Retrofit Insulation for London Victorian Terraces: Solid Wall, Roof, and Floor Insulation Options

LONDON VICTORIAN TERRACES WERE BUILT WITH SOLID BRICK EXTERNAL WALLS — TYPICALLY 215-225MM OF SOLID BRICKWORK (ONE BRICK THICK — THE BRICK LAID AS ALTERNATING STRETCHERS AND HEADERS — FLEMISH BOND OR ENGLISH BOND) WITH NO CAVITY AND NO INSULATION — THE U-VALUE OF THE ORIGINAL UNINSULATED SOLID BRICK WALL IS APPROXIMATELY 2.0-2.3 W/M²K — COMPARED WITH PART L 2021 TARGET U-VALUE 0.18 W/M²K FOR A NEW EXTERNAL WALL — THE SOLID BRICK WALL LOSES HEAT AT APPROXIMATELY 10-12 TIMES THE RATE OF A NEW COMPLIANT WALL.

SOLID WALL INSULATION OPTIONS FOR A LONDON VICTORIAN TERRACE FALL INTO TWO MAIN CATEGORIES: EXTERNAL WALL INSULATION (EWI — APPLIED TO THE OUTSIDE FACE OF THE WALL) AND INTERNAL WALL INSULATION (IWI — APPLIED TO THE INSIDE FACE OF THE WALL — SOMETIMES CALLED DRY LINING). THE CRITICAL DIFFERENCE IS: EWI DOES NOT AFFECT INTERNAL ROOM DIMENSIONS AND DOES NOT REQUIRE MOVING INTERNAL SERVICES AND FINISHES — BUT IT CHANGES THE EXTERNAL APPEARANCE OF THE PROPERTY (AND MAY REQUIRE PLANNING PERMISSION — PARTICULARLY IN CONSERVATION AREAS) — IWI DOES NOT CHANGE THE EXTERNAL APPEARANCE — BUT IT REDUCES INTERNAL ROOM DIMENSIONS (EACH TREATED WALL LOSES 50-100MM OF INTERNAL WIDTH DEPENDING ON THE SYSTEM USED) AND REQUIRES MOVING ALL SKIRTING BOARDS, ELECTRICAL SOCKETS AND SWITCHES, WINDOW CILLS, AND DOOR ARCHITRAVES ON THE TREATED WALLS.

EXTERNAL WALL INSULATION (EWI): EWI IS GENERALLY CONSIDERED THE HIGHER-PERFORMANCE SOLID WALL INSULATION OPTION — BECAUSE IT INSULATES THE FULL WALL SURFACE (INCLUDING PARTY WALL RETURNS, WINDOW REVEALS, AND OTHER JUNCTIONS) FROM THE OUTSIDE — ELIMINATING MORE THERMAL BRIDGES THAN IWI WHICH IS LIMITED TO THE FLAT INTERNAL WALL SURFACE; TYPICAL EWI SYSTEM FOR LONDON VICTORIAN TERRACE (MINERAL WOOL OR EPS SLAB SYSTEMS): WETHERBY EWI SYSTEM: MINERAL WOOL SLAB (ROCKWOOL RWA45 OR KNAUF DP3 MINERAL WOOL — 100MM TYPICAL — U-VALUE APPROXIMATELY 0.28-0.32 W/M²K — OR 120-150MM FOR 0.18-0.22 W/M²K) MECHANICALLY FIXED AND ADHESIVE-BONDED TO THE EXISTING EXTERNAL WALL FACE — BASECOAT (WETHERBY ADHESIVE AND BASECOAT) + FIBREGLASS MESH REINFORCEMENT + SILICONE RENDER FINISH (WETHERBY SILICONE TC15 OR K-REND TC15 — AVAILABLE IN A RANGE OF COLOURS); PERMAROCK EWI SYSTEM (EXPANDED POLYSTYRENE — EPS — SLAB: PERMAROCK EPS SLAB 100MM — U-VALUE APPROXIMATELY 0.30-0.36 W/M²K AT 100MM — OR 120MM FOR 0.25-0.30 W/M²K — HIGHER THERMAL PERFORMANCE PER MM THAN MINERAL WOOL — BUT EPS IS COMBUSTIBLE — SPECIAL ATTENTION TO FIRE BARRIERS REQUIRED AT WINDOW AND DOOR PERIMETERS, ABOVE 18M HEIGHT, AND AT ALL FLOOR LEVELS — MINERAL WOOL EWI IS PREFERRED WHERE NON-COMBUSTIBLE CLADDING IS REQUIRED UNDER BUILDING REGULATIONS PART B); PLANNING PERMISSION FOR EWI IN LONDON: EWI ON A LONDON VICTORIAN TERRACE CONSTITUTES AN ALTERATION TO THE EXTERNAL APPEARANCE OF THE BUILDING — IN CONSERVATION AREAS OR ON LISTED BUILDINGS, EWI REQUIRES PLANNING PERMISSION AND/OR LISTED BUILDING CONSENT — MANY LONDON CONSERVATION AREA APPRAISALS SPECIFICALLY PROHIBIT EWI ON THE PRIMARY FRONT ELEVATION — EWI TO THE REAR ELEVATION ONLY IS SOMETIMES PERMITTED — CHECK WITH THE LOCAL PLANNING AUTHORITY.

INTERNAL WALL INSULATION (IWI — INSULATION APPLIED TO THE INNER FACE OF THE EXTERNAL WALL — ALLOWING THE WALL'S EXTERNAL APPEARANCE TO BE PRESERVED WHILE IMPROVING THERMAL PERFORMANCE FROM INSIDE): THE MOST COMMONLY APPLICABLE SOLID WALL INSULATION OPTION FOR LONDON VICTORIAN TERRACES IN CONSERVATION AREAS OR WHERE THE EXTERNAL BRICKWORK APPEARANCE MUST BE PRESERVED.

IWI SYSTEM OPTIONS FOR A LONDON VICTORIAN TERRACE: (1) INSULATED PLASTERBOARD (THE SIMPLEST AND MOST WIDELY USED IWI SYSTEM — DOT-AND-DABB TO EXISTING WALL OR MECHANICALLY FIXED — LOW INSTALLATION DISRUPTION — BUT LIMITED THERMAL PERFORMANCE PER MM): KINGSPAN K18 (KOOLTHERM — PHENOLIC CORE — 37.5MM BOARD INCLUDING 12.5MM PLASTERBOARD — U-VALUE FOR 215MM SOLID BRICK + K18 37.5MM ≈ 0.55-0.65 W/M²K — FOR 0.30 W/M²K WOULD NEED APPROXIMATELY 80MM TOTAL IWI); KNAUF INSULATED PLASTERBOARD (GLASS MINERAL WOOL CORE — 60MM TOTAL INCLUDING 12.5MM PLASTERBOARD — U-VALUE 215MM SOLID BRICK + 60MM ≈ 0.45-0.55 W/M²K); NOTE: INSULATED PLASTERBOARD DOT-AND-DABB CREATES A SIGNIFICANT THERMAL BRIDGE AT EACH DOT OF ADHESIVE — AND AT THE WALL-FLOOR JUNCTION WHERE THE IWI TERMINATES — PARTIAL THERMAL BRIDGING AT THESE JUNCTIONS REDUCES THE EFFECTIVE U-VALUE IMPROVEMENT BY APPROXIMATELY 20-40% COMPARED WITH THE UNFACTORED VALUE; (2) STUD FRAME IWI (TIMBER STUD FRAME FIXED TO OR SLIGHTLY SEPARATED FROM THE EXISTING WALL FACE — INSULATION INSTALLED BETWEEN STUDS — PLASTERBOARD FIN ISH — HIGHER PERFORMANCE — BUT GREATER ROOM DIMENSION LOSS): 89MM C16 STUD AT 400MM CENTRES + 80MM KNAUF DRITHERM 32 MINERAL WOOL BATT BETWEEN STUDS + 12.5MM PLASTERBOARD: U-VALUE APPROXIMATELY 0.25-0.35 W/M²K (215MM SOLID BRICK + 89MM STUD FRAME IWI); (3) MINERAL WOOL SLAB ON FRAME (STEICO OR PAVATEX WOOD FIBRE BOARD — USED IN HERITAGE RETROFIT APPLICATIONS WHERE VAPOUR PERMEABILITY IS IMPORTANT — WOOD FIBRE IS VAPOUR-OPEN AND ALLOWS THE SOLID BRICK WALL TO BREATHE — CRITICAL FOR LIME-POINTED SOLID BRICK WALLS WHERE MOISTURE MUST BE ABLE TO PASS THROUGH THE WALL ASSEMBLY — AVOIDS THE RISK OF INTERSTITIAL CONDENSATION THAT CAN OCCUR WITH IMPERMEABLE INSULATION ON THE INSIDE FACE OF A SOLID BRICK WALL IN COLD AND TEMPERATE CLIMATES): STEICO FLEX 036 WOOD FIBRE BATT (80-100MM) IN STUD FRAME: U-VALUE APPROXIMATELY 0.30-0.40 W/M²K (215MM SOLID BRICK + 100MM STEICO); CRITICAL MOISTURE CONSIDERATION FOR IWI: WHERE PIR OR EPS INSULATION IS APPLIED TO THE INSIDE FACE OF A SOLID BRICK WALL (IMPERMEABLE VAPOUR BARRIER EFFECT) — THE MOISTURE BEHAVIOUR OF THE WALL ASSEMBLY MUST BE CHECKED WITH A GLASER DIAGRAM OR WUFI DYNAMIC MOISTURE SIMULATION — THE RISK IS THAT INTERSTITIAL CONDENSATION FORMS AT THE INTERFACE BETWEEN THE COLD BRICK AND THE WARM INSULATION — WOOD FIBRE (VAPOUR-OPEN) AVOIDS THIS RISK.

THE ROOF IS THE SECOND MAJOR HEAT-LOSS ELEMENT IN A LONDON VICTORIAN TERRACE — HEAT RISES AND EXITS THROUGH THE ROOF — THE ORIGINAL UNINSULATED ROOF OF A VICTORIAN TERRACE (COLD PITCHED ROOF WITH UNINSULATED COLD LOFT) HAS A U-VALUE OF APPROXIMATELY 2.5-3.0 W/M²K AT CEILING LEVEL — PART L 2021 BACKSTOP U-VALUE FOR ROOFS IS 0.15 W/M²K — THE UNINSULATED VICTORIAN ROOF LOSES HEAT AT APPROXIMATELY 15-20 TIMES THE RATE OF A PART L 2021 COMPLIANT ROOF.

ROOF INSULATION OPTIONS FOR A LONDON VICTORIAN TERRACE: (A) COLD ROOF LOFT INSULATION (INSULATION AT CEILING LEVEL — LEAVING THE LOFT SPACE UNHEATED (COLD) ABOVE THE INSULATION — THE STANDARD APPROACH FOR A VICTORIAN TERRACE LOFT THAT IS USED ONLY FOR STORAGE OR IS UNUSED): GLASS MINERAL WOOL ROLL (KNAUF EARTHWOOL LOFT ROLL 44 OR ISOVER SPACESAVER 2 — KNAUF EARTHWOOL ROLL AT £2-4/M² FOR 100MM — THE STANDARD MATERIAL): 100MM BETWEEN CEILING JOISTS PLUS 200-270MM ACROSS THE TOP OF THE CEILING JOISTS PERPENDICULAR TO THE FIRST LAYER (TOTAL 300-370MM MINERAL WOOL — ACHIEVING U-VALUE APPROXIMATELY 0.13-0.15 W/M²K AT CEILING LEVEL — COMPLIANT WITH PART L 2021 BACKSTOP 0.15 W/M²K); ENSURE ACCESS HATCH IS INSULATED AND DRAUGHT-PROOFED (LOFT HATCH ACCOUNTS FOR SIGNIFICANT HEAT LOSS IF UNINSULATED — WARMUP LOFT HATCH COVER OR SIMILAR — 50MM PIR INSULATION PAD ON TOP OF EXISTING HATCH); NOTE: 200-270MM ACROSS THE TOP OF THE JOISTS MEANS THAT SERVICES RUNNING ACROSS THE LOFT FLOOR (COLD WATER TANK, PIPEWORK) MUST BE INSULATED TO PROTECT AGAINST FROST — THE INSULATION IS BELOW THE PIPEWORK IN A COLD ROOF CONFIGURATION — LAGGING WITH ARMAFLEX CLOSED-CELL PIPE INSULATION (25MM THICKNESS) IS REQUIRED ABOVE THE INSULATION LEVEL; (B) WARM ROOF LOFT INSULATION (INSULATION AT RAFTER LEVEL — THE LOFT SPACE IS ENCLOSED AND WARM — TYPICALLY USED WHEN THE LOFT IS CONVERTED TO HABITABLE SPACE — OR WHERE A COLD ROOF IS NOT PRACTICAL DUE TO THE ROOF GEOMETRY): FOR A LOFT CONVERSION OR HABITABLE LOFT: BETWEEN AND BELOW RAFTER INSULATION (XTRATHERM THIN-R XT/UF OR KINGSPAN KOOLTHERM K7 100MM BETWEEN EXISTING RAFTERS — THEN 50MM KINGSPAN K18 INSULATED PLASTERBOARD BELOW — TOTAL THERMAL RESISTANCE SUFFICIENT TO ACHIEVE APPROXIMATELY 0.18-0.22 W/M²K); ABOVE RAFTER INSULATION (FULLY BONDED PIR BOARD ABOVE THE ROOF DECK — USED IN A RE-ROOFING SITUATION — IKO ENERTHERM ABOVE RAFTER — 100MM PIR — 0.15-0.18 W/M²K — THE CLEANEST THERMAL PERFORMANCE APPROACH).

THE GROUND FLOOR OF A LONDON VICTORIAN TERRACE IS TYPICALLY EITHER A SUSPENDED TIMBER FLOOR (JOISTS ON SLEEPER WALLS OVER A VENTILATED VOID — TYPICAL GROUND FLOOR CONSTRUCTION IN LONDON VICTORIAN TERRACES) OR A SOLID CONCRETE SLAB (IN PROPERTIES WHERE THE ORIGINAL SUSPENDED TIMBER FLOOR HAS BEEN REPLACED — OR IN A REAR GROUND FLOOR EXTENSION). THE ORIGINAL UNINSULATED SUSPENDED TIMBER GROUND FLOOR HAS A U-VALUE OF APPROXIMATELY 0.7-1.0 W/M²K — PART L 2021 BACKSTOP U-VALUE FOR GROUND FLOORS IS 0.15 W/M²K.

INSULATING A SUSPENDED TIMBER GROUND FLOOR (BETWEEN-JOIST INSULATION — THE STANDARD APPROACH): LIFT EXISTING FLOORBOARDS — INSTALL INSULATION BETWEEN THE JOISTS — RELAY THE FLOORBOARDS: PIR RIGID BOARD (CELOTEX GA4000 OR KINGSPAN KOOLTHERM K3 — CUT TO FIT BETWEEN JOISTS — SUPPORTED BY NYLON NETTING STAPLED TO THE JOIST SIDES OR SMALL BATTENS FIXED TO THE JOIST SIDES): 80MM PIR BETWEEN JOISTS: U-VALUE APPROXIMATELY 0.25-0.35 W/M²K; 100MM PIR BETWEEN JOISTS: U-VALUE APPROXIMATELY 0.20-0.28 W/M²K; CRITICAL VENTILATION NOTE: INSTALLING INSULATION IN THE UNDERFLOOR VOID MUST NOT COMPROMISE THE VENTILATION OF THE VOID — AIRBRICKS MUST REMAIN FULLY CLEAR — THE INSULATION FILLS THE SPACE BETWEEN THE JOISTS BUT MUST NOT BLOCK THE AIRBRICK OPENINGS OR RESTRICT AIR MOVEMENT THROUGH THE VOID — FAILURE TO MAINTAIN VOID VENTILATION AFTER INSULATING WILL LEAD TO ELEVATED MOISTURE CONTENT IN THE JOISTS AND RISK OF WET ROT; MINERAL WOOL BATT (KNAUF DRITHERM 32 OR ROCKWOOL RW3 100MM — CUT TO FIT BETWEEN JOISTS — HELD BY NETTING OR BATTENS): U-VALUE APPROXIMATELY 0.30-0.40 W/M²K AT 100MM — CHEAPER THAN PIR BUT LESS THERMALLY EFFICIENT PER MM; INSULATING A CONCRETE SLAB (EITHER EXISTING SLAB OR NEW EXTENSION SLAB): PIR INSULATION LAYER ABOVE THE EXISTING SLAB (KINGSPAN KOOLTHERM K3 OR CELOTEX GA4000 AT 100MM) + SAND-CEMENT SCREED OR LIQUID SCREED OVER: RAISES FLOOR LEVEL BY APPROXIMATELY 175-200MM (100MM INSULATION + 65-75MM SCREED) — MUST CHECK DOOR THRESHOLD HEIGHTS, SKIRTING BOARD HEIGHTS, AND STAIR FIRST RISER HEIGHT BEFORE SPECIFYING.

RETROFIT INSULATION COSTS FOR A LONDON VICTORIAN TERRACE (LONDON 2025 — APPROXIMATE RANGE — SPECIALIST CONTRACTOR — SUPPLY AND FIT): COLD ROOF LOFT INSULATION (MINERAL WOOL — 300-370MM TOTAL DEPTH — CEILING LEVEL — UNHEATED LOFT): APPROXIMATELY £300-800 (DIY MATERIAL ONLY — KNAUF EARTHWOOL ROLLS FROM B&Q OR TRAVIS PERKINS) OR £600-1,500 (PROFESSIONAL INSTALLATION INCLUDING INSULATING LOFT HATCH AND LAYING ACCESS BOARDS — TWO-STOREY LONDON VICTORIAN TERRACE); SUSPENDED TIMBER GROUND FLOOR INSULATION (BETWEEN JOISTS — PIR — INCLUDING BOARD LIFT AND RELAY): APPROXIMATELY £1,000-2,500 (FRONT HALF OF GROUND FLOOR — TYPICAL 25-35M² — SPECIALIST CONTRACTOR); INTERNAL WALL INSULATION (IWI — INSULATED PLASTERBOARD DOT-AND-DABB 37.5MM SYSTEM — SINGLE ROOM — ALL EXTERNAL WALLS — INCLUDING SKIRTING BOARDS, SOCKETS, AND SKIM COAT): APPROXIMATELY £800-2,000 (SINGLE ROOM — SPECIALIST CONTRACTOR); STUD FRAME IWI (89MM STUD + MINERAL WOOL OR WOOD FIBRE — SINGLE ROOM): APPROXIMATELY £1,200-3,000 (SINGLE ROOM); EXTERNAL WALL INSULATION (EWI — MINERAL WOOL OR EPS SLAB — 100MM — FULL REAR ELEVATION EXCLUDING FRONT — SPECIALIST EWI CONTRACTOR — INCLUDING RENDER FINISH): APPROXIMATELY £3,500-8,000 (REAR ELEVATION ONLY — TWO-STOREY VICTORIAN TERRACE — FULL HOUSE FRONT AND REAR: £8,000-20,000+); GOVERNMENT FUNDING FOR SOLID WALL INSULATION: THE GREAT BRITISH INSULATION SCHEME (GBIS — GOVERNMENT OF THE UK — ADMINISTERED BY OFGEM — PROVIDES GRANTS FOR INSULATION UPGRADES TO PROPERTIES WITH AN EPC RATING OF D, E, F, OR G — ELIGIBILITY: OCCUPANTS RECEIVING CERTAIN BENEFITS OR IN LOWER-INCOME HOUSEHOLDS — OR IN LOWER EPC BAND PROPERTIES OUTSIDE MEANS-TESTED CRITERIA — SOLID WALL INSULATION (BOTH EWI AND IWI) IS A KEY ELIGIBLE MEASURE UNDER GBIS); THE ENERGY COMPANY OBLIGATION (ECO4 — THE FOURTH PHASE OF THE ENERGY COMPANY OBLIGATION SCHEME — ADMINISTERED BY OFGEM — OBLIGES LARGE ENERGY SUPPLIERS TO FUND ENERGY EFFICIENCY IMPROVEMENTS FOR ELIGIBLE LOWER-INCOME HOUSEHOLDS INCLUDING SOLID WALL INSULATION); PLANNING CONSIDERATIONS FOR EWI: CONSERVATION AREA: EWI TYPICALLY REQUIRES PLANNING PERMISSION IN A CONSERVATION AREA — SOME LONDON CONSERVATION AREAS SPECIFICALLY PROHIBIT EWI ON THE FRONT ELEVATION (AS IT ALTERS THE CHARACTER OF THE BRICKWORK THAT DEFINES THE CONSERVATION AREA) — SOME ALLOW EWI TO THE REAR — CHECK WITH THE BOROUGH PLANNING DEPARTMENT BEFORE STARTING EWI WORK IN A CONSERVATION AREA; LISTED BUILDINGS: LISTED BUILDING CONSENT REQUIRED FOR ANY INSULATION WORK THAT AFFECTS THE FABRIC OF A LISTED VICTORIAN TERRACE — IWI ON LISTED BUILDINGS REQUIRES A SENSITIVE APPROACH (VAPOUR-OPEN MATERIALS PREFERRED — SPECIALIST HERITAGE ARCHITECT OR CONSERVATION OFFICER INPUT REQUIRED).