External Wall Insulation in London: EWI Systems, Planning Rules & Costs 2025

AN EXTERNAL WALL INSULATION SYSTEM CONSISTS OF THREE PRINCIPAL LAYERS APPLIED TO THE EXTERNAL FACE OF THE EXISTING WALL:

1. INSULATION BOARD LAYER: FIXED TO THE EXISTING WALL USING A COMBINATION OF ADHESIVE MORTAR ("BASECOAT" APPLIED WITH NOTCHED TROWEL) AND MECHANICAL FIXINGS (HELICAL ANCHOR BOLTS OR SPIRAL ANCHORS DRIVEN THROUGH THE INSULATION INTO THE MASONRY — TYPCIALLY 6-10 PER M2 ON A SOLID BRICK WALL).

INSULATION TYPES FOR EWI: — EPS (EXPANDED POLYSTYRENE): THE MOST WIDELY USED EWI INSULATION BOARD IN THE UK. THERMAL CONDUCTIVITY: 0.032-0.038 W/MK. LAMBDA 0.032: 100MM ACHIEVES U=0.28 W/M2K FROM 225MM SOLID BRICK STARTING POINT. FIRE CLASSIFICATION: REACTION TO FIRE — MOST EPS SYSTEMS ARE COMBINED WITH A FULL BASECOAT/MESH/TOPCOAT SYSTEM THAT PROVIDES FIRE PROTECTION UNDER BS EN 13501-1 CLASS B-S2-D0. COST: £40-£65/M2 (SYSTEM INSTALLED).

— MINERAL WOOL (STONE WOOL OR GLASS WOOL EWI SLAB): HIGHER FIRE PERFORMANCE (A1/A2 NON-COMBUSTIBLE). THERMAL CONDUCTIVITY: 0.035-0.040 W/MK. MANUFACTURERS: ROCKWOOL FRONT ROCK EWI SLAB, KNAUF AUSSENPUTZ, ISOVER EWI SLAB. HIGHER COST THAN EPS (£65-£90/M2 INSTALLED) BUT PREFERRED WHERE FIRE SAFETY IS A PRIMARY CONCERN (BUILDINGS OVER 18M HIGH — MANDATORY UNDER THE BUILDING SAFETY ACT 2022 AMENDMENT TO APPROVED DOCUMENT B; ALSO PREFERRED ON BUILDINGS ADJACENT TO COMMERCIAL PREMISES OR ON FIRE ESCAPE ROUTES).

— PIR (POLYISOCYANURATE) RIGID FOAM EWI BOARD: HIGHEST THERMAL PERFORMANCE — THERMAL CONDUCTIVITY 0.022-0.023 W/MK. 60MM PIR ACHIEVES U=0.3 W/M2K FROM SOLID BRICK. LOWER FIRE RATING THAN MINERAL WOOL. MANUFACTURERS: KINGSPAN KOOLTHERM K5 EWI, CELOTEX EWI SYSTEM. COST: £70-£100/M2 INSTALLED.

2. REINFORCEMENT MESH AND BASECOAT: GLASS-FIBRE REINFORCEMENT MESH EMBEDDED IN POLYMER-MODIFIED ADHESIVE BASECOAT (PERMAROCK, WEBERTHERM, WETHERBY EWI). MINIMUM 2-COAT BASECOAT WITH MESH FULLY EMBEDDED (NOT BRIDGING ACROSS THE SURFACE). MESH PREVENTS CRACK PROPAGATION FROM THERMAL MOVEMENT.

3. PRIMER AND FINISH RENDER: THIN-COAT ACRYLIC OR SILICONE RENDER (1.5-3MM THICKNESS): — SILICONE RENDER: K-REND SILICONE TC15, WEBER SILICONE ONE COAT, PAREX MONOREX GM. HYDROPHOBIC (WATER-REPELLENT) AND BREATHABLE. 25-YEAR COLOUR GUARANTEE FROM K-REND. MOST WIDELY SPECIFIED EWI FINISH IN UK. — ACRYLIC RENDER: CHEAPER BUT LESS DURABLE AND LESS BREATHABLE THAN SILICONE. MAINLY USED IN SOCIAL HOUSING RETROFITS UNDER ECO4. — MINERAL RENDER: NON-COMBUSTIBLE, SUITABLE FOR MINERAL WOOL EWI SYSTEMS ON HIGHER-RISK BUILDINGS.

EWI ADDS TYPICALLY 100-130MM TO THE EXTERNAL FACE OF THE WALL — CHANGING THE EXTERNAL APPEARANCE OF THE PROPERTY (DIFFERENT FINISH MATERIAL, INCREASED WIDTH, WINDOW AND DOOR REVEALS BECOME DEEPER). IN LONDON, THIS MEANS:

PERMITTED DEVELOPMENT: IN MOST CASES, EWI ON A DWELLING IN A NON-CONSERVATION AREA, NON-LISTED BUILDING, NON-ARTICLE 4 AREA QUALIFIES AS PERMITTED DEVELOPMENT UNDER GPDO 2015 CLASS A (ADDITIONS, ALTERATIONS TO ROOFS AND EXTERNAL WALLS OF DWELLINGHOUSES) PROVIDED: — THE MATERIALS ARE OF A SIMILAR APPEARANCE TO THE EXISTING (THIS IS WHERE RENDER EWI CAN BE CONTENTIOUS — IF THE ORIGINAL BUILDING IS BRICK-FACED AND THE EWI RENDERS OVER IT IN A DIFFERENT MATERIAL, THE LPA MAY ARGUE IT DOES NOT QUALIFY AS PD) — NOT ON A LISTED BUILDING — NOT IN A CONSERVATION AREA

CONSERVATION AREAS: EWI ON A PROPERTY IN A CONSERVATION AREA ALMOST CERTAINLY REQUIRES PLANNING PERMISSION — AND IS VERY LIKELY TO BE REFUSED ON THE FRONT ELEVATION WHERE IT WOULD AFFECT THE CHARACTER OF THE AREA BY COVERING ORIGINAL BRICKWORK WITH RENDER. ALTERNATIVE APPROACH: INTERNAL WALL INSULATION (IWI) — WOOD FIBRE BOARD, HEMP LIME, OR PIR WITH VAPOUR CONTROL LAYER — IS THE APPROPRIATE APPROACH IN CONSERVATION AREAS.

LISTED BUILDINGS: EWI ON A LISTED BUILDING REQUIRES LISTED BUILDING CONSENT AND IS ALMOST ALWAYS REFUSED — IT COVERS AND POTENTIALLY DAMAGES HISTORIC FABRIC. BREATHABLE INTERNAL INSULATION IS THE ONLY APPROPRIATE OPTION.

ECO4 FUNDING CONTEXT: EWI IS ONE OF THE PRIMARY MEASURES FUNDED UNDER THE UK GOVERNMENT'S ECO4 SCHEME AND THE GREAT BRITISH INSULATION SCHEME — BUT ONLY FOR OWNER-OCCUPIERS OR PRIVATE LANDLORDS IN LOWER INCOME BANDS (ECO4 ELIGIBLE) OR FOR PROPERTIES WITH EPC RATING D OR BELOW (GBIS). IN LONDON, MANY BOROUGHS ALSO HAVE LOCAL AUTHORITY DELIVERY (LAD) SCHEMES — CHECK WITH THE GLA ENERGY TEAM AND YOUR BOROUGH FOR AVAILABLE GRANT FUNDING BEFORE COMMITTING TO PRIVATE PAYMENT.

THE PRIMARY OBJECTIVE OF EWI IS TO SIGNIFICANTLY REDUCE THE U-VALUE OF THE EXTERNAL WALLS — REDUCING HEAT LOSS AND IMPROVING ENERGY EFFICIENCY.

STARTING U-VALUES FOR COMMON LONDON WALL TYPES: — 225MM (9-INCH) SOLID BRICK WALL (MOST COMMON VICTORIAN TERRACE): U=2.0-2.1 W/M2K — 340MM (13.5-INCH) SOLID BRICK WALL (DOUBLE-FRONTED VICTORIAN): U=1.5 W/M2K — UNINSULATED CAVITY WALL (POST-1920, PRE-1978): U=1.5-1.7 W/M2K — PARTIALLY FILLED CAVITY (50MM MINERAL WOOL CAVITY): U=0.5 W/M2K

TARGET U-VALUES: — BUILDING REGULATIONS PART L (NEW EXTENSIONS): 0.18 W/M2K (WALLS) — DECENT HOMES STANDARD (SOCIAL HOUSING): 0.35 W/M2K — ECO4 / PAS 2035 RETROFIT STANDARD: 0.3 W/M2K — BS EN ISO 9869 THERMAL PERFORMANCE — EWI INSTALLER TARGET: 0.3 W/M2K OR BETTER

EWI THICKNESS TO ACHIEVE 0.3 W/M2K FROM 225MM SOLID BRICK: — EPS (LAMBDA 0.032): 90-100MM EPS BOARD — MINERAL WOOL (LAMBDA 0.036): 100-110MM MINERAL WOOL — PIR (LAMBDA 0.022): 60-70MM PIR BOARD

THERMAL BRIDGING: ONE OF THE KEY ADVANTAGES OF EWI OVER INTERNAL WALL INSULATION IS THAT EWI WRAPS THE ENTIRE WALL — ELIMINATING THERMAL BRIDGES AT FLOOR JUNCTIONS, WINDOW REVEALS, AND STRUCTURAL BEAMS. INTERNAL INSULATION, APPLIED TO THE INTERNAL FACE, CANNOT ELIMINATE THERMAL BRIDGING AT FLOOR/WALL AND CEILING/WALL JUNCTIONS — THESE REMAIN COLD SPOTS AND POTENTIAL CONDENSATION RISK AREAS.

EWI INSTALLATION QUALITY IS LARGELY DETERMINED BY HOW WELL THE INSTALLER HANDLES THE DETAILS — PARTICULARLY AROUND WINDOW AND DOOR OPENINGS, SILLS, EAVES, AND GROUND LEVEL JUNCTIONS. POOR DETAIL WORK IS THE PRIMARY CAUSE OF EWI FAILURES IN THE UK.

WINDOW REVEAL DETAILING: WINDOWS AND DOORS REMAIN IN THEIR EXISTING POSITIONS (AGAINST THE ORIGINAL WALL FACE). THE EWI SYSTEM IS RETURNED INTO THE REVEALS — TYPICALLY WITH A THINNER INSULATION THICKNESS (50MM RATHER THAN 100MM) TO AVOID EXCESSIVE SHADOW AND REDUCTION OF DAYLIGHT. THE REVEAL INSULATION IS FINISHED WITH THE SAME BASECOAT/MESH/RENDER AS THE MAIN FACE.

SILL EXTENSION: WINDOW SILLS MUST BE EXTENDED FORWARD TO OVERHANG THE NEW EWI FACE — ALUMINIUM OR PVC SILL EXTENSIONS (EGGER OR ROCKFLASH ALUMINIUM SILL SYSTEM) WITH CORRECT DRIP EDGE AND END DAM DETAILS. WATER RUNNING OFF THE SILL MUST NOT GET BEHIND THE EWI SYSTEM.

GROUND LEVEL TERMINATION: THE EWI SYSTEM MUST TERMINATE ABOVE GROUND LEVEL (MINIMUM 150MM ABOVE DPC LEVEL) WITH A BEADED STARTER TRACK THAT PREVENTS WIND UPLIFT AND ALLOWS DRAINAGE. THE GAP BETWEEN THE BOTTOM OF EWI AND GROUND IS A COMMON ENTRY POINT FOR MOISTURE IF NOT CORRECTLY DETAILED.

EAVES AND VERGE DETAILING: BROCKS OR EXTENDED SOFFITS MAY BE REQUIRED IF THE EWI THICKNESS REDUCES THE OVERHANG OF THE EXISTING EAVES. THE EWI MUST NOT BE TAKEN OVER AN UNVENTILATED ROOF EAVES — ROOF VENTILATION MUST BE MAINTAINED.

EWI INSTALLATION COSTS IN LONDON (2025):

SYSTEM COMPARISON (SUPPLY AND FIT, PER M2 OF EXTERNAL WALL AREA): — EPS SYSTEM (PERMAROCK, WEBERTHERM, WETHERBY): £80-£120/M2 — MINERAL WOOL SYSTEM (ROCKWOOL FRONT ROCK, KNAUF): £100-£145/M2 — PIR SYSTEM (KINGSPAN KOOLTHERM K5): £110-£155/M2

TYPICAL LONDON TERRACE (80M2 EXTERNAL WALL AREA, EXCLUDING FRONT ELEVATION): — EPS SYSTEM: £6,400-£9,600 — MINERAL WOOL: £8,000-£11,600 — PIR: £8,800-£12,400

FULL HOUSE (200M2 EXTERNAL WALL AREA — ALL ELEVATIONS EXCEPT FRONT IF CONSERVATION): — EPS: £16,000-£24,000 — MINERAL WOOL: £20,000-£29,000

ADDITIONAL COSTS: — SCAFFOLDING (MANDATORY FOR EWI): £3,000-£8,000 DEPENDING ON HOUSE SIZE AND CONFIGURATION — WINDOW SILL EXTENSIONS: £200-£400 PER WINDOW — METER BOX AND EXTERNAL FITTING RELOCATIONS: £150-£300 PER ITEM — RENDER FINISH PREMIUM (SILICONE OVER ACRYLIC): +£10-£20/M2

GRANT FUNDING CHECK: BEFORE PAYING PRIVATELY FOR EWI, CHECK: 1. ECO4 (GREAT BRITISH INSULATION SCHEME): FREE EWI FOR ELIGIBLE LOW-INCOME HOUSEHOLDS IN PROPERTIES WITH EPC D-G RATING 2. LONDON COUNCIL WARMER HOMES PROGRAMMES: MANY BOROUGHS HAVE SUBSIDISED EWI PROGRAMMES 3. GREAT BRITISH INSULATION SCHEME (GBIS): PROPERTIES WITH EPC D-G RATING AND COUNCIL TAX BANDS A-D MAY QUALIFY WEBSITE: SIMPLEENERGYADVICE.ORG FOR ELIGIBILITY CHECK