Warm Roof Insulation for London Victorian Terraces: Rafter Insulation for Loft Conversions

IN A LONDON VICTORIAN TERRACE LOFT CONVERSION, THE WARM ROOF INSULATION IS PLACED AT RAFTER LEVEL. THE ORIGINAL VICTORIAN ROOF RAFTERS ARE TYPICALLY 100MM × 50MM OR 75MM × 50MM — MUCH TOO SHALLOW TO ACHIEVE THE TARGET U-VALUE WITH INSULATION BETWEEN THE RAFTERS ALONE. THE SOLUTION IS A TWO-LAYER SYSTEM:

LAYER 1 — BETWEEN THE RAFTERS: - FULL-FILL RIGID PIR (POLYISOCYANURATE) BOARD BETWEEN THE RAFTERS — FRICTION-FIT WITH TIGHTLY CUT BOARDS TO ELIMINATE GAPS (KINGSPAN KOOLTHERM K107 100MM, RECTICEL EUROTHANE GP 100MM). WITH 100MM RAFTERS AND FULL-FILL PIR BETWEEN THE RAFTERS, THE CONTRIBUTION FROM THE BETWEEN-RAFTER LAYER IS APPROXIMATELY 0.022-0.025 W/MK (PIR LAMBDA VALUE) FOR 90-100MM EFFECTIVE DEPTH (ALLOWING FOR RAFTER WIDTH AND BRIDGING LOSSES). - ALTERNATIVELY: SEMI-RIGID MINERAL WOOL BETWEEN THE RAFTERS (ROCKWOOL FLEXI BATT, KNAUF EARTHWOOL RAFTER ROLL) — LESS EFFICIENT THERMALLY (ΛVALUE ~ 0.040 W/MK) BUT VAPOUR-OPEN AND MORE COMPATIBLE WITH VENTILATED RAFTER CONSTRUCTION WHERE A VENTILATION CHANNEL IS MAINTAINED ABOVE THE INSULATION (SEE BELOW).

LAYER 2 — BELOW THE RAFTERS (COUNTER-BATTEN LAYER): RIGID PIR BOARDS FIXED BELOW THE RAFTERS ON COUNTER-BATTENS (WHICH CREATE A CLEAR CAVITY FOR SERVICES — ELECTRICAL CABLING, SURFACE-MOUNTED LIGHTING): - KINGSPAN KOOLTHERM K18 INSULATED PLASTERBOARD (A COMPOSITE BOARD — PIR INSULATION BONDED TO PLASTERBOARD — AVAILABLE IN 62.5MM, 72.5MM, 82.5MM OVERALL THICKNESS). THE PLASTERBOARD FACE IS THE FINISHED CEILING — TAPED AND SKIMMED DIRECTLY. - SEPARATE PIR BOARD (60-80MM) + COUNTER-BATTEN + 12.5MM PLASTERBOARD: MORE FLEXIBLE (SERVICES CAN BE RUN IN THE COUNTER-BATTEN CAVITY) BUT MORE COMPONENTS. - FOIL-FACED PIR BELOW RAFTERS (KINGSPAN THERMAROOF TR27 LPC/FM, RECTICEL EUROWALL+): THE FOIL FACE REFLECTS RADIANT HEAT AND ACTS AS A VAPOUR CONTROL LAYER — BUT ALL JOINTS AND PENETRATIONS MUST BE SEALED WITH FOIL TAPE FOR THE VCL TO BE EFFECTIVE.

TYPICAL WARM ROOF BUILD-UP FOR A LONDON VICTORIAN TERRACE LOFT CONVERSION: (FROM OUTSIDE TO INSIDE — WORKING DOWN THROUGH THE ROOF CONSTRUCTION) 1. EXISTING CLAY PLAIN TILES (OR CONCRETE TILES, SLATES) ON TILE BATTENS 2. BREATHABLE SARKING MEMBRANE (VAPOUR-PERMEABLE UNDERLAY — ALLOWS MOISTURE TO ESCAPE FROM THE CONSTRUCTION OUTWARD WHILE RESISTING LIQUID WATER INGRESS FROM OUTSIDE): KLOBER PERMO AIR, GLIDEVALE PROTECT TF200 THERMO. THE BREATHABLE MEMBRANE IS FIXED TO THE TOP OF THE RAFTERS AND LAPS OVER THE FASCIA INTO THE GUTTER. 3. BETWEEN RAFTERS: 100MM FULL-FILL PIR (KINGSPAN K7 PITCHED ROOF BOARD OR RECTICEL EUROTHANE GP) — CUT TO FRICTION-FIT TIGHTLY IN THE RAFTER BAY. 4. BELOW RAFTERS: COUNTER-BATTENS (38MM × 38MM TREATED TIMBER COUNTER-BATTEN — FIXED THROUGH THE PIR INTO THE RAFTERS BELOW; SERVICES CAVITY). 5. BELOW COUNTER-BATTENS: 62.5MM OR 72.5MM KINGSPAN KOOLTHERM K18 INSULATED PLASTERBOARD (COMPOSITE PIR + PLASTERBOARD) — JOINTS STAGGERED AND ALL JOINTS TAPED. 6. SKIM COAT ON PLASTERBOARD FACE.

ACHIEVED U-VALUE FOR THIS BUILD-UP: - BETWEEN RAFTERS: 100MM PIR (KOOLTHERM K7 — Λ = 0.022 W/MK) - BELOW RAFTERS: 52MM PIR WITHIN K18 INSULATED PLASTERBOARD (Λ = 0.021 W/MK) - TOTAL THERMAL RESISTANCE (ACCOUNTING FOR RAFTER BRIDGING AND COUNTER-BATTEN BRIDGING): APPROXIMATELY 0.14-0.16 W/M²K — CLOSE TO THE PART L (2022) TARGET OF 0.11-0.15 W/M²K FOR A PITCHED ROOF IN A MATERIAL CHANGE OF USE (LOFT CONVERSION).

VENTILATION — WARM ROOF CONSTRUCTION IS UNVENTILATED: IN A WARM ROOF, THE INSULATION IS CONTINUOUS FROM THE TOP SURFACE OF THE RAFTERS (OR FROM THE BREATHABLE MEMBRANE ON TOP OF THE RAFTERS) TO THE FINISHED CEILING — THERE IS NO VENTILATION CAVITY WITHIN THE INSULATION BUILD-UP. THE BREATHABLE (VAPOUR-PERMEABLE) MEMBRANE ABOVE THE RAFTERS ALLOWS ANY MOISTURE THAT DIFFUSES OUTWARD THROUGH THE INSULATION TO PASS THROUGH THE MEMBRANE AND DISPERSE INTO THE SPACE BETWEEN THE MEMBRANE AND THE TILE BATTENS (WHICH IS VENTILATED BY THE NATURAL AIR MOVEMENT BELOW THE TILES). THIS IS THE TECHNICALLY CORRECT AND CURRENT BEST-PRACTICE APPROACH FOR A WARM ROOF IN A LOFT CONVERSION — IT AVOIDS THE DIFFICULTIES OF MAINTAINING A 50MM CLEAR VENTILATION CHANNEL ABOVE THE INSULATION IN A VENTILATED RAFTER CONSTRUCTION.

SPRAY FOAM INSULATION IN LOFT CONVERSIONS — THE MORTGAGE PROBLEM: SOME LONDON VICTORIAN TERRACE LOFT CONVERSIONS (AND SOME OLDER PROPERTIES WHERE THE COLD ROOF HAS BEEN UPGRADED BY A PREVIOUS OWNER) HAVE SPRAY POLYURETHANE FOAM (SPF) APPLIED BETWEEN THE RAFTERS. WHILE SPF (OPEN-CELL OR CLOSED-CELL) CAN ACHIEVE GOOD THERMAL PERFORMANCE, IT CREATES A SERIOUS PROBLEM WITH MORTGAGE LENDERS: - RICS (ROYAL INSTITUTION OF CHARTERED SURVEYORS) AND MOST UK HIGH-STREET MORTGAGE LENDERS (NATIONWIDE, BARCLAYS, HALIFAX, SANTANDER) WILL NOT LEND ON PROPERTIES WHERE SPRAY FOAM HAS BEEN APPLIED TO THE ROOF TIMBERS — BECAUSE THE FOAM BONDS TO THE TIMBERS, MAKING IT IMPOSSIBLE TO INSPECT THE TIMBER CONDITION WITHOUT REMOVING THE FOAM, AND BECAUSE REMOVAL IS EXPENSIVE (£5,000-15,000+). - RESALE IMPACT: A PROPERTY WITH SPRAY FOAM IN THE ROOF STRUCTURE MAY BE UNMORTGAGEABLE FOR A BUYER USING A MAINSTREAM LENDER — SEVERELY AFFECTING RESALE VALUE AND MARKETABILITY. - REMOVAL COST: SPRAY FOAM REMOVAL SPECIALISTS (VIMOR WINDOW VINYLS, NATIONAL FOAM REMOVAL): £2,500-8,000+ FOR A TYPICAL LOFT, DEPENDING ON ACCESSIBILITY AND FOAM TYPE. - RECOMMENDATION: DO NOT USE SPRAY FOAM AS THE PRIMARY WARM ROOF INSULATION SYSTEM IN A LONDON VICTORIAN TERRACE. USE RIGID PIR BOARDS (BETWEEN AND BELOW RAFTERS) OR RIGID MINERAL WOOL BOARDS INSTEAD.

STRUCTURAL ISSUES IN VICTORIAN TERRACE RAFTER LOFT CONVERSIONS: BEFORE APPLYING WARM ROOF INSULATION AS PART OF A LOFT CONVERSION, THE EXISTING ROOF STRUCTURE MUST BE ASSESSED BY A STRUCTURAL ENGINEER: - RAFTER ADEQUACY: ORIGINAL VICTORIAN RAFTERS (100MM × 50MM) ARE TYPICALLY NOT ADEQUATE AS THE PRIMARY FLOOR STRUCTURE FOR A HABITABLE LOFT ROOM — NEW STRUCTURAL FLOOR BEAMS (TYPICALLY 200MM × 50MM C24 KD TIMBER JOISTS OR LVL BEAMS) MUST BE INSTALLED BEFORE THE WARM ROOF BUILD-UP IS APPLIED. - RAFTER STRENGTHENING: THE RAFTERS THEMSELVES MAY NEED DOUBLING UP (SISTER RAFTERS FIXED ALONGSIDE THE EXISTING RAFTERS) TO CARRY THE INCREASED WIND AND SNOW LOADS ON THE ENLARGED HABITABLE ROOF AREA. - PARTY WALL: THE ROOF STRUCTURE WILL BEAR ON THE PARTY WALL — PARTY WALL ACT AWARD MAY BE REQUIRED BEFORE STRUCTURAL WORKS BEGIN.

COSTS (SUPPLY AND FIT, LONDON 2025-2026): WARM ROOF INSULATION (100MM PIR BETWEEN RAFTERS + K18 INSULATED PLASTERBOARD BELOW — PER M² OF ROOF AREA, SUPPLY AND FIT): £80-150 WARM ROOF INSULATION (FULL LOFT CONVERSION ROOF — APPROXIMATELY 30-40M² OF ROOF AREA): £3,000-7,000 SPRAY FOAM REMOVAL (PER M² OF ROOF AREA — SPECIALIST): £25-60/M² BREATHABLE SARKING MEMBRANE (RE-SARKING BELOW TILES — PER M²): £15-35