Cavity Walls in London Homes: Understanding, Filling and Repairing 2025

THE STANDARD CAVITY WALL CONSTRUCTION IN LONDON HOUSES BUILT FROM APPROXIMATELY 1920 ONWARDS: OUTER LEAF — 102MM BRICK (STANDARD LONDON STOCK, FACING BRICK OR FLETTON) — THE VISIBLE EXTERNAL SKIN OF THE WALL; CAVITY — THE AIR GAP BETWEEN THE LEAVES; TYPICALLY 50MM IN PRE-1960 LONDON HOUSING, INCREASING TO 75-100MM IN POST-1965 CONSTRUCTION AND UP TO 150MM IN LONDON HOUSING FROM THE 1980S ONWARDS AS CAVITY FILL INSULATION BECAME STANDARD; INNER LEAF — EITHER 102MM BRICK (MOST COMMON IN LONDON VICTORIAN EXPANSION TERRACES AND INTER-WAR HOUSING) OR MEDIUM-DENSE CONCRETE BLOCK (STANDARD IN LONDON HOUSING FROM THE 1950S ONWARDS); PLASTER — 13MM LIME OR GYPSUM PLASTER TO THE INTERNAL FACE OF THE INNER LEAF. WALL TIES — THE TWO LEAVES OF A CAVITY WALL ARE STRUCTURALLY TIED TOGETHER BY WALL TIES (EMBEDDED IN THE BED JOINTS OF BOTH LEAVES AT APPROXIMATELY 900MM HORIZONTAL AND 450MM VERTICAL CENTRES). ORIGINAL TIES IN LONDON HOUSING (PRE-1980) WERE FLAT FISH-TAIL MILD STEEL TIES WHICH ARE PRONE TO CORROSION.

CAVITY WALL INSULATION CAN BE INJECTED INTO THE CAVITY OF AN EXISTING LONDON HOUSE THROUGH SMALL HOLES DRILLED IN THE OUTER LEAF AND THEN RE-POINTED. THE MAIN TYPES OF CAVITY FILL USED IN LONDON IN 2025: MINERAL FIBRE (BLOWN MINERAL WOOL) — KNAUF SUPAFILL, SAINT-GOBAIN ISOVER AND THERMABEAD WOOL ARE THE BLOWN MINERAL FIBRE PRODUCTS USED IN LONDON CAVITY WALL INSULATION; INJECTED AS A LOOSE FIBRE FILL; PROVIDES THERMAL RESISTANCE OF APPROXIMATELY 0.034 W/MK; CIGA (CAVITY INSULATION GUARANTEE AGENCY) GUARANTEE IS STANDARD FOR ALL PROPERLY INSTALLED CAVITY FILL IN LONDON. EPS BEAD (EXPANDED POLYSTYRENE BEADS) — BONDED EPS BEAD SYSTEMS (THERMABEAD, KNAUFF JETFILL EPS) ARE THE MOST WIDELY INSTALLED CAVITY FILL MATERIAL IN LONDON CAVITY WALLS WITH NARROWER CAVITIES (50-75MM); THE EPS BEADS ARE INJECTED WITH AN ADHESIVE AND BOND IN PLACE TO FILL THE CAVITY COMPLETELY; R-VALUE: 0.032-0.040 W/MK. CLOSED-CELL PIR FOAM INJECTION — A PREMIUM OPTION FOR LONDON HOUSES WITH VERY NARROW OR BRIDGED CAVITIES WHERE BEADS OR FIBRE CANNOT REACH; MORE EXPENSIVE BUT HIGHER THERMAL PERFORMANCE. CAVITY FILL IS NOT APPROPRIATE WHERE THE OUTER LEAF IS ALREADY SHOWING SIGNS OF DAMP PENETRATION (FACING BRICKWORK IN VERY EXPOSED LONDON LOCATIONS CAN ALLOW WIND-DRIVEN RAIN TO BRIDGE THE INSULATION).

WALL TIE FAILURE IS ONE OF THE MOST SERIOUS STRUCTURAL DEFECTS IN LONDON CAVITY WALL PROPERTIES BUILT BEFORE APPROXIMATELY 1980. ORIGINAL FLAT FISH-TAIL MILD STEEL WALL TIES IN LONDON INTERWAR AND POST-WAR HOUSING CORRODE OVER TIME, EXPANDING AS THEY RUST AND PUSHING APART THE BED JOINTS OF THE OUTER BRICK LEAF. SIGNS OF FAILED WALL TIES IN LONDON HOUSES: HORIZONTAL CRACKING IN THE MORTAR JOINTS OF THE OUTER BRICK FACE AT REGULAR (450MM) VERTICAL INTERVALS — THE CLASSIC SIGNATURE OF EXPANDED CORRODING WALL TIES; OUTWARD BOWING OF THE OUTER LEAF OF BRICKWORK; STRUCTURAL ENGINEER OR SPECIALIST WALL TIE CONTRACTOR INVESTIGATION USING METAL DETECTOR OR BORESCOPE. WALL TIE REPLACEMENT IN LONDON HOUSES: NEW REMEDIAL WALL TIES ARE INSTALLED BY DRILLING THROUGH THE OUTER LEAF INTO THE CAVITY AND INTO THE INNER LEAF, AND INJECTING A STAINLESS STEEL HELICAL TIE WITH RESIN ANCHOR (HELIFIX HELIBAR OR THOR HELICAL TIES ARE THE STANDARD LONDON WALL TIE REPLACEMENT PRODUCTS). COSTS IN LONDON IN 2025: WALL TIE INVESTIGATION SURVEY — £200-£400; WALL TIE REPLACEMENT — £20-£30 PER TIE INSTALLED (TYPICALLY 10-15 TIES PER M² OF WALL AREA); FOR A TYPICAL LONDON MID-TERRACE REAR ELEVATION — £800-£2,500 INCLUDING MORTAR CRACK STITCHING.

INCORRECTLY INSTALLED OR DEGRADED CAVITY WALL INSULATION CAN CAUSE SEVERE DAMP PROBLEMS IN LONDON HOMES. THE MOST COMMON CAUSES: BRIDGING — IF THE FILL MATERIAL (ESPECIALLY MINERAL FIBRE) BECOMES SATURATED OR BRIDGES GAPS IN THE CAVITY, IT CAN TRANSMIT MOISTURE FROM THE OUTER LEAF TO THE INNER LEAF; PARTICULARLY A RISK ON LONDON PROPERTIES FACING PREVAILING SOUTH-WEST WIND-DRIVEN RAIN. SUBSIDENCE OF FILL — OLDER BLOWN MINERAL FIBRE INSTALLATIONS FROM THE 1970S-1990S CAN SUBSIDE WITHIN THE CAVITY, LEAVING VOIDS AT THE TOP OF THE WALL WHERE HEAT LOSS INCREASES AND INTERSTITIAL CONDENSATION CAN OCCUR. FAILED EXTRACTION AND REMOVAL — IN SEVERE CASES, CAVITY FILL EXTRACTION (USING HIGH-POWERED VACUUM SYSTEMS THROUGH DRILL HOLES) IS REQUIRED BEFORE THE OUTER FACE IS REINVESTIGATED. THIS TYPICALLY COSTS £1,500-£4,000 FOR A LONDON MID-TERRACE AND IS COVERED UNDER CIGA WARRANTY IF THE ORIGINAL INSTALLATION WAS REGISTERED. CIGA CLAIM PROCESS — IF CAVITY WALL INSULATION WAS INSTALLED WITH A CIGA GUARANTEE (CHECK PAPERWORK OR SEARCH THE CIGA DATABASE AT CIGA.CO.UK) AND HAS FAILED, A CIGA CLAIM CAN FUND REMEDIATION AT NO COST TO THE HOMEOWNER.

LONDON CAVITY WALL WORKS COSTS IN 2025: CAVITY WALL INSULATION FILL (MINERAL FIBRE OR EPS BEAD, CIGA REGISTERED, SUPPLY AND INSTALL, 75MM CAVITY, PER M² OF WALL AREA) — £10-£18/M²; TYPICAL SEMI-DETACHED LONDON HOUSE (80M² OF EXTERNAL WALL AREA) — £800-£1,500 INSTALLED; FULLY FUNDED SCHEMES AVAILABLE THROUGH ECO4 (ENERGY COMPANY OBLIGATION) FOR ELIGIBLE LONDON HOUSEHOLDS (CHECK ELIGIBLE.CO.UK OR THROUGH LOCAL COUNCIL). WALL TIE SURVEY — £200-£400. WALL TIE REPLACEMENT — £20-£30 PER TIE; TYPICAL LONDON MID-TERRACE REAR ELEVATION — £800-£2,500. CAVITY FILL EXTRACTION — £1,500-£4,000 FOR A MID-TERRACE; COVERED UNDER VALID CIGA CLAIM WHERE APPLICABLE. EXTERNAL WALL REPOINTING AFTER DRILL-AND-FILL — INCLUDED IN A GOOD-QUALITY CAVITY FILL INSTALLATION QUOTE; IF QUOTED SEPARATELY, EXPECT £30-£60/M² FOR LONDON REPOINTING.