Cavity Wall Insulation in London: Types, Costs, Grants & What to Avoid

NOT ALL LONDON HOMES WITH CAVITY WALLS ARE SUITABLE FOR CWI — A PROFESSIONAL CAVITY WALL ASSESSMENT (BORESCOPE SURVEY) IS ESSENTIAL BEFORE INSTALLATION.

CRITERIA FOR SUITABLE PROPERTIES:

1. MINIMUM CAVITY WIDTH: MINIMUM 50MM CLEAR CAVITY IS REQUIRED FOR BLOWN MINERAL WOOL. NARROWER CAVITIES (BELOW 50MM) ARE NOT SUITABLE FOR CONVENTIONAL BLOWN-FIBRE CWI AND MAY REQUIRE EPS BEAD (WHICH CAN FILL NARROWER CAVITIES) OR RECONSIDERATION.

2. EXPOSURE ZONE: BRE DIGEST 236 AND BS 8208 CLASSIFY WALLS INTO FOUR EXPOSURE ZONES FROM SHELTERED TO SEVERE. LONDON IS GENERALLY IN EXPOSURE ZONE 3 (MODERATE TO SEVERE) — MEANING THAT THE OUTER LEAF OF BRICKWORK MUST BE IN GOOD CONDITION AND THE MORTAR JOINTS MUST BE SOUND AND REPOINTED WHERE NECESSARY BEFORE CWI IS INSTALLED. WIND-DRIVEN RAIN CAN BRIDGE A POORLY CONSTRUCTED OR COMPROMISED OUTER LEAF AND CARRY MOISTURE TO THE NOW-INSULATED CAVITY WALL INNER FACE — CAUSING DAMP PENETRATION.

3. WALL CONDITION: BOREHOLE SURVEY (BORESCOPE INSPECTION THROUGH A SERIES OF 22MM HOLES DRILLED IN THE MORTAR JOINTS) ASSESSES: — CAVITY CLEAR OF DEBRIS AND MORTAR DROPPINGS (EXCESSIVE DEBRIS BLOCKS UNIFORM FILL) — CAVITY TIES IN SOUND CONDITION (CORRODED CAVITY TIES PRE-EXISTING IN PROPERTIES BUILT 1920-1980 WITH GALVANISED TIE IS A COMMON LONDON PROBLEM — MUST BE ADDRESSED SEPARATELY) — OUTER LEAF BRICKWORK SOUND (NO SPALLING, CRACKING, OR DAMP STAINING) — CAVITY FREE OF EXISTING INSULATION (SOME LONDON HOMES HAVE PARTIAL FILL ALREADY)

4. HISTORICAL FLOOD RISK: PROPERTIES THAT HAVE FLOODED OR ARE IN FLOOD RISK ZONES (MANY LONDON PROPERTIES NEAR THE THAMES IN RICHMOND, KEW, TWICKENHAM, HAMMERSMITH, PUTNEY) MAY NOT BE SUITABLE FOR MINERAL WOOL CWI AS THE MATERIAL CAN ABSORB AND RETAIN WATER — EPS BEAD IS MORE RESILIENT TO FLOODING AS IT DOES NOT ABSORB WATER.

THREE MAIN MATERIALS ARE USED FOR CAVITY WALL INSULATION IN THE UK:

1. BLOWN MINERAL WOOL (KNAUF ECOBEAD MINERAL WOOL, ISOVER OPTIM-R, ROCKWOOL CAVITY FILL): THE MOST WIDELY INSTALLED CWI IN LONDON. MINERAL WOOL FIBRE IS BLOWN UNDER CONTROLLED PRESSURE THROUGH A SERIES OF HOLES DRILLED AT 1M CENTRES IN THE MORTAR JOINTS OF THE OUTER LEAF. THE FIBRE FILLS THE CAVITY UNIFORMLY. AFTER INJECTION, HOLES ARE RESEALED WITH MATCHING MORTAR. PERFORMANCE: THERMAL CONDUCTIVITY 0.032-0.044 W/MK. WALL U-VALUE IMPROVEMENT: FROM 1.5-1.7 W/M2K TO 0.45-0.55 W/M2K. ADVANTAGES: LOW COST, WELL-ESTABLISHED PRODUCT, NON-COMBUSTIBLE (A1 FIRE RATING), WIDELY AVAILABLE THROUGH CIGA-REGISTERED INSTALLERS. DISADVANTAGES: ABSORBS AND CAN RETAIN MOISTURE IF CAVITY EXPERIENCES WATER INGRESS — THE PRIMARY CAUSE OF FAILURE IN INAPPROPRIATE APPLICATIONS.

2. EPS BEAD (EXPANDED POLYSTYRENE BEAD WITH GRAPHITE — "GRAPHITE EPS"): GRAPHITE-ENHANCED EPS BEAD (NEOWALL, PERMA-FILL GRAPHITE EPS, RECTICEL EUROBEAD) BLOWN INTO CAVITY AND BONDED WITH ADHESIVE TO PREVENT SETTLEMENT OR MIGRATION. THERMAL CONDUCTIVITY: 0.031-0.033 W/MK (BETTER THAN STANDARD MINERAL WOOL). HYDROPHOBIC — DOES NOT ABSORB WATER. ADVANTAGES: BETTER THERMAL PERFORMANCE THAN MINERAL WOOL; HYDROPHOBIC (SUITABLE FOR FLOOD-RISK ZONES OR EXPOSED ELEVATIONS); GOOD FOR NARROW CAVITIES (CAN FILL DOWN TO 40MM). DISADVANTAGES: HIGHER COST THAN MINERAL WOOL (15-25% PREMIUM); FIRE RATING — EPS IS COMBUSTIBLE (REACTION TO FIRE CLASS E UNPROTECTED); MUST BE ENCLOSED WITHIN MASONRY CAVITY FOR FIRE SAFETY.

3. INJECTION FOAM (UF FOAM — UREA FORMALDEHYDE, OR POLYURETHANE): FOAM INJECTED INTO CAVITY — EXPANDS AND SETS TO FILL CAVITY. HISTORICALLY THE CHEAPEST OPTION. HOWEVER: UF FOAM: WAS WIDELY USED IN THE 1970S-1980S AND HAS PROVEN PROBLEMATIC — SHRINKAGE OVER TIME CREATES VOIDS; HYGROSCOPIC (ABSORBS MOISTURE); FORMALDEHYDE OFFGASSING WAS A HISTORIC CONCERN. POLYURETHANE FOAM: MODERN VERSION — BETTER PERFORMANCE THAN UF. HOWEVER, CONCERNS REMAIN ABOUT LONG-TERM PERFORMANCE AND FIRE BEHAVIOUR IF CAVITY IS BREACHED. RECOMMENDATION: MINERAL WOOL OR GRAPHITE EPS BEAD IS THE PREFERRED MATERIAL FOR LONDON RESIDENTIAL PROPERTIES — AVOID FOAM INJECTION UNLESS A SPECIALIST WITH BHETA CERTIFICATION AND STRONG TRACK RECORD RECOMMENDS IT FOR A SPECIFIC REASON.

THE CAVITY INSULATION GUARANTEE AGENCY (CIGA) IS THE UK'S SCHEME FOR GUARANTEEING CWI INSTALLATION QUALITY. ALL CWI INSTALLED IN ACCORDANCE WITH BS 8208 BY A CIGA-REGISTERED INSTALLER SHOULD BE BACKED BY A 25-YEAR CIGA GUARANTEE.

KEY POINTS ABOUT CIGA: — ALL CWI INSTALLED UNDER ECO4 OR GREAT BRITISH INSULATION SCHEME (GBIS) MUST BE INSTALLED BY A CIGA-REGISTERED INSTALLER — CIGA GUARANTEES ARE TRANSFERABLE TO NEW OWNERS ON PROPERTY SALE (IMPORTANT FOR LONDON PROPERTY RESALE) — CIGA HAS A COMPLAINTS AND REMEDIATION PROCESS FOR FAILED INSTALLATIONS — CHECK CIGA.CO.UK FOR INSTALLER REGISTER AND TO MAKE A COMPLAINT IF AN EXISTING INSTALLATION IS CAUSING PROBLEMS

HOW TO CHOOSE A CWI INSTALLER: 1. CIGA-REGISTERED: VERIFY REGISTRATION ON CIGA.CO.UK 2. BEIS/TRUSTMARK REGISTERED: ALL ECO4 INSTALLERS MUST BE TRUSTMARK REGISTERED 3. BORESCOPE SURVEY INCLUDED: INSIST ON A FULL BORESCOPE SURVEY BEFORE ANY QUOTE — REJECT ANY INSTALLER WHO QUOTES WITHOUT DRILLING AND INSPECTING THE CAVITY 4. AVOID DOORSTEP CANVASSERS: THE MAJORITY OF MISINSTALLATION CASES FROM THE 2009-2013 PERIOD INVOLVED CANVASSERS TARGETING HOMES FOR SUBSIDISED INSTALLATION WITHOUT PROPER CAVITY ASSESSMENT 5. GET 3 QUOTES AND COMPARE ASSESSMENT REPORTS: DIFFERENT INSTALLERS MAY IDENTIFY DIFFERENT SUITABILITY ISSUES

SEVERAL GRANT SCHEMES ARE AVAILABLE TO LONDON HOMEOWNERS FOR CAVITY WALL INSULATION — SOME PROVIDING FREE INSTALLATION FOR ELIGIBLE HOUSEHOLDS:

ECO4 (ENERGY COMPANY OBLIGATION 4): OBLIGATION ON UK'S LARGEST ENERGY COMPANIES TO FUND ENERGY EFFICIENCY IMPROVEMENTS IN LOWER-INCOME HOUSEHOLDS. CAVITY WALL INSULATION IS A CORE ECO4 MEASURE. ELIGIBILITY: — HOUSEHOLD IN RECEIPT OF QUALIFYING BENEFIT (UNIVERSAL CREDIT, PENSION CREDIT, CHILD TAX CREDIT, WORKING TAX CREDIT) — OR PROPERTY WITH EPC RATING D-G — OR VULNERABLE HOUSEHOLD (IDENTIFIED BY HEALTH CONDITION) APPLY: THROUGH YOUR ENERGY SUPPLIER OR VIA SIMPLEENERGYADVICE.ORG

GREAT BRITISH INSULATION SCHEME (GBIS): BROADER ELIGIBILITY THAN ECO4 — AVAILABLE TO PROPERTIES WITH EPC RATING D-G IN COUNCIL TAX BANDS A-D (OR E-H FOR ECO4-ELIGIBLE HOUSEHOLDS). LAUNCHED 2023, RUNNING TO 2026. APPLY: SIMPLEENERGYADVICE.ORG OR DIRECTLY WITH A REGISTERED INSTALLER.

LONDON-SPECIFIC SCHEMES: — WARMER HOMES PROGRAMME (GLA): THE MAYOR OF LONDON'S WARMER HOMES PROGRAMME PROVIDES GRANTS FOR ENERGY EFFICIENCY IMPROVEMENTS INCLUDING CWI FOR LOWER-INCOME LONDON HOUSEHOLDS. CHECK: LONDON.GOV.UK/WARMER-HOMES — BOROUGH SCHEMES: SEVERAL LONDON BOROUGHS (ISLINGTON, HACKNEY, LAMBETH, LEWISHAM) HAVE RUN LOCAL INSULATION PROGRAMMES — CHECK YOUR BOROUGH COUNCIL WEBSITE.

WHEN NOT GRANT-ELIGIBLE: FOR PRIVATELY-FUNDED CWI IN LONDON, EXPECT TO PAY: — SEMI-DETACHED HOUSE (TYPICAL 3-BED LONDON PROPERTY): £600-£1,200 FOR MINERAL WOOL CWI — DETACHED HOUSE: £800-£1,500 — GRAPHITE EPS BEAD (PREMIUM): ADD 20-30% TO ABOVE FIGURES

A SIGNIFICANT NUMBER OF LONDON HOMES HAD CWI INSTALLED UNDER PRE-ECO4 SUBSIDY SCHEMES IN THE 2000S-2010S THAT HAS SUBSEQUENTLY FAILED — CAUSING DAMPNESS, MOULD, AND IN SOME CASES STRUCTURAL DAMAGE. UNDERSTANDING THE CAUSES OF FAILURE IS IMPORTANT FOR BOTH BUYERS OF LONDON PROPERTIES AND HOMEOWNERS EXPERIENCING PROBLEMS.

COMMON CAUSES OF CWI FAILURE: 1. INSTALLATION IN AN UNSUITABLE PROPERTY: EXPOSED LOCATION, ALREADY DAMP OUTER LEAF, MORTAR JOINT DEFECTS, OR A NARROW CAVITY — ALL MAKE CWI MORE LIKELY TO FAIL 2. USE OF INAPPROPRIATE MATERIAL: HYDROPHILIC MINERAL WOOL INSTALLED IN A HIGH-EXPOSURE-ZONE LOCATION OR IN A PROPERTY WITH DAMAGED OUTER LEAF 3. POOR INSTALLATION: INADEQUATE DRILLING PATTERN OR UNDER-PRESSURE BLOWING LEAVING VOIDS IN THE FILL 4. CAVITY TIE CORROSION: PRE-EXISTING CORRODED TIES COMBINED WITH CWI CAN ACCELERATE DAMP PENETRATION THROUGH TIE RUST STAINING AND MORTAR CRACKING

SIGNS OF FAILED CWI: — DAMP PATCHES ON INTERNAL WALLS APPEARING AFTER CWI INSTALLATION — MOULD GROWTH ON EXTERNAL WALLS, ESPECIALLY IN COLD SPOTS — DAMP SMELL IN ROOMS WITH EXTERNAL WALLS — SALT CRYSTALLISATION (EFFLORESCENCE) ON INTERNAL WALL PLASTER

REMEDIATION: — REPORT TO CIGA IF INSTALLED UNDER A GUARANTEE — SPECIALIST CWI EXTRACTION (CAVITY CLEARANCE) COMPANIES CAN REMOVE FAILED INSULATION BY DRILLING AND VACUUMING OR WASHING OUT THE CAVITY — REPAIR OUTER LEAF DEFECTS, REPOINT WITH NHL LIME MORTAR, AND ALLOW TO DRY BEFORE REINSTALLING — COST OF EXTRACTION: £2,000-£6,000 DEPENDING ON PROPERTY SIZE