Condensation and Damp in London Homes: A Complete Guide

CONDENSATION IS CAUSED BY WARM MOIST AIR CONTACTING COLD SURFACES. IT IS BY FAR THE MOST COMMON DAMP PROBLEM IN LONDON — RESPONSIBLE FOR AN ESTIMATED 80-85% OF ALL DAMP COMPLAINTS. IT APPEARS AS BLACK MOULD (ASPERGILLUS NIGER, CLADOSPORIUM) ON WINDOW REVEALS, EXTERNAL CORNERS, AND NORTH-FACING WALLS. THE FIX IS PRIMARILY VENTILATION AND LIFESTYLE MODIFICATION: A FAMILY OF FOUR GENERATES APPROXIMATELY 10-15 LITRES OF WATER VAPOUR PER DAY FROM BREATHING, COOKING, AND BATHING. PENETRATING DAMP TRAVELS HORIZONTALLY OR DIAGONALLY THROUGH WALLS OR ROOFS. CAUSES INCLUDE FAILED POINTING, CRACKED RENDER, DEFECTIVE FLASHINGS, FAULTY GUTTERS, AND BRIDGED CAVITY WALLS. IT TYPICALLY CREATES TIDE MARKS, SALTING (EFFLORESCENCE), AND ISOLATED WET PATCHES THAT CORRESPOND WITH RAINFALL. RISING DAMP IS THE LEAST COMMON OF THE THREE — LESS THAN 5% OF DAMP CASES WHEN PROPERLY DIAGNOSED — YET IT IS THE TYPE MOST OFTEN SOLD TO LONDON HOMEOWNERS BY DAMP-PROOFING COMPANIES. TRUE RISING DAMP OCCURS WHEN GROUND MOISTURE WICKS UP A MASONRY WALL THROUGH CAPILLARY ACTION, TYPICALLY TO A HEIGHT OF NO MORE THAN 1.2M. IT IS RARE IN POST-1900 LONDON PROPERTY BECAUSE VIRTUALLY ALL BRICK BUILDINGS HAVE A SLATE OR LEAD DAMP-PROOF COURSE (DPC) AT LOW LEVEL.

A PROFESSIONAL CONDENSATION DIAGNOSIS USES A PROTIMETER SURVEYMASTER OR TRAMEX ENCOUNTER PLUS MOISTURE METER IN RESISTANCE MODE. TRUE MOISTURE-LADEN MASONRY GIVES READINGS ABOVE 17-20% WME; DRY WALLS WITH HYGROSCOPIC SALT CONTAMINATION CAN GIVE FALSE HIGH READINGS. A CARBIDE BOMB TEST (SPEEDY MOISTURE METER) GIVES A GRAVIMETRIC MOISTURE CONTENT READING AND IS DEFINITIVE. KEY VISUAL SIGNS OF CONDENSATION RATHER THAN PENETRATING DAMP: MOULD STARTING AT CORNERS AND REVEALS, NOT ISOLATED PATCHES; MOULD ON COLD THERMAL BRIDGES SUCH AS JOIST ENDS, STEEL LINTELS, AND WINDOW FRAMES; BLACK SPECKLING ON GROUT AND SILICON SEALANT IN BATHROOMS; WATER ON WINDOWS AND COLD PIPES. IN LONDON TERRACED HOUSES, COMMON CONDENSATION HOT SPOTS ARE: (1) REAR BEDROOM ABOVE A FLAT-ROOF KITCHEN EXTENSION WHERE THE COLD FLAT ROOF CREATES A MASSIVE COLD BRIDGE THROUGH THE WALL; (2) LOFT ROOMS WHERE INSULATION INSTALLED BETWEEN RAFTERS ONLY LEAVES THE RIDGE TIMBER AS A COLD BRIDGE; (3) GROUND-FLOOR FLATS WITH SOLID MASONRY WALLS AND NO EFFECTIVE BACKGROUND VENTILATION. THE PSYCHROMETRIC CALCULATION THAT DAMP SURVEYORS USE IS: AT 20°C AND 65% RELATIVE HUMIDITY, DEW POINT IS APPROXIMATELY 12.6°C. ANY WALL SURFACE BELOW 12.6°C WILL ATTRACT CONDENSATION. LONDON SOLID 9-INCH BRICK WALLS CAN HAVE INTERNAL FACE TEMPERATURES OF 8-12°C IN WINTER.

THE BUILDING REGULATIONS PART F 2022 (ENGLAND) SETS MINIMUM VENTILATION RATES FOR DWELLINGS. FOR A 3-BED HOUSE THE WHOLE-DWELLING VENTILATION RATE IS 29 L/S (LITRES PER SECOND) EQUIVALENT. PRODUCT-SPECIFIC SOLUTIONS INSTALLED IN LONDON HOMES: POSITIVE INPUT VENTILATION (PIV) — THE MOST COST-EFFECTIVE RETROFIT FOR MOST LONDON HOUSES. NUAIRE DRIMASTER-ECO HEAT (DFEV HEAT) DRAWS AIR THROUGH A ROOFSPACE HEAT EXCHANGER, WARMS IT SLIGHTLY, AND PRESSURISES THE HOUSE TO DRIVE MOIST AIR OUT THROUGH ADVENTITIOUS GAPS. COSTS £450-£650 SUPPLY; £200-£350 INSTALL. SUITABLE FOR HOUSES WITH A LOFT SPACE. NUAIRE CYFAN AXIAL KITCHEN FAN (15L/S CONTINUOUS, 30L/S BOOST): £95-£130 UNIT, £150-£250 INSTALL — REQUIRED IN EVERY KITCHEN UNDER PART F. TITON TRIMVENT XCITE 2500 ACOUSTIC TRICKLE VENT (3,000 MM² EQVA): RETROFITTABLE INTO WINDOW FRAMES, £12-£18 PER VENT, CRITICAL FOR CONTINUOUSLY-OCCUPIED ROOMS. FOR WHOLE-HOUSE MVHR IN NEW EXTENSIONS OR MAJOR REFURBISHMENTS: ZEHNDER COMFOAIR Q150 (93.1% HEAT RECOVERY EFFICIENCY) — SUITABLE FOR 1-2 BED FLATS; ZEHNDER COMFOAIR Q350 FOR 3-4 BED HOUSES. DUCT WORK TYPICALLY ADDS £800-£1,500 TO INSTALLATION COSTS. MVHR BECOMES ECONOMICALLY JUSTIFIED FOR EXTENSIONS ABOVE £60,000 IN VALUE WHERE THE FABRIC-FIRST APPROACH HAS ALREADY BEEN APPLIED.

LONDON'S PREDOMINANTLY VICTORIAN AND EDWARDIAN BRICK CONSTRUCTION CREATES SPECIFIC PENETRATING DAMP VULNERABILITIES. TOP SIX CAUSES IN ORDER OF FREQUENCY: (1) FAILED OR MISSING LEAD FLASHINGS — PARTICULARLY AT CHIMNEY STACKS, ABUTMENTS, AND FLAT ROOF/WALL JUNCTIONS. LEAD FLASHINGS HAVE A DESIGN LIFE OF 50-60 YEARS; MOST VICTORIAN HOUSES HAVE SEEN TWO GENERATIONS OF LEAD REPLACEMENT. BS EN 12588 CODE 4 LEAD (1.8MM THICKNESS) IS THE MINIMUM STANDARD. A TYPICAL CHIMNEY STACK FLASH REPOINT AND LEAD REPLACEMENT COSTS £800-£1,800. (2) FAILED OR OPEN MORTAR POINTING — COMMON IN LONDON STOCK BRICK WALLS WHERE CEMENT REPOINTING HAS BEEN USED ON ORIGINALLY LIME-POINTED WALLS. CEMENT IS HARDER THAN THE BRICK AND TRAPS MOISTURE BEHIND IT, ACCELERATING SPALLING. CORRECT REMEDY IS HACK OUT CEMENT AND REPOINT WITH NHL 2 OR NHL 3.5 LIME MORTAR. ST ASTIER NHL 3.5 LIME MORTAR MIXED 1:2.5 (LIME PUTTY:SHARP SAND) IS THE STANDARD SPECIFICATION FOR LONDON STOCK BRICK. REPOINTING COSTS £60-£120 PER M2 FOR HAND-RAKED LIME POINTING. (3) DEFECTIVE OR BLOCKED GUTTERS — CAST IRON OGEE AND HALF-ROUND GUTTERS ON VICTORIAN TERRACE HOUSES ARE TYPICALLY OVER 100 YEARS OLD. JOINTS FAIL, BRACKETS CORRODE, AND OUTLETS BLOCK WITH MOSS. OVERFLOWING GUTTERS SATURATE THE WALL IMMEDIATELY BELOW. ALUMINIUM OR CAST-IRON REPLACEMENT GUTTER SYSTEM (INCLUDING BRACKETS AND RUNNING OUTLETS) COSTS £1,200-£2,500 FOR A TYPICAL 3-STOREY TERRACE FRONTAGE. (4) BRIDGED CAVITY WALLS — IN LONDON'S SMALL NUMBER OF 1930S-50S CAVITY WALL PROPERTIES, CAVITY WALL INSULATION (CWI) CAN BRIDGE THE CAVITY WHEN INSTALLED INCORRECTLY OR WHERE WALLS ARE ALREADY SATURATED. ALSO, MORTAR DROPPINGS AT THE BASE OF THE CAVITY FROM ORIGINAL CONSTRUCTION ACT AS BRIDGES AT GROUND LEVEL. (5) CRACKED OR DAMAGED RENDER — CEMENT RENDER ON SOLID BRICK WALLS CRACKS OVER TIME, ALLOWING RAIN PENETRATION BEHIND THE RENDER LAYER. (6) FAILED WINDOW AND DOOR SEALS — DETERIORATED SEALANT AT WINDOW FRAME PERIMETERS.

TRUE RISING DAMP IN LONDON IS FAR RARER THAN ADVERTISED. THE PROPERTY CARE ASSOCIATION (PCA) ESTIMATES THAT 40-60% OF PROPERTIES WHERE A CHEMICAL DPC INJECTION IS RECOMMENDED DO NOT ACTUALLY HAVE RISING DAMP — THEY HAVE CONDENSATION, PENETRATING DAMP, OR HYGROSCOPIC SALTS FROM A HISTORIC DAMP PROBLEM. HYGROSCOPIC SALTS (CHLORIDES AND NITRATES) THAT HAVE BEEN DEPOSITED IN PLASTER BY PAST MOISTURE MOVEMENT WILL ATTRACT MOISTURE FROM THE ATMOSPHERE EVEN AFTER THE ORIGINAL DAMP SOURCE IS RESOLVED — GIVING FALSE ELECTRICAL-RESISTANCE METER READINGS THAT INEXPERIENCED SURVEYORS MISINTERPRET AS ACTIVE RISING DAMP. THE DEFINITIVE TEST FOR RISING DAMP IS THE CARBIDE BOMB TEST ON PLASTER AND MASONRY CORES TAKEN AT MULTIPLE HEIGHTS — IF MOISTURE CONTENT IS HIGHER AT LOW LEVEL AND DECREASING WITH HEIGHT, TRUE RISING DAMP IS PROBABLE. CHEMICAL DPC INJECTION: PERMAGARD DRYZONE CREAM INJECTED AT 75MM CENTRES INTO 12MM-DIAMETER HOLES DRILLED AT 115MM HEIGHT IN EACH MORTAR COURSE. SAFEGUARD DAMP PROOF CREAM (LATER BRAND: SAFEGUARD DRYZONE) IS THE MOST COMMONLY SPECIFIED. CHEMICAL DAMP-PROOF COURSE INJECTION COSTS £1,800-£4,500 FOR A TYPICAL TERRACE HOUSE PARTY WALL LINE. CORRECT REMEDIAL PLASTER SPECIFICATION AFTER DPC: REMOVE ALL PLASTER TO 300MM ABOVE HIGHEST DAMP READING. APPLY SAND AND CEMENT SCRATCH COAT (1:3) WITH RENDERED COAT (1:6) PLUS STORMDRY MASONRY PROTECTION CREAM ON THE EXTERNAL FACE. VANDEX SUPER CEMENTITIOUS TANKING SLURRY CAN BE USED ON INTERNAL FACES WHERE EXTERNAL TREATMENT IS NOT POSSIBLE. REMMERS KIESOL C SILICONE MICRO-EMULSION IS AN ALTERNATIVE INJECTION SYSTEM.

INDEPENDENT PCA-MEMBER DAMP SURVEYOR: £250-£600 DEPENDING ON PROPERTY SIZE. THIS IS ALWAYS WORTH COMMISSIONING INDEPENDENTLY RATHER THAN ACCEPTING A FREE SURVEY FROM A DAMP-PROOFING COMPANY (WHOSE SURVEYOR HAS AN OBVIOUS COMMERCIAL INTEREST). CONDENSATION REMEDIATION: PIV UNIT INSTALLATION £650-£1,000 ALL-IN; TRICKLE VENT UPGRADES £300-£800; EXTRACTOR FAN REPLACEMENTS £150-£300 PER FAN; MOULD TREATMENT WITH BIOBURN MICROBIOCIDAL WASH + DULUX TRADE MOULDSHIELD EMULSION DECORATION £400-£900 PER ROOM. PENETRATING DAMP — COMMON REPAIR PACKAGES: LEAD FLASHINGS TO CHIMNEY + REPOINT STACK: £1,000-£2,500; FULL GUTTER REPLACEMENT: £1,500-£3,500; REPOINTING FRONT ELEVATION IN LIME: £3,000-£8,000 FOR A STANDARD 3-STOREY TERRACE; RENDER REPAIR (LOCALISED): £800-£2,500; FULL RENDER REPLACEMENT WITH K-REND SILICONE TC15: £8,000-£18,000 FOR STANDARD SEMI. RISING DAMP — REMEDIATION PACKAGE: CHEMICAL INJECTION + REPLASTERING 1 ROOM (GROUND FLOOR): £2,500-£4,500; CHEMICAL INJECTION + REPLASTERING FULL GROUND FLOOR 3-BED TERRACE: £4,500-£9,000. INTERSTITIAL CONDENSATION IN FLAT ROOF EXTENSION: REMOVE FLAT ROOF FINISH, ADD RIGID PIR INSULATION ABOVE DECK TO KEEP DECK WARM SIDE OF INSULATION, RELAY SARNAFIL — £6,000-£12,000 FOR A 20M2 EXTENSION ROOF. ALLOW 15% CONTINGENCY ON ALL DAMP REMEDIATION WORK — OPENING UP FREQUENTLY REVEALS ADDITIONAL ISSUES.