Spray Foam Insulation in London Homes: What to Know Before You Buy or Mortgage

SPRAY POLYURETHANE FOAM (SPF) IS PRODUCED BY MIXING TWO CHEMICAL COMPONENTS (ISOCYANATE AND POLYOL) AT THE POINT OF APPLICATION — THE REACTION PRODUCES AN EXPANDING FOAM THAT SETS TO A RIGID OR SEMI-RIGID INSULATING MATERIAL. THERE ARE TWO MAIN TYPES OF SPF RELEVANT TO LONDON RESIDENTIAL APPLICATIONS.

OPEN CELL SPF (LOWER DENSITY, LOWER LAMBDA, VAPOUR-PERMEABLE): OPEN CELL SPF (SOMETIMES CALLED "SOFT FOAM" OR "LOW-DENSITY SPF") EXPANDS MORE AND HAS A LOWER DENSITY (APPROXIMATELY 8 TO 30 KG/M³). THE CELL STRUCTURE IS PARTIALLY OPEN — THE FOAM IS PERMEABLE TO WATER VAPOUR (VAPOUR OPEN). THERMAL CONDUCTIVITY (LAMBDA): APPROXIMATELY 0.035 TO 0.040 W/MK. TYPICAL THICKNESS REQUIRED FOR ROOF RAFTERS IN A LONDON LOFT: 100 TO 150MM FOR U-VALUES APPROACHING 0.18 W/M²K. OPEN CELL SPF IS VAPOUR-PERMEABLE — MEANING MOISTURE IN ROOFING TIMBERS CAN STILL MOVE, WHICH IS IMPORTANT IN EXISTING ROOFS WHERE SOME MOISTURE CYCLING IS NORMAL.

CLOSED CELL SPF (HIGHER DENSITY, BETTER INSULATION, VAPOUR BARRIER): CLOSED CELL SPF (HARD FOAM OR HIGH-DENSITY SPF) HAS A HIGHER DENSITY (TYPICALLY 30 TO 60 KG/M³) AND A CLOSED CELL STRUCTURE THAT MAKES IT HIGHLY RESISTANT TO VAPOUR AND LIQUID WATER. THERMAL CONDUCTIVITY: APPROXIMATELY 0.022 TO 0.028 W/MK (BETTER THAN OPEN CELL — LESS THICKNESS REQUIRED). CLOSED CELL SPF ACTS AS A VAPOUR BARRIER — PREVENTING MOISTURE MOVEMENT THROUGH THE TREATED SURFACE. WHERE APPLIED INSIDE A PITCHED ROOF, CLOSED CELL SPF CAN TRAP MOISTURE IN THE ROOFING TIMBERS BEHIND THE FOAM, PREVENTING DRYING AND ACCELERATING DECAY. THIS IS THE DOMINANT MECHANISM BY WHICH SPRAY FOAM CAUSES STRUCTURAL DAMAGE TO LONDON VICTORIAN ROOFS.

THE UK MORTGAGE LENDING MARKET HAS DEVELOPED A STRONG NEGATIVE POSITION ON SPRAY FOAM INSULATION IN RESIDENTIAL PROPERTIES — AND THIS DIRECTLY AFFECTS LONDON HOMEOWNERS TRYING TO SELL OR REMORTGAGE PROPERTIES WITH SPRAY FOAM INSTALLED.

WHY MORTGAGE LENDERS DECLINE PROPERTIES WITH SPRAY FOAM: RICS (ROYAL INSTITUTION OF CHARTERED SURVEYORS) ISSUED GUIDANCE IN 2023 NOTING THAT SPRAY FOAM APPLIED TO THE INTERNAL FACE OF PITCHED ROOF RAFTERS CAN CAUSE SIGNIFICANT PROBLEMS INCLUDING: CONCEALMENT OF STRUCTURAL DEFECTS (CRACKS, DAMAGED RAFTERS, FAILED FIXINGS) THAT CANNOT BE INSPECTED THROUGH THE FOAM. PREVENTION OF NATURAL MOISTURE MOVEMENT IN ROOF TIMBERS — LEADING TO ACCELERATED DECAY. DIFFICULTY OR IMPOSSIBILITY OF REMOVING THE FOAM WITHOUT REPLACING ALL ROOF TIMBERS. DIFFICULTY IN OBTAINING ACCURATE ROOF STRUCTURAL SURVEYS WHEN FOAM IS PRESENT. AS A RESULT OF THESE CONCERNS, MANY MAINSTREAM UK MORTGAGE LENDERS (LLOYDS, NATWEST, SANTANDER, NATIONWIDE, BARCLAYS) HAVE ISSUED INTERNAL GUIDANCE THAT THEY WILL DECLINE MORTGAGE APPLICATIONS OR REMORTGAGES ON PROPERTIES WHERE SPRAY FOAM HAS BEEN APPLIED TO THE ROOF STRUCTURE. THIS EFFECTIVELY LIMITS THE POOL OF BUYERS FOR LONDON PROPERTIES WITH SPRAY FOAM — SIGNIFICANTLY AFFECTING SALEABILITY AND VALUE.

SURVEYOR APPROACHES: RICS-QUALIFIED VALUATION SURVEYORS HAVE BEEN INSTRUCTED BY LENDERS TO NOTE THE PRESENCE OF SPRAY FOAM IN THEIR REPORTS. WHERE FOAM IS PRESENT IN THE ROOF, MANY SURVEYORS WILL REPORT THE PROPERTY AS "MORTGAGE RETAINED" OR WILL FLAG A REDUCTION IN VALUE UNTIL THE FOAM IS REMOVED AND AN INDEPENDENT STRUCTURAL TIMBER SURVEY IS COMPLETED.

LONDON VICTORIAN TERRACES (BUILT 1840 TO 1910) HAVE SPECIFIC CHARACTERISTICS THAT MAKE THEM PARTICULARLY VULNERABLE TO PROBLEMS CAUSED BY SPRAY FOAM.

COMPLEX ROOF TIMBERWORK: VICTORIAN LONDON TERRACES HAVE RELATIVELY COMPLEX PITCHED ROOF STRUCTURES — PRINCIPAL RAFTERS, COMMON RAFTERS, VALLEY RAFTERS, JACK RAFTERS, HIP AND VALLEY BOARDS, AND KING POST OR QUEEN POST TRUSSES. THE STRUCTURAL ADEQUACY OF THESE TIMBERS DEPENDS ON THEIR CONDITION AND CROSS-SECTION — BOTH OF WHICH ARE IMPOSSIBLE TO ASSESS ONCE COVERED IN SPRAY FOAM. RAFTER DECAY CAUSED BY THE MOISTURE-TRAPPING EFFECT OF CLOSED CELL SPF IS THE CRITICAL RISK IN LONDON VICTORIAN ROOFS.

LACK OF SARKING FELT IN OLDER LONDON TERRACES: MANY LONDON VICTORIAN TERRACE ROOFS WERE BUILT WITHOUT SARKING FELT (UNDERLAY BELOW THE SLATES OR TILES) — THE SLATES OR TILES WERE LAID DIRECTLY ON BATTENS WITH NO SECONDARY WATERPROOFING LAYER. IN THESE ROOFS, THERE IS NO BARRIER BETWEEN THE EXTERNAL ENVIRONMENT AND THE RAFTER VOID — WINDBLOWN RAIN AND CONDENSATION DIRECTLY CONTACT THE RAFTER SURFACES. APPLYING CLOSED CELL SPF TO THESE RAFTERS COMPLETELY SEALS THE RAFTER IN AN IMPERMEABLE COCOON — ANY MOISTURE PRESENT IN THE TIMBER AT THE TIME OF APPLICATION (WHICH MAY BE SIGNIFICANT IN AN OLD ROOF WITH OCCASIONAL MINOR WATER INGRESS) CANNOT ESCAPE AND WILL PROMOTE DECAY.

SOCIAL HOUSING GREEN DEAL AND ENERGY SCHEME LEGACY: A SIGNIFICANT NUMBER OF LONDON PROPERTIES WERE TREATED WITH SPRAY FOAM AS PART OF GOVERNMENT GREEN DEAL SCHEMES (2012 TO 2015) OR SUBSEQUENT ECO SCHEMES. IN MANY OF THESE CASES, THE INSTALLATION WAS DONE WITHOUT ADEQUATE PRE-INSPECTION OF ROOF TIMBER CONDITION, WITHOUT HOMEOWNER EDUCATION ABOUT MORTGAGE IMPLICATIONS, AND WITHOUT ADEQUATE CONSIDERATION OF LONG-TERM STRUCTURAL RISK.

FOR LONDON HOMEOWNERS WHO NEED TO SELL OR REMORTGAGE A PROPERTY WITH SPRAY FOAM AND WHO ARE FACING LENDER RESISTANCE, REMOVAL OF THE SPRAY FOAM IS THE PRIMARY REMEDIATION ROUTE.

SPRAY FOAM REMOVAL METHODS: SPRAY FOAM CANNOT SIMPLY BE PAINTED OVER OR HIDDEN — IT MUST BE PHYSICALLY REMOVED FROM THE ROOFING TIMBERS. THE MAIN METHODS ARE: MANUAL REMOVAL (HAND SCRAPING, ANGLE GRINDER, HAMMER AND CHISEL): THE FOAM IS PHYSICALLY REMOVED FROM THE RAFTER FACES. THIS IS EXTREMELY LABOUR-INTENSIVE — A 50 M² LONDON TERRACE ROOF SPACE WITH FULL FOAM COVERAGE CAN TAKE 3 TO 8 DAYS OF SPECIALIST LABOUR FOR REMOVAL. THE FOAM CANNOT BE FULLY REMOVED FROM TIMBER GRAIN — RESIDUAL FOAM CONTAMINATION OF THE TIMBER SURFACE WILL REMAIN. DRY ICE (CO2) BLASTING: PELLETS OF SOLID CO2 ARE PROJECTED AT HIGH VELOCITY AGAINST THE FOAM — THE THERMAL SHOCK CAUSES THE FOAM TO BECOME BRITTLE AND DETACH FROM THE TIMBER WITHOUT DAMAGING THE TIMBER SURFACE. MORE EXPENSIVE THAN MANUAL REMOVAL BUT GIVES CLEANER TIMBER SURFACE. REQUIRES SPECIALIST EQUIPMENT.

POST-REMOVAL STRUCTURAL SURVEY: AFTER FOAM REMOVAL, A STRUCTURAL TIMBER SURVEY OF ALL ROOF TIMBERS (BY A STRUCTURAL ENGINEER OR SPECIALIST TIMBER SURVEYOR USING MOISTURE METER, PROBE TESTING, AND VISUAL ASSESSMENT) IS REQUIRED TO ESTABLISH THAT THE ROOF TIMBERS ARE STRUCTURALLY ADEQUATE. WHERE DECAY IS FOUND IN RAFTERS, HIPS, OR VALLEYS, THESE WILL NEED TO BE SPLICED OR REPLACED BEFORE THE PROPERTY CAN BE MORTGAGED.

COST OF SPRAY FOAM REMOVAL IN LONDON: MANUAL REMOVAL: £2,000 TO £6,000 FOR A STANDARD LONDON TERRACE ROOF SPACE. DRY ICE BLASTING: £3,500 TO £8,000 FOR A STANDARD ROOF SPACE. POST-REMOVAL STRUCTURAL SURVEY: £500 TO £1,500. ADDITIONAL RAFTER REPAIRS IF DECAY FOUND: £2,000 TO £10,000+ DEPENDING ON EXTENT.

THE MORTGAGE AND STRUCTURAL PROBLEMS ASSOCIATED WITH SPRAY FOAM MAKE IT A POOR CHOICE FOR MOST LONDON RESIDENTIAL APPLICATIONS. THE FOLLOWING ALTERNATIVES ARE APPROPRIATE AND MORTGAGE-COMPLIANT.

PITCHED ROOF INSULATION (WARM ROOF — INSULATION BETWEEN AND OVER RAFTERS): FOR A WARM PITCHED ROOF (LOFT CONVERSION WITH HABITABLE SPACE BELOW), THE CORRECT INSULATION APPROACH IS PARTIAL-FILL BETWEEN RAFTERS WITH RIGID PIR BOARDS (CELOTEX RR4000, XTRATHERM RF-GS, RECTICEL EUROWALL) LEAVING A MINIMUM 50MM VENTILATED AIR SPACE ABOVE, PLUS OVER-RAFTER PIR BOARDS ABOVE THE SARKING FELT IF POSSIBLE. THIS APPROACH IS FULLY INSPECTABLE, EASILY REPAIRABLE, AND HAS NO MORTGAGE IMPLICATIONS.

COLD LOFT FLOOR INSULATION (MOST EFFICIENT FOR LONDON UNOCCUPIED LOFT SPACES): FOR LONDON TERRACES WHERE THE LOFT IS USED ONLY FOR STORAGE AND NOT AS HABITABLE SPACE, INSULATING THE LOFT FLOOR (BETWEEN AND OVER JOISTS) RATHER THAN THE ROOF SLOPE IS FAR MORE ENERGY-EFFICIENT AND COST-EFFECTIVE. SPECIFICATION: KNAUF EARTHWOOL LOFT ROLL 44 (MINERAL WOOL, LAMBDA 0.044 W/MK) AT 100MM BETWEEN JOISTS + 150MM TO 200MM PERPENDICULAR LAYER OVER JOISTS = 250 TO 300MM TOTAL INSULATION. THIS ACHIEVES A LOFT FLOOR U-VALUE OF 0.11 TO 0.14 W/M²K AT MUCH LOWER COST THAN SPRAY FOAM OR RIGID BOARD, WITH NO MORTGAGE IMPLICATIONS AND FULL ACCESSIBILITY TO ROOF TIMBERS FOR FUTURE INSPECTION AND REPAIR.

ACCESSED COLD LOFT INSULATION: WHERE THE LOFT IS SOMETIMES ACCESSED, USE SPLIT-LEVEL MINERAL WOOL (100MM BETWEEN JOISTS, 200MM ABOVE JOISTS WITH LOFT BOARDS LAID OVER 25MM EPS FOAM LAYER ON LOFT LEGS — THE LOFT LEGS RAISE THE BOARDS ABOVE THE INSULATION TO ALLOW FULL-DEPTH FILL AND STILL PROVIDE STORAGE ACCESS).

FOR LONDON HOMEOWNERS CONSIDERING SPRAY FOAM, THE FOLLOWING COST COMPARISON IS RELEVANT.

SPRAY FOAM INSTALLATION (OPEN OR CLOSED CELL, APPLIED TO RAFTERS — SUPPLY AND APPLY INCLUDING SURVEY): OPEN CELL SPF: £800 TO £2,500 FOR STANDARD LONDON TERRACE ROOF SPACE (50 TO 80 M² CEILING AREA). CLOSED CELL SPF: £1,500 TO £4,000. THESE COSTS DO NOT INCLUDE FUTURE REMOVAL COSTS OR IMPACT ON PROPERTY VALUE.

KNAUF EARTHWOOL LOFT ROLL 44 (COLD LOFT FLOOR INSULATION — THE RECOMMENDED ALTERNATIVE): MATERIALS (100MM + 200MM LAYERS, 80 M² LOFT FLOOR): £500 TO £900. INSTALLATION (PROFESSIONAL INSTALLER): £600 TO £1,500. TOTAL: £1,100 TO £2,400. NO MORTGAGE IMPLICATIONS. PRESERVES ROOF TIMBER INSPECTION ACCESS. QUALIFIES FOR ECO SCHEME FUNDING UNDER CERTAIN CONDITIONS.

PIR RIGID BOARD BETWEEN-RAFTER INSULATION (FOR LOFT CONVERSION WITH HABITABLE SPACE): SUPPLY AND INSTALL (CELOTEX RR4000 OR XTRATHERM, 150MM TOTAL THICKNESS): £30 TO £55 PER M² INSTALLED. FOR 50 M² ROOF SLOPE: £1,500 TO £2,750. FULLY MORTGAGE-COMPATIBLE.