External Wall Insulation for London Victorian Terraces: EWI Systems and Considerations

EXTERNAL WALL INSULATION SYSTEMS CONSIST OF: AN INSULATION BOARD FIXED TO THE EXISTING EXTERNAL WALL; A REINFORCED RENDER COAT (BASE COAT + FIBREGLASS MESH + FINISH COAT) ON TOP; OR A VENTILATED CLADDING SYSTEM. THERE ARE SEVERAL SYSTEM TYPES:

EPS (EXPANDED POLYSTYRENE) EWI: THE MOST COMMON AND MOST COST-EFFECTIVE EWI SYSTEM FOR RESIDENTIAL PROPERTIES: - INSULATION: 100MM EPS BOARD (LAMBDA VALUE 0.032 W/M·K TYPICALLY) GIVES A U-VALUE OF APPROXIMATELY 0.30 W/M²K FOR A 215MM SOLID BRICK WALL (IMPROVED FROM APPROXIMATELY 2.1 W/M²K UNINSULATED) - 120MM EPS: U-VALUE APPROXIMATELY 0.26 W/M²K - FIXING: MECHANICALLY FIXED (RAWLPLUG EJOT WDH ANCHORS AT 6-8 PER M²) + ADHESIVE (KNAUF MARMORIT EPS ADHESIVE) - FINISH: SILICONE RENDER (WEBER SILICONE THROUGH-COLOUR RENDER, K-REND SILICONE TC15) IN CHOICE OF COLOUR - SYSTEM PROVIDERS: WETHERBY EWI (WETHERBY BUILDING SYSTEMS), PERMAROCK EWI, KNAUF EWI 011 SYSTEM, PAREX EWI (PAREX UK)

MINERAL WOOL (ROCKWOOL) EWI: A SEMI-RIGID MINERAL FIBRE BOARD — NON-COMBUSTIBLE (A1 FIRE CLASSIFICATION) AND MORE BREATHABLE THAN EPS: - THERMAL PERFORMANCE SLIGHTLY LOWER THAN EPS PER MM (LAMBDA 0.034-0.036 W/M·K) — REQUIRES SLIGHTLY THICKER BOARD FOR EQUIVALENT U-VALUE - PREFERRED FOR PROPERTIES OVER 11M HEIGHT (FIRE SAFETY REGULATIONS REQUIRE NON-COMBUSTIBLE EWI ON HIGH-RISE BUILDINGS — BUT ALSO RECOMMENDED ON LOWER-RISE PROPERTIES WHERE FIRE RISK MANAGEMENT IS A CONSIDERATION) - SYSTEM: ROCKWOOL FIXROCK EWI BOARD + ROCKWOOL EWI ADHESIVE MORTAR + SILICONE OR MINERAL RENDER FINISH - ALSO PREFERRED WHERE THE WALL IS DAMP OR SLIGHTLY WET — MINERAL WOOL ALLOWS MORE VAPOUR MOVEMENT THAN EPS

PIR BOARD EWI (PREMIUM HIGH-PERFORMANCE): WHERE SPACE IS VERY CONSTRAINED (E.G., THE PROPERTY IS AT OR CLOSE TO THE BOUNDARY — COMMON IN LONDON TERRACE SIDE WALLS), PIR (POLYISOCYANURATE) BOARDS ACHIEVE BETTER THERMAL PERFORMANCE PER MM: - KINGSPAN KOOLTHERM K5 EXTERNAL WALL BOARD (LAMBDA 0.020 W/M·K): 80MM THICKNESS GIVES EQUIVALENT U-VALUE TO 120MM EPS - NOTE: PIR EWI SYSTEMS REQUIRE A NON-COMBUSTIBLE PROTECTIVE CLADDING LAYER (NOT FLAMMABLE RENDER) — CHECK SYSTEM SPECIFICATION CAREFULLY FOR FIRE COMPLIANCE

VENTILATED CLADDING EWI: AN ALTERNATIVE TO RENDERED EWI — INSULATION BOARDS FIXED TO THE WALL; HORIZONTAL OR VERTICAL BATTENS OVER THE INSULATION; CLADDING FIXED TO THE BATTENS WITH AN AIR GAP BETWEEN INSULATION AND CLADDING (THE VENTILATED CAVITY ALLOWS MOISTURE TO ESCAPE): - CLADDING OPTIONS: LARCH OR CEDAR TIMBER; FIBRE CEMENT (CEDRAL CLICK C06 WEATHERBOARDING); COMPRESSED CALCIUM SILICATE (TRESPA METEON); ARCHITECTURAL PROFILED METAL (COLORCOAT PRISMA, RAL 7016 STEEL) - MORE EXPENSIVE THAN RENDERED EWI; BETTER FOR CONTEMPORARY EXTENSION AESTHETICS; DOES NOT SUIT TRADITIONAL VICTORIAN TERRACE FRONT ELEVATIONS IN CONSERVATION AREAS

EWI ON LONDON VICTORIAN TERRACES RAISES SIGNIFICANT PLANNING CONSIDERATIONS:

PERMITTED DEVELOPMENT FOR EWI: EWI FALLS UNDER PERMITTED DEVELOPMENT (CLASS A — ENLARGEMENT, IMPROVEMENT OR OTHER ALTERATION OF A DWELLINGHOUSE) IF: - THE EXTERNAL CLADDING MATERIAL IS OF A SIMILAR APPEARANCE TO THE EXISTING FINISH (E.G., RENDER ON A RENDERED WALL) - THE PROPERTY IS NOT IN A CONSERVATION AREA OR ARTICLE 4 DIRECTION AREA - THE WORK DOES NOT REDUCE THE PERMITTED DEVELOPMENT SETBACK FROM THE FRONT BOUNDARY (TYPICAL EWI ADDS 100-150MM TO THE WALL FACE)

CONSERVATION AREA CONSTRAINTS: IN A LONDON CONSERVATION AREA (AND MOST INNER LONDON VICTORIAN TERRACE STREETS ARE IN OR ADJACENT TO CONSERVATION AREAS), ADDING EWI TO THE FRONT ELEVATION ALMOST CERTAINLY REQUIRES PLANNING PERMISSION AND ALMOST CERTAINLY WILL NOT BE GRANTED: - THE FRONT ELEVATION OF A LONDON VICTORIAN TERRACE IN A CONSERVATION AREA IS A CONTRIBUTION TO THE STREET SCENE — ADDING A RENDER FINISH OVER STOCK BRICK FUNDAMENTALLY CHANGES THE CHARACTER OF THE CONSERVATION AREA - HISTORIC ENGLAND GUIDANCE: INTERNAL INSULATION (ADDING INSULATION TO THE INSIDE FACE OF THE EXTERNAL WALL) IS THE PREFERRED INTERVENTION FOR LISTED BUILDINGS AND CONSERVATION AREA PROPERTIES

REAR ELEVATION EWI: WHERE EWI IS PROPOSED TO THE REAR ELEVATION ONLY (NOT VISIBLE FROM THE STREET — THE TYPICAL APPROACH IN LONDON TERRACES): - MORE LIKELY TO BE ACCEPTABLE (PLANNING PERMISSION USUALLY NOT REQUIRED FOR REAR ELEVATION WORK NOT IN A CONSERVATION AREA — CONFIRM WITH LOCAL AUTHORITY) - REAR EWI ALSO ADDS INSULATION TO THE REAR EXTENSION WALLS — SIGNIFICANT THERMAL BENEFIT WHERE THE REAR EXTENSION HAS SOLID SINGLE-SKIN BRICKWORK (COMMON IN OLDER EXTENSIONS) - JUNCTION DETAILING IS CRITICAL: WHERE THE EWI RETURNS AROUND THE REAR CORNER TO CONNECT WITH THE PARTY WALL, CAREFUL DETAILING PREVENTS THERMAL BRIDGING AT THE JUNCTION

IMPACT ON PROPERTY DIMENSIONS AND BOUNDARIES: EWI ADDS 100-150MM TO THE EXTERNAL WALL FACE — THIS REDUCES THE INTERIOR FLOOR AREA SLIGHTLY (IF APPLIED INTERNALLY) OR EXTENDS THE EXTERNAL FOOTPRINT SLIGHTLY (IF APPLIED EXTERNALLY). WHERE THE PROPERTY IS AT OR NEAR THE PARTY WALL BOUNDARY (VERY COMMON IN LONDON TERRACES), CONFIRM THAT THE EWI THICKNESS DOES NOT ENCROACH ON THE PARTY WALL BOUNDARY BEYOND THE PARTY WALL ACT ALLOWANCE.

FOR LONDON VICTORIAN TERRACES IN CONSERVATION AREAS WHERE EXTERNAL EWI IS NOT POSSIBLE, INTERNAL WALL INSULATION (IWI) IS THE ALTERNATIVE:

INTERNAL WALL INSULATION OPTIONS: 1. DRY LINING (PLASTERBOARD + PIR BOARD LAMINATE — "INS-I-BOARD" OR KINGSPAN KOOLTHERM K18 INSULATED PLASTERBOARD): THE SIMPLEST SYSTEM — 52.5MM BOARD (25MM PIR + 12.5MM PLASTERBOARD) BONDED TO THE EXISTING WALL FACE. U-VALUE OF SOLID BRICK WALL IMPROVED FROM 2.1 TO APPROXIMATELY 0.60 W/M²K. EASY TO INSTALL; LOSES 52.5MM OF ROOM DEPTH ON EACH INSULATED WALL.

2. TIMBER FRAME WITH MINERAL WOOL BETWEEN (BATT INSULATION — ROCKWOOL RWA45, KNAUF EARTHWOOL): A TIMBER STUD FRAME (50X50MM CLS) FIXED 25-50MM FROM THE WALL FACE; MINERAL WOOL BATTS BETWEEN THE STUDS; PLASTERBOARD FINISH. MORE EFFECTIVE THERMALLY THAN GLUED LAMINATE BOARD; ALSO ALLOWS SERVICE RUNS (ELECTRICAL CABLES, PIPES) WITHIN THE CAVITY; LOSES MORE ROOM DEPTH (75-100MM FROM WALL FACE).

3. VACUUM INSULATION PANEL (VIP — KINGSPAN KOOLTHERM K8, OPTIM-R): ULTRA-THIN INSULATION (LAMBDA AS LOW AS 0.007 W/M·K) THAT ACHIEVES GOOD U-VALUES IN VERY THIN APPLICATIONS — USEFUL WHERE ROOM DEPTH LOSS IS A CRITICAL CONSTRAINT (E.G., NARROW VICTORIAN TERRACE ROOMS).

COLD BRIDGE AND MOISTURE RISK IN INTERNAL WALL INSULATION: THE MAIN TECHNICAL CHALLENGES WITH IWI: - INTERSTITIAL CONDENSATION RISK: WARM MOIST INTERIOR AIR REACHING THE COLD SOLID BRICK WALL THROUGH THE INSULATION LAYER — MITIGATED BY CONTINUOUS VAPOUR CONTROL LAYER (VCL) ON THE WARM SIDE (INSIDE FACE) OF THE INSULATION - COLD BRIDGING AT FLOOR JUNCTION (SOLID GROUND FLOOR EDGE BEAM OR TIMBER JOIST ENDS ARE EXPOSED AND COLD — THERMAL BRIDGE BYPASSES THE WALL INSULATION) - FITTING/SKIRTING: SKIRTING BOARDS, ELECTRICAL SOCKET POSITIONS, WINDOW REVEALS, AND PIPE PENETRATIONS ALL MUST BE CAREFULLY DETAILED TO MAINTAIN THE VCL INTEGRITY

COSTS FOR EWI AND IWI IN LONDON VICTORIAN TERRACES (SUPPLY AND FIT, 2025-2026): EPS EWI (REAR ELEVATION ONLY — 100MM, SILICONE RENDER FINISH, 30M²): £5,000-9,000 MINERAL WOOL EWI (REAR ELEVATION, 100MM, SILICONE RENDER, 30M²): £6,000-10,500 INTERNAL WALL INSULATION (KINGSPAN K18 DRY LINING — 52.5MM — PER M² SUPPLY AND FIT): £35-55 INTERNAL WALL INSULATION (TIMBER FRAME + ROCKWOOL, PER M² SUPPLY AND FIT): £45-70 WHOLE-HOUSE REAR AND FLANK EWI (100MM EPS, SILICONE RENDER, 60M²): £10,000-18,000