not internal walls)
- Problem started after CWI installation
- Higher energy bills (damp walls harder to heat)
Key Diagnostic Question:
Did the damp problem start AFTER cavity wall insulation was installed? If yes, there's a 90% chance the insulation is the cause. Get a professional damp survey to prove causation - essential for compensation claims.
Solutions for CWI Damp Problems
Cavity Wall Insulation Extraction (Full Removal)
The most effective solution. Specialized extraction equipment removes 95-98% of the insulation, restoring the cavity gap and allowing walls to dry naturally.
Process:
- • External wall drilled (100-200 holes)
- • Industrial vacuum extracts insulation
- • Cavity inspected with borescope
- • Holes filled and made good
- • External redecoration (if needed)
Cost: £3,000 - £12,000
- • Semi-detached: £3,000-£5,000
- • Detached: £6,000-£9,000
- • Large/complex: £10,000-£12,000
- • Usually completed in 1-3 days
Partial Extraction (Targeted Removal)
Removing insulation only from affected areas (e.g., most exposed walls). Cheaper but may not solve the whole problem if damp migrates to other areas.
Best for:
- • Damp confined to one or two walls
- • Budget constraints
- • South/east walls performing well
Cost: £1,500 - £5,000
40-60% cheaper than full extraction but carries risk of needing full extraction later.
Improved Ventilation & Dehumidification
For condensation-driven damp (not penetrating damp). Installing mechanical ventilation, dehumidifiers, and improving air circulation can manage symptoms without extraction.
Solutions:
- • Positive input ventilation (PIV) system: £800-£1,500
- • Mechanical extract ventilation (MEV): £1,200-£2,500
- • Whole-house dehumidifier: £300-£800
- • Additional trickle vents: £200-£600
Limitations:
- • Only works for condensation, not penetrating damp
- • Ongoing running costs (electricity)
- • Treats symptoms, not cause
- • May not fully eliminate mould
Solutions That DON'T Work (Avoid These)
- Chemical damp treatments: Don't address the insulation bridging water across cavity
- Replastering/tanking internal walls: Traps damp inside, makes it worse
- Just painting over mould: Mould returns within weeks
- Waiting it out: CWI damp gets worse over time, not better
Getting Compensation for CWI Extraction
Most cavity wall insulation installed in the last 25 years comes with a CIGA guarantee. If your installer provided a CIGA certificate, you can claim for extraction even if the installer has ceased trading.
CIGA Guarantee Claims
CIGA provides 25-year insurance-backed guarantees. Success rate: 72%. Average payout: £3,000-£12,000.
- • Covers extraction and making good
- • No cost if installer still trading (they pay)
- • CIGA pays if installer defunct
- • Claims process: 6-12 weeks typically
Direct Installer Claims
If installer still trading, contact them first. They're obligated to rectify defects for 10 years under consumer law.
Legal Action (No-Win-No-Fee)
If other routes fail, pursue through specialist solicitors. Success rate: 94% for CWI cases.
Learn About Compensation ClaimsGet Help with Cavity Wall Insulation Damp
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