Internal Wall Insulation (IWI) for London Victorian Terraces: Slim Profile and Breathable Systems

INTERNAL WALL INSULATION FOR A LONDON VICTORIAN TERRACE MUST BE CHOSEN TO BALANCE THERMAL PERFORMANCE (U-VALUE IMPROVEMENT), THICKNESS (FLOOR AREA LOSS), CONDENSATION RISK (INTERSTITIAL CONDENSATION AT THE COLD WALL FACE), AND BREATHABILITY (COMPATIBILITY WITH THE ORIGINAL LIME PLASTER WALLS).

SLIM-PROFILE AEROGEL INSULATION (THE THINNEST IWI SYSTEM FOR LONDON TERRACES — MAXIMUM THERMAL IMPROVEMENT PER MM OF THICKNESS): CABOT THERMAL WRAP 10 (AEROGEL COMPOSITE BLANKET — 10MM THICKNESS, λ = 0.0155 W/MK): ACHIEVES A THERMAL RESISTANCE OF 0.64 M²K/W FOR A 10MM LAYER. INSTALLED WITH MECHANICALLY FIXED 10MM PLASTERBOARD OVER (TOTAL WALL BUILD-UP: 10MM AEROGEL + 12.5MM PLASTERBOARD = 22.5MM FROM WALL FACE). THERMALLY, 22.5MM OF AEROGEL COMPOSITE SYSTEM ACHIEVES A COMPARABLE IMPROVEMENT TO 50-60MM OF PIR BOARD — MAKING IT IDEAL FOR LONDON TERRACES WHERE FLOOR AREA LOSS IS A PRIMARY CONCERN. SPACETHERM A2 AEROGEL INSULATION BOARD (A2 FIRE-RATED — NON-COMBUSTIBLE): MANUFACTURED BY PROCTOR GROUP, SPACETHERM A2 IS A 10MM OR 25MM AEROGEL BLANKET BONDED TO CALCIUM SILICATE BOARD — NON-COMBUSTIBLE (CLASS A2) AND BUILDABLE (CAN BE MECHANICALLY FIXED OR BONDED DIRECTLY TO THE WALL). USEFUL FOR LONDON LISTED BUILDINGS AND CONSERVATION AREA PROPERTIES WHERE FIRE PERFORMANCE OF INTERNAL INSULATION IS IMPORTANT.

KINGSPAN KOOLTHERM K18 INTERNAL INSULATION BOARD (THE STANDARD RIGID PIR BOARD IWI CHOICE FOR LONDON VICTORIAN TERRACE RENOVATION): KOOLTHERM K18 (FACING OF ALUMINIUM FOIL ON BOTH SIDES — ONE SIDE ACTS AS A VAPOUR CONTROL LAYER): λ = 0.020 W/MK. TYPICAL THICKNESS FOR IWI IN A LONDON TERRACE: 37.5MM (KOOLTHERM K18 AT 25MM + PLASTERBOARD 12.5MM BONDED) — ACHIEVES APPROXIMATELY R 1.25 M²K/W ADDITIONAL THERMAL RESISTANCE. INSTALLED WITH DOT-AND-DAB ADHESIVE (BRITISH GYPSUM GYPROC BOND-IT) DIRECT TO THE EXISTING WALL SURFACE (AFTER CLEANING, TREATING FOR DAMP, AND PRIMING). TOTAL DEPTH FROM WALL: 37.5MM (INCLUDING PLASTERBOARD FACE).

LIME-HEMP OR WOOD FIBRE BOARD (BREATHABLE IWI — FOR OLDER LONDON TERRACES WHERE BREATHABILITY IS CRITICAL): ISOCHANVRE HEMP-LIME COMPOSITE BOARD (60-80MM — λ = 0.058 W/MK): A BREATHABLE, NATURAL-MATERIAL INSULATION BOARD — COMPATIBLE WITH LIME PLASTER WALLS (DOES NOT TRAP MOISTURE). SLOWER DRYING RATE — THE BOARD ABSORBS AND RELEASES MOISTURE WITHOUT DEGRADING, PREVENTING CONDENSATION ACCUMULATION. PAVATEX PAVAFLEX WOOD FIBRE INSULATION BOARD (60-100MM — λ = 0.038 W/MK): SOFT WOOD FIBRE BOARD — BREATHABLE, GOOD ACOUSTIC PERFORMANCE, COMPATIBLE WITH BREATHABLE (LIME) PLASTERS AND PAINTS.

THE MOST IMPORTANT TECHNICAL ISSUE IN ANY INTERNAL WALL INSULATION INSTALLATION IN A LONDON VICTORIAN TERRACE IS INTERSTITIAL CONDENSATION — MOISTURE WITHIN THE WALL FABRIC CONDENSING AT THE COLD INTERFACE BETWEEN THE INSULATION AND THE ORIGINAL BRICK WALL.

WHY CONDENSATION RISK IS SIGNIFICANT IN IWI: ADDING INSULATION TO THE INSIDE OF A SOLID BRICK WALL CHANGES THE THERMAL PROFILE THROUGH THE WALL — THE BRICK WALL BECOMES COLDER (BECAUSE THE INSULATION NOW KEEPS THE INTERIOR WARM, BUT THE BRICK WALL ITSELF IS EXPOSED TO OUTDOOR COLD AIR). AS THE BRICK WALL BECOMES COLDER, IT APPROACHES AND EVENTUALLY FALLS BELOW THE DEW POINT OF MOIST INTERIOR AIR — PARTICULARLY IF WARM MOIST AIR FROM THE INTERIOR CAN DIFFUSE OR LEAK INTO THE WALL-INSULATION INTERFACE. CONDENSATION AT THIS INTERFACE: RISKS ROT IN TIMBER ELEMENTS BUILT INTO THE ORIGINAL LONDON STOCK BRICK WALL (E.G. FLOOR JOIST ENDS BUILT INTO THE PARTY WALL ARE ALSO BUILT INTO EXTERNAL WALLS IN SOME VICTORIAN TERRACE CONFIGURATIONS). CAN CAUSE MOULD GROWTH IN THE INSULATION VOID IF BREATHABILITY IS INSUFFICIENT. CAN CAUSE EFFLORESCENCE ON THE EXTERNAL BRICK FACE AS MOISTURE CYCLES THROUGH THE WALL.

BREATHABILITY VS VAPOUR CONTROL — THE TWO APPROACHES: APPROACH 1: VAPOUR CONTROL (FOIL-BACKED PIR BOARD — KOOLTHERM K18, CELOTEX PL4000): THE FOIL FACING ON THE WARM SIDE OF THE INSULATION BOARD ACTS AS A VAPOUR CONTROL LAYER (VCL) — PREVENTING WARM MOIST INTERNAL AIR FROM REACHING THE COLD WALL FACE. THIS APPROACH IS THEORETICALLY CORRECT IF THE VCL IS CONTINUOUS AND WELL-SEALED — BUT ANY GAPS, PENETRATIONS (SOCKETS, LIGHT SWITCHES, PIPES), OR DAMAGE TO THE FOIL VCL CREATES A BYPASS FOR MOIST AIR. APPROACH 2: BREATHABLE INSULATION (HEMP-LIME, WOOD FIBRE): RATHER THAN BLOCKING MOISTURE MOVEMENT WITH A VCL, BREATHABLE INSULATION ALLOWS MOISTURE TO MOVE THROUGH THE WALL AND RELEASE TO THE INTERIOR OR EXTERIOR WITHOUT ACCUMULATING. THE BREATHABLE APPROACH IS MORE FORGIVING OF VCL IMPERFECTIONS BUT GENERALLY PROVIDES LESS THERMAL RESISTANCE PER MM THAN PIR. GLASER METHOD CONDENSATION RISK ASSESSMENT (BS EN ISO 13788): ALL IWI DESIGNS FOR LONDON VICTORIAN TERRACES SHOULD BE SUBJECTED TO A GLASER METHOD CALCULATION OR, BETTER, A WUFI HYGROTHERMAL MODEL (FRAUNHOFER IBP METHOD) — TO VERIFY THAT INTERSTITIAL CONDENSATION RISK IS ACCEPTABLE UNDER THE DESIGN CONDITIONS.

INTERNAL WALL INSULATION INSTALLATION IN A LONDON VICTORIAN TERRACE REQUIRES CAREFUL MANAGEMENT OF SEVERAL CRITICAL DETAILS BEYOND THE INSULATION BOARD ITSELF.

DPC AND EXISTING DAMP: BEFORE INSTALLING IWI, THE ORIGINAL LIME PLASTER MUST BE CHECKED FOR DAMP — A CARBIDE BOMB TEST (SPEEDY MOISTURE METER) CONFIRMS WHETHER RISING DAMP OR PENETRATING DAMP IS PRESENT. IF DAMP IS PRESENT, IT MUST BE RESOLVED FIRST (DRYZONE DPC CREAM INJECTION FOR RISING DAMP — OR RESOLVE THE SOURCE OF PENETRATING DAMP): INSTALLING IWI OVER A DAMP WALL TRAPS MOISTURE IN THE WALL AND ACCELERATES DETERIORATION OF BOTH THE ORIGINAL BRICKWORK AND THE INSULATION.

ELECTRICAL SERVICES RELOCATION: WHEN IWI IS INSTALLED, EXISTING SURFACE-MOUNTED OR SHALLOW-FLUSH ELECTRICAL SOCKETS AND SWITCHES ON THE EXTERNAL WALL MUST BE: RELOCATED TO THE NEW INSULATED WALL FACE (USING ROUND CONDUIT BACK-BOXES — PLASTIC SURFACE MOUNT IF THE INSULATION DEPTH IS INSUFFICIENT FOR RECESSING). SERVICES MUST NOT PENETRATE THE VCL (FOIL FACE OF PIR BOARD) — USE AIRTIGHT BOX GASKETS (ECOTEC BOX OR PRESSOFLEX AIRTIGHT BOXES) TO MAINTAIN VCL CONTINUITY AT SOCKET LOCATIONS.

SKIRTING AND ARCHITRAVE RELOCATION: EXISTING SKIRTING BOARDS AND ARCHITRAVES MUST BE REMOVED BEFORE IWI INSTALLATION AND REINSTATED ON THE NEW WALL FACE AFTER — AT THE CORRECT HEIGHT. WINDOW REVEALS: THE INTERNAL REVEALS OF WINDOWS IN THE EXTERNAL WALL SHOULD ALSO BE INSULATED (MINIMUM 25MM AEROGEL OR SLIM PIR ON REVEALS) — OTHERWISE THE REVEAL ACTS AS A THERMAL BRIDGE AND COLD SPOT WHERE CONDENSATION AND MOULD GROWTH WILL OCCUR. WALL-FLOOR JUNCTION (COLD BRIDGING AT FLOOR LEVEL): THE JUNCTION BETWEEN THE EXTERNAL WALL AND THE GROUND OR SUSPENDED FLOOR IS A SIGNIFICANT THERMAL BRIDGE — INSULATION AT THE BASE OF THE IWI SHOULD EXTEND TO LINK WITH THE FLOOR INSULATION OR SKIRTING BOARD LEVEL TO MINIMISE THIS BRIDGE.

IWI INSTALLATIONS IN LONDON VICTORIAN TERRACES MUST ACHIEVE THE MINIMUM THERMAL PERFORMANCE REQUIRED BY PART L OF THE BUILDING REGULATIONS — SUBJECT TO THE "CONSEQUENTIAL IMPROVEMENTS" AND "SIMPLE PAYBACK" RULES FOR EXISTING DWELLINGS.

PART L 2021 MINIMUM U-VALUE FOR IWI IN EXISTING DWELLINGS: 0.30 W/M²K — THE SAME TARGET AS FOR EWI (TABLE 5, APPROVED DOCUMENT L1B 2021). THIS IS THE MINIMUM. ACHIEVING 0.30 W/M²K WITH IWI ON A 225MM SOLID LONDON STOCK BRICK WALL: STARTING U-VALUE: APPROXIMATELY 2.0 W/M²K (UNINSULATED). KOOLTHERM K18 AT 25MM (λ = 0.020): ADDS R 1.25 M²K/W — REDUCES U-VALUE TO APPROXIMATELY 0.50 W/M²K (NOT SUFFICIENT FOR 0.30 W/M²K ALONE — ADDITIONAL THICKNESS OR HIGHER-PERFORMING BOARD NEEDED). KOOLTHERM K18 AT 50MM (TOTAL WALL DEPTH INCREASE 62.5MM WITH PLASTERBOARD): REDUCES U-VALUE TO APPROXIMATELY 0.29 W/M²K — MARGINALLY BELOW 0.30 TARGET. AEROGEL SPACETHERM (25MM + 12.5MM PLASTERBOARD = 37.5MM TOTAL): REDUCES U-VALUE TO APPROXIMATELY 0.40 W/M²K — BETTER THAN UNINSULATED BUT SHORT OF 0.30 W/M²K TARGET — AEROGEL AT 50MM (+ 12.5MM BOARD = 62.5MM) APPROACHES 0.25 W/M²K.

CONSEQUENTIAL IMPROVEMENTS RULE: UNDER PART L 2021, WHEN INSULATING MORE THAN 25% OF THE THERMAL ELEMENT (EXTERNAL WALL) OF AN EXISTING DWELLING, THE ELEMENT MUST BE UPGRADED TO THE PART L MINIMUM UNLESS: THE IMPROVEMENT WOULD NOT ACHIEVE A 15-YEAR SIMPLE PAYBACK PERIOD (ENERGY COST SAVINGS RELATIVE TO INSTALLATION COST). THE IMPROVEMENT WOULD NOT ACHIEVE THE TARGET U-VALUE DUE TO TECHNICAL OR COST CONSTRAINTS (E.G. A LISTED BUILDING OR A ROOM WHERE IWI WOULD MAKE THE ROOM TOO SMALL TO BE HABITABLE). IN THESE CASES, THE MAXIMUM TECHNICALLY AND ECONOMICALLY FEASIBLE IMPROVEMENT SHOULD BE INSTALLED.

INTERNAL WALL INSULATION COSTS IN LONDON ARE LOWER THAN EWI COSTS PER M² OF WALL AREA — BUT THE INTERNAL DISRUPTION (ROOM CLEARANCE, SERVICES RELOCATION, SKIRTING AND ARCHITRAVE REPLACEMENT, REPLASTERING, REDECORATION) OFTEN MAKES THE FULL PROJECT COST COMPARABLE TO EWI FOR THE SAME WALL AREA.

IWI MATERIAL COSTS (SUPPLY ONLY PER M²): AEROGEL (CABOT THERMAL WRAP 10 OR SPACETHERM A2 — 10-25MM): £40 TO £90 PER M² SUPPLY. KOOLTHERM K18 PIR BOARD (25MM + PLASTERBOARD INCLUDED): £15 TO £25 PER M² SUPPLY. HEMP-LIME OR WOOD FIBRE BOARD (60-80MM): £25 TO £50 PER M² SUPPLY.

IWI INSTALLATION LABOUR (PER M² OF WALL AREA): PIR BOARD DOT-AND-DAB INSTALLATION: £15 TO £30 PER M². AEROGEL COMPOSITE SYSTEM (FIXING + BOARDING): £25 TO £50 PER M².

TOTAL IWI SUPPLY AND INSTALL (MATERIAL + LABOUR — PER M²): KOOLTHERM K18 25MM: £30 TO £55 PER M². AEROGEL SLIM-PROFILE 10-25MM: £65 TO £140 PER M². HEMP-LIME 60MM: £50 TO £90 PER M².

ADDITIONAL COSTS PER ROOM (LONDON VICTORIAN TERRACE FRONT RECEPTION — APPROXIMATELY 16 M² OF EXTERNAL WALL): SERVICES RELOCATION (ELECTRICAL — 3 SOCKETS AND 1 SWITCH): £300 TO £600. SKIRTING BOARD REPLACEMENT (16M PERIMETER): £200 TO £500. DECORATION (PAINT — WALLS AFTER IWI INSTALLATION): £300 TO £600. TOTAL ADDITIONAL COSTS PER ROOM: £800 TO £1,700.

TYPICAL LONDON TERRACE FRONT ROOM IWI (16 M² WALL, KOOLTHERM K18 25MM): SUPPLY AND INSTALL: £480 TO £880. PLUS ADDITIONAL WORKS: £800 TO £1,700. TOTAL ROOM IWI PROJECT: £1,280 TO £2,580.