Thermal Bridging in London Victorian Terraces: Identifying Cold Spots and How to Fix Them

WHAT IS A THERMAL BRIDGE? A THERMAL BRIDGE IS A PART OF THE BUILDING ENVELOPE WHERE HEAT FLOW IS SIGNIFICANTLY HIGHER THAN THROUGH THE SURROUNDING AREA. IN A LONDON VICTORIAN TERRACE, THE \'BACKGROUND\' WALL (SOLID BRICK OR CAVITY BRICK — PRE-1920S CONSTRUCTION IS ALMOST ALWAYS SOLID) HAS A U-VALUE OF APPROXIMATELY 2.0-2.5 W/M²K. A THERMAL BRIDGE CREATES A ZONE WHERE THE LOCAL U-VALUE IS MUCH HIGHER — IN SOME CASES 5-10 W/M²K — AND WHERE THE INTERNAL SURFACE TEMPERATURE IN WINTER DROPS BELOW THE DEW POINT OF THE INDOOR AIR (TYPICALLY BELOW 12-14°C FOR AN INDOOR RELATIVE HUMIDITY OF 60%), TRIGGERING CONDENSATION AND MOULD.

TYPES OF THERMAL BRIDGE: 1. GEOMETRIC THERMAL BRIDGES (ALSO CALLED \'LINEAR THERMAL BRIDGES\'): THESE OCCUR AT JUNCTIONS BETWEEN ELEMENTS WHERE THE EXTERNAL SURFACE AREA IS LARGER THAN THE INTERNAL SURFACE AREA — HEAT \'FANS OUT\' TOWARD THE OUTSIDE: - EXTERNAL CORNERS (E.G., THE CORNER WHERE TWO EXTERNAL WALLS MEET): THE EXTERNAL CORNER HAS A LARGER EXTERNAL THAN INTERNAL SURFACE. IN A SOLID BRICK VICTORIAN TERRACE, AN EXTERNAL CORNER CAN HAVE AN INTERNAL SURFACE TEMPERATURE 3-5°C LOWER THAN THE ADJACENT FLAT WALL IN WINTER — HENCE THE CLASSIC MOULD PATTERN IN THE CORNER OF EXTERNAL WALLS. - WINDOW REVEALS (THE RETURN OF THE EXTERNAL WALL AROUND THE WINDOW FRAME): THE WINDOW REVEAL IS A THIN SECTION OF MASONRY EXPOSED ON BOTH THE EXTERNAL AND THE INTERNAL FACE. IN A SOLID BRICK TERRACE (225MM BRICK WALL), THE REVEAL IS ONLY 75-100MM DEEP — A VERY SHORT HEAT PATH FROM OUTSIDE TO INSIDE. THE REVEAL SURFACE TEMPERATURE CAN BE 6-8°C LOWER THAN THE ADJACENT WALL. - WALL/FLOOR JUNCTIONS: WHERE A SUSPENDED TIMBER FLOOR MEETS AN EXTERNAL WALL (THE FRONT GROUND-FLOOR EXTERNAL WALL IN A VICTORIAN TERRACE, WHERE THE FLOOR JOISTS RUN FROM THE FRONT TO THE REAR AND THE JOIST ENDS ARE BUILT INTO THE EXTERNAL MASONRY), THE FLOOR JOIST END BRIDGES THE WALL INSULATION. ALSO AT THE PARTY WALL/EXTERNAL WALL JUNCTION.

2. REPEATING THERMAL BRIDGES (IN THE FABRIC OF INSULATED ELEMENTS): - WALL TIES IN CAVITY WALL CONSTRUCTION (NOT TYPICAL IN SOLID-WALL VICTORIAN TERRACES, BUT RELEVANT IN EARLY EDWARDIAN CAVITY WALLS): METAL WALL TIES CONDUCT HEAT DIRECTLY FROM THE INNER TO THE OUTER LEAF ACROSS THE CAVITY. - TIMBER STUDS IN INTERNAL WALL INSULATION (IWI) SYSTEMS: WHERE INTERNAL WALL INSULATION IS INSTALLED ON A TIMBER BATTEN FRAME AGAINST THE EXTERNAL WALL, THE BATTENS THEMSELVES CONDUCT HEAT AND BYPASS THE INSULATION. A SYSTEM WITH 50MM TIMBER BATTENS AND 50MM MINERAL WOOL BETWEEN THE BATTENS HAS A COMBINED U-VALUE APPROXIMATELY 25-30% WORSE THAN A CONTINUOUS RIGID INSULATION SYSTEM OF THE SAME THICKNESS. - JOIST HANGERS, RESTRAINT STRAPS, LINTELS: ANY METAL ELEMENT THAT PENETRATES FROM INSIDE TO OUTSIDE OF AN INSULATION LAYER.

3. REPEATING BRIDGES SPECIFIC TO LONDON VICTORIAN TERRACES: - CHIMNEY BREAST: THE CHIMNEY BREAST PROJECTS INTO THE ROOM FROM THE EXTERNAL OR PARTY WALL — IT CONDUCTS HEAT FROM THE INTERIOR OF THE FLUE (OPEN OR CLOSED) TO OUTSIDE. WHERE THE FLUE IS OPEN, THE HEAT LOSS IS PREDOMINANTLY CONVECTIVE (AIR MOVEMENT UP THE FLUE) RATHER THAN CONDUCTIVE. - TIMBER LINTELS OVER WINDOW AND DOOR OPENINGS: TIMBER LINTELS (STILL PRESENT IN SOME VICTORIAN CONSTRUCTION — COMBINED WITH A RELIEVING ARCH) BRIDGE THE WALL INSULATION ABOVE WINDOWS AND CONDUCT HEAT OUT THROUGH THE LINTEL. - SUSPENDED TIMBER FLOOR COLD BRIDGE AT PERIMETER: WHERE THE SUSPENDED GROUND FLOOR MEETS THE EXTERNAL WALL, THERE IS A THERMAL BRIDGE AT THE FLOOR PERIMETER. THIS IS ALSO A COMMON SITE FOR PENETRATING DAMP AND MOULD AT THE FLOOR/WALL JUNCTION.

QUANTIFYING THERMAL BRIDGES — PSI (Ψ) VALUES: IN UK BUILDING ENERGY CALCULATIONS (SAP — STANDARD ASSESSMENT PROCEDURE), THERMAL BRIDGES ARE QUANTIFIED AS LINEAR THERMAL TRANSMITTANCE VALUES (PSI — Ψ — W/MK): - THE Ψ VALUE REPRESENTS THE ADDITIONAL HEAT LOSS PER METRE LENGTH OF JUNCTION PER DEGREE TEMPERATURE DIFFERENCE BETWEEN INSIDE AND OUTSIDE. - ACCREDITED DETAILS (UK): THE BUILDING REGULATIONS APPROVED DOCUMENT L (CONSERVATION OF FUEL AND POWER) ALLOWS DESIGNERS TO USE GENERIC Ψ VALUES FROM THE ACCREDITED THERMAL BRIDGE DETAILS (PREVIOUSLY \'THERMAL BRIDGE LIBRARY\' — NOW THE ENERGY EFFICIENCY DESIGN STANDARD DETAILS, AVAILABLE FROM BEIS). A WELL-DETAILED WALL/FLOOR JUNCTION HAS Ψ ≈ 0.08 W/MK; A POORLY DETAILED JUNCTION CAN HAVE Ψ ≈ 0.20-0.40 W/MK OR MORE. - THERMAL BRIDGE FACTOR (Y VALUE — OVERALL): SAP ALSO USES AN OVERALL THERMAL BRIDGING Y-VALUE TO REPRESENT THE EFFECT OF ALL JUNCTIONS IN A BUILDING. A TYPICAL VICTORIAN TERRACE WITH NO THERMAL BRIDGE TREATMENT HAS Y ≈ 0.15 W/M²K (NOTIONAL); A WELL-DETAILED RETROFIT CAN ACHIEVE Y ≈ 0.05 W/M²K.

MOULD RISK CRITERION — TEMPERATURE FACTOR (fRSI): THE MOULD RISK AT A THERMAL BRIDGE IS ASSESSED USING THE TEMPERATURE FACTOR (fRSI — TEMPERATURE RATIO AT THE SURFACE): - fRSI = (TSI - TE) / (TI - TE) WHERE TSI = SURFACE TEMPERATURE, TI = INDOOR AIR TEMPERATURE, TE = OUTDOOR AIR TEMPERATURE. - MINIMUM REQUIREMENT (BS EN ISO 13788): fRSI ≥ 0.75 FOR INTERNAL SURFACES TO AVOID MOULD GROWTH RISK UNDER UK DESIGN CONDITIONS (TI = 20°C, TE = -5°C, INDOOR RH = 65%). - A COLD CORNER IN A VICTORIAN TERRACE MAY HAVE fRSI = 0.55-0.65 — BELOW THE MOULD RISK THRESHOLD AND A CONFIRMED HIGH-RISK MOULD LOCATION.

RETROFIT SOLUTIONS FOR THERMAL BRIDGES IN LONDON VICTORIAN TERRACES: 1. INTERNAL WALL INSULATION (IWI) WITH CONTINUOUS RIGID PIR: - WHERE IWI IS BEING INSTALLED ON THE EXTERNAL WALLS, CONTINUOUS RIGID PIR (RATHER THAN MINERAL WOOL BETWEEN BATTENS) ELIMINATES THE REPEATING STUD BRIDGE. THE PIR BOARDS ARE BONDED DIRECTLY TO THE MASONRY WALL SURFACE (DOT AND DAB OR FULL ADHESIVE BED) — NO BATTENS. - RETURNING THE INSULATION AROUND THE WINDOW REVEALS (THE \'WARM REVEAL\'): INSTALLING IWI AROUND THE WINDOW REVEAL (TYPICALLY 12-25MM PIR ON THE REVEAL FACE) IS THE MOST EFFECTIVE WAY TO ELIMINATE THE WINDOW REVEAL THERMAL BRIDGE. THIS NARROWS THE VISIBLE WINDOW REVEAL BY 12-25MM — WHICH REDUCES NATURAL LIGHT SLIGHTLY BUT DRAMATICALLY REDUCES THE COLD SPOT AND MOULD RISK. - EXTERNAL CORNER RETURN: WRAPPING THE IWI AROUND THE EXTERNAL CORNER (THE INSULATION RETURNS AROUND THE CORNER BY AT LEAST THE WALL THICKNESS) ELIMINATES THE GEOMETRIC CORNER BRIDGE. 2. EXTERNAL WALL INSULATION (EWI) — THE BEST THERMAL BRIDGE SOLUTION: - EXTERNAL INSULATION (RENDER SYSTEM ON MINERAL WOOL OR PIR BOARDS APPLIED TO THE EXTERNAL FACE OF THE WALL) WRAPS ALL JUNCTIONS, CORNERS, REVEALS, AND JOIST ENDS FROM THE OUTSIDE — EFFECTIVELY ELIMINATING ALL THE GEOMETRIC AND REPEATING BRIDGES IN THE WALL. THE INTERNAL SURFACE TEMPERATURE IS RAISED TO NEAR-ROOM TEMPERATURE THROUGHOUT. COST: £100-200/M² SUPPLY AND FIT. - PLANNING NOTE: EXTERNAL WALL INSULATION CHANGES THE EXTERNAL APPEARANCE OF THE BUILDING AND REQUIRES PLANNING PERMISSION IN CONSERVATION AREAS. 3. LOCALISED COLD BRIDGE TREATMENT AT CORNERS AND REVEALS: - FOR A TARGETED INTERVENTION AT THE MOULD-PRONE CORNER ONLY (WITHOUT FULL IWI): A 25-50MM WEDGE OR TAPERED PIR BOARD BONDED INTO THE CORNER AND FINISHED WITH LIME OR GYPSUM SKIM CAN RAISE THE CORNER SURFACE TEMPERATURE ABOVE THE MOULD RISK THRESHOLD. THIS IS LOWER COST THAN FULL IWI (APPROXIMATELY £100-300 PER CORNER) AND ADDRESSES THE IMMEDIATE MOULD RISK.

COSTS (SUPPLY AND FIT, LONDON 2025-2026): CORNER THERMAL BRIDGE TREATMENT (PIR WEDGE AND SKIM — PER CORNER): £100-300 WINDOW REVEAL IWI RETURN (WARM REVEAL — PER WINDOW): £80-200 FULL IWI (CONTINUOUS PIR — PER M²): £80-200 FULL EWI (EXTERNAL RENDER SYSTEM ON MINERAL WOOL — PER M²): £100-200 THERMAL BRIDGE ASSESSMENT AND SAP CALCULATION (SPECIALIST): £300-800