External Wall Insulation (EWI) with Render for London Victorian Terraces

AN EWI (EXTERNAL WALL INSULATION) SYSTEM APPLIED WITH A SILICONE OR MINERAL RENDER FINISH IS A MULTI-LAYER SYSTEM — EACH LAYER HAS A SPECIFIC TECHNICAL FUNCTION.

LAYER 1: INSULATION BOARD. FOR LONDON VICTORIAN TERRACE SOLID-WALL EWI, THREE MAIN INSULATION BOARD TYPES ARE USED: EPS (EXPANDED POLYSTYRENE — WETHERBY EXTERNAL WALL INSULATION EPS BOARD, SAINT-GOBAIN PERFORMANCE WEBER BOARD EPS): MOST WIDELY USED AND COST-EFFECTIVE. WHITE OR GRAPHITE-ENHANCED EPS (GRAPHITE EPS — ALSO CALLED GREY EPS OR NEOPOR — HAS λ = 0.031 W/MK VS STANDARD EPS λ = 0.039 W/MK — 20% THINNER FOR SAME U-VALUE). MINERAL WOOL (KNAUF EARTHWOOL EWI SLAB OR ROCKWOOL FRONTROCK MAX E — λ = 0.040 W/MK): USED WHERE BREATHABILITY, FIRE PERFORMANCE (CLASS A1 NON-COMBUSTIBLE), OR ACOUSTIC PERFORMANCE OF THE WALL IS PRIORITISED OVER THICKNESS MINIMISATION. PHENOLIC FOAM (KINGSPAN KOOLTHERM K5 EXTERNAL BOARD — λ = 0.020 W/MK): THE THINNEST EWI BOARD FOR A GIVEN U-VALUE — 60MM PHENOLIC ACHIEVES 0.30 W/M²K ON A SOLID BRICK WALL, VERSUS 100MM GRAPHITE EPS OR 140MM MINERAL WOOL.

LAYER 2: ADHESIVE AND MECHANICAL FIXING. THE INSULATION BOARDS ARE FIXED TO THE EXISTING BRICKWORK WITH: EWI ADHESIVE (K-SYSTEM BASE COAT ADHESIVE, WETHERBY EWI ADHESIVE — APPLIED IN A PERIPHERAL BEAD AND DOT PATTERN). MECHANICAL FIXINGS (PLASTIC CAVITY-FREE FIXINGS — SCHÖCK ISOLINK OR EJOT SPIRAL ANCHOR AT 6 TO 8 FIXINGS PER M² — WIND-UPLIFT RESISTANCE AND DEFENCE AGAINST ADHESIVE FAILURE).

LAYER 3: BASECOAT + REINFORCEMENT MESH. A POLYMER-MODIFIED BASECOAT (K-REND SILICONE BASE COAT, WETHERBY EWI REINFORCEMENT ADHESIVE) IS TROWELLED OVER THE INSULATION BOARDS. A 160 G/M² ALKALINE-RESISTANT GLASS FIBRE MESH (PERMAROCK GLASS FIBRE MESH) IS EMBEDDED IN THE WET BASECOAT — THIS CRACK-BRIDGING MESH LAYER IS CRITICAL FOR PREVENTING IMPACT CRACKS AND DIFFERENTIAL MOVEMENT CRACKS IN THE RENDER FINISH. LAYER 4: PRIMER AND TOP COAT. A PRIMER (K-REND SILICONE PRIMER OR WETHERBY EWI PRIMER) IS APPLIED AFTER BASECOAT CURE (24 TO 48 HOURS). SILICONE TOPCOAT (K-REND SILICONE TC15 IN SCRAPED FINISH) OR MINERAL TOPCOAT (WETHERBY MINERAL RENDER).

THE TARGET U-VALUE FOR AN EWI UPGRADE ON A LONDON VICTORIAN TERRACE SOLID BRICK WALL DEPENDS ON THE REGULATORY CONTEXT AND THE ENERGY PERFORMANCE AMBITION.

STARTING U-VALUE OF A TYPICAL UNINSULATED LONDON STOCK BRICK WALL: 225MM SINGLE LEAF LONDON STOCK BRICK + INTERNAL LIME PLASTER: APPROXIMATELY 1.6 TO 2.1 W/M²K (DEPENDING ON BRICK AND MORTAR THERMAL CONDUCTIVITY — LONDON STOCK BRICK IS RELATIVELY POROUS AND SLIGHTLY BETTER THAN DENSE ENGINEERING BRICK).

PART L 2021 MINIMUM RETAINED EWI U-VALUE FOR EXISTING DWELLING REFURBISHMENT: 0.30 W/M²K (TABLE 5, APPROVED DOCUMENT L1B 2021). THIS IS THE MINIMUM — IT IS OFTEN COST-EFFECTIVE TO SPECIFY A BETTER U-VALUE FOR ENHANCED ENERGY SAVINGS AND LONGER-TERM FUEL BILL REDUCTION.

INSULATION THICKNESS REQUIRED TO ACHIEVE 0.30 W/M²K ON LONDON STOCK BRICK (225MM, U = 2.0 W/M²K STARTING): GRAPHITE EPS (λ = 0.031 W/MK): 70MM — ACHIEVES 0.29 W/M²K. STANDARD EPS (λ = 0.039 W/MK): 100MM — ACHIEVES 0.31 W/M²K. MINERAL WOOL (λ = 0.040 W/MK): 100MM — ACHIEVES 0.31 W/M²K. PHENOLIC FOAM (λ = 0.020 W/MK): 50MM — ACHIEVES 0.28 W/M²K. TO ACHIEVE 0.20 W/M²K (DEEP ENERGY RETROFIT AMBITION — PAS 2035 RETROFIT COORDINATOR RECOMMENDATION FOR EPC BAND B): GRAPHITE EPS: 120MM. MINERAL WOOL: 140MM.

EXTERNAL WALL INSULATION ON A LONDON VICTORIAN TERRACE CHANGES THE EXTERNAL APPEARANCE OF THE BUILDING — IT ADDS 80 TO 160MM OF THICKNESS TO THE EXTERNAL WALLS AND PROVIDES A RENDERED FINISH OVER THE ORIGINAL BRICKWORK. THIS HAS SIGNIFICANT PLANNING IMPLICATIONS FOR MANY LONDON PROPERTIES.

PERMITTED DEVELOPMENT FOR EWI: HOUSEHOLDER PERMITTED DEVELOPMENT RIGHTS ALLOW EXTERNAL WALL INSULATION ON THE PRINCIPAL ELEVATION (FRONT ELEVATION) ONLY WHERE THE RENDER OR CLADDING FINISH USES MATERIALS "OF A SIMILAR APPEARANCE TO THOSE USED IN THE CONSTRUCTION OF THE EXTERIOR OF THE EXISTING DWELLINGHOUSE." THIS IS A VERY HIGH BAR FOR LONDON VICTORIAN TERRACES — A BRICK-FRONTED TERRACE COVERED IN WHITE RENDER IS NOT OF A SIMILAR APPEARANCE TO THE ORIGINAL BRICK.

CONSEQUENCE: IN PRACTICE, EWI ON THE FRONT ELEVATION OF A LONDON VICTORIAN TERRACE ALMOST ALWAYS REQUIRES A PLANNING APPLICATION (HOUSEHOLDER PLANNING APPLICATION — FEE £258 IN ENGLAND AS OF 2025). REAR ELEVATION EWI MAY BENEFIT FROM PD WHERE NOT VISIBLE FROM THE HIGHWAY, BUT PLANNING ADVICE SHOULD BE SOUGHT.

CONSERVATION AREA ADDITIONAL RESTRICTIONS: IN LONDON CONSERVATION AREAS (COVERING MANY INNER LONDON VICTORIAN TERRACES IN ISLINGTON, HACKNEY, LAMBETH, CAMDEN, KENSINGTON, WANDSWORTH, AND SOUTHWARK), ANY CHANGE TO THE EXTERNAL APPEARANCE THAT WOULD ALTER THE CHARACTER OF THE CONSERVATION AREA REQUIRES CONSENT. EWI OVER FRONT BRICKWORK IN A CONSERVATION AREA WILL TYPICALLY BE REFUSED UNLESS THE BRICK IS ALREADY NON-ORIGINAL (E.G. ALREADY RENDERED, OR PREVIOUSLY CLAD IN AN INAPPROPRIATE MATERIAL). EWI ON REAR ELEVATIONS IS MORE FEASIBLE IN CONSERVATION AREAS — CONTACT THE LOCAL PLANNING AUTHORITY FOR PRE-APPLICATION ADVICE.

FOR EWI PROJECTS FUNDED THROUGH UK GOVERNMENT ENERGY EFFICIENCY SCHEMES — INCLUDING THE ECO4 SCHEME (ENERGY COMPANY OBLIGATION — FUNDED BY THE BIG SIX ENERGY SUPPLIERS), THE GREAT BRITISH INSULATION SCHEME (GBIS), OR THE LOCAL AUTHORITY DELIVERY (LAD) SCHEME — THE INSTALLATION MUST COMPLY WITH PAS 2035:2019 (RETROFITTING DWELLINGS FOR IMPROVED ENERGY EFFICIENCY — SPECIFICATION AND GUIDANCE).

PAS 2035 REQUIRES: RETROFIT ASSESSOR: A PAS 2035 REGISTERED RETROFIT ASSESSOR MUST ASSESS THE PROPERTY BEFORE WORK BEGINS — PRODUCING A RETROFIT ASSESSMENT REPORT THAT IDENTIFIES THE PROPERTY'S CURRENT ENERGY PERFORMANCE AND ANY RISKS (DAMP, VENTILATION ISSUES, STRUCTURAL DEFECTS) THAT MUST BE ADDRESSED BEFORE EWI IS INSTALLED. RETROFIT COORDINATOR: A PAS 2035 REGISTERED RETROFIT COORDINATOR MUST OVERSEE THE PROJECT — ENSURING THE MEASURES ARE DESIGNED APPROPRIATELY (AVOIDING UNINTENDED CONSEQUENCES SUCH AS INTERSTITIAL CONDENSATION WITHIN THE WALL BUILD-UP), SPECIFYING THE APPROPRIATE INSULATION PRODUCT, AND ENSURING THE INSTALLATION IS COMPLETED BY A REGISTERED INSTALLER. REGISTERED INSTALLER: THE EWI INSTALLATION MUST BE CARRIED OUT BY AN INSTALLER REGISTERED UNDER A COMPETENT PERSON SCHEME (CIGA, NAPIT, NICEIC, OR BBA-APPROVED INSTALLER SCHEME FOR EWI). FOR SELF-FUNDED EWI (NOT ECO4 OR OTHER GOVERNMENT SCHEME), PAS 2035 IS NOT LEGALLY MANDATORY — BUT THE TECHNICAL REQUIREMENTS AND INSTALLATION QUALITY STANDARDS SHOULD STILL BE FOLLOWED AS BEST PRACTICE.

ONE OF THE MOST IMPORTANT TECHNICAL DETAILS IN AN EWI INSTALLATION ON A LONDON VICTORIAN TERRACE IS THE TREATMENT OF THERMAL BRIDGES — POINTS WHERE THE INSULATION IS PENETRATED OR BYPASSED, CREATING A PATH FOR HEAT LOSS AND CONDENSATION RISK.

TYPICAL THERMAL BRIDGE LOCATIONS IN A LONDON VICTORIAN TERRACE EWI INSTALLATION: WINDOW AND DOOR REVEALS: THE WINDOW FRAME IS TYPICALLY SET INTO THE ORIGINAL BRICK REVEAL — WHEN EWI IS APPLIED TO THE MAIN WALL, THE REVEAL BEHIND THE WINDOW FRAME REMAINS UNINSULATED, CREATING A THERMAL BRIDGE AT EACH WINDOW. SOLUTION: INSTALL EWI OVER THE REVEAL (MINIMUM 25MM GRAPHITE EPS ON THE WINDOW REVEAL) TO REDUCE THE REVEAL COLD BRIDGE. PARAPET AND EAVES DETAILS: THE JUNCTION BETWEEN THE EWI ON THE WALL AND THE UNINSULATED ROOF CONSTRUCTION CREATES A THERMAL BRIDGE AT PARAPET OR EAVES LEVEL. EXTERNAL STRUCTURAL ELEMENTS: LINTELS OVER WINDOWS AND DOORS, CONCRETE SLABS AT INTERMEDIATE FLOORS, AND JOIST ENDS BUILT INTO THE WALL — ALL REQUIRE CAREFUL INSULATION DETAILING TO MINIMISE COLD BRIDGING. WINDOW SILL EXTENSION PROFILES: WHEN THE EWI ADDS 80MM+ TO THE EXTERNAL WALL, THE ORIGINAL WINDOW SILL NO LONGER PROJECTS SUFFICIENTLY OVER THE NEW EXTERNAL FACE — SILL EXTENSION PROFILES (ALUMINIUM OR COMPOSITE — PERMAROCK WINDOW SILL EXTENSION OR WETHERBY EQUIVALENT) ARE REQUIRED AT EVERY WINDOW AND DOOR OPENING.

EWI COSTS IN LONDON DEPEND ON THE INSULATION THICKNESS, THE RENDER FINISH SPECIFICATION, SCAFFOLDING REQUIREMENTS, AND THE EXTENT OF PREPARATORY WORK (MASONRY REPAIRS, WINDOW SILL EXTENSIONS, LINTEL TREATMENT).

TYPICAL LONDON EWI SYSTEM COSTS (SUPPLY AND INSTALL — PER M² OF WALL AREA, EXCLUDING SCAFFOLDING): GRAPHITE EPS 70MM (K-REND SILICONE TC15 FINISH — WETHERBY OR PERMAROCK K-SYSTEM): £60 TO £90 PER M² SUPPLY AND INSTALL. MINERAL WOOL 100MM (KNAUF EARTHWOOL EWI SLAB — WEBER OR PERMAROCK MINERAL RENDER FINISH): £70 TO £100 PER M² SUPPLY AND INSTALL. PHENOLIC FOAM 50MM (KINGSPAN KOOLTHERM K5 — K-REND TC15 FINISH): £80 TO £120 PER M² SUPPLY AND INSTALL.

SCAFFOLDING (FOR A TYPICAL LONDON 2-STOREY VICTORIAN TERRACE — FRONT AND REAR): £1,500 TO £3,500 FOR THE SCAFFOLDING ELEMENT ALONE.

TYPICAL LONDON TERRACE (SEMI-DETACHED, 2-STOREY, FRONT + REAR ELEVATIONS ONLY — APPROXIMATELY 80 M² OF WALL AREA): GRAPHITE EPS 70MM: £4,800 TO £7,200 SYSTEM COST + £1,500 TO £3,500 SCAFFOLDING = £6,300 TO £10,700 TOTAL. MINERAL WOOL 100MM: £5,600 TO £8,000 SYSTEM COST + SCAFFOLDING = £7,100 TO £11,500 TOTAL.

ECO4 SCHEME FUNDING: FOR LONDON HOUSEHOLDS QUALIFYING FOR ECO4 (FUEL POVERTY — BENEFITS-ELIGIBLE OWNER-OCCUPIERS OR PRIVATE TENANTS WITH LANDLORD CONSENT, EPC D OR BELOW), EWI ON SOLID WALLS MAY BE FULLY OR PARTIALLY FUNDED THROUGH THE ENERGY COMPANY OBLIGATION SCHEME. CHECK ELIGIBILITY AT GOV.UK/ECO4 OR THROUGH A REGISTERED EWI INSTALLER.