Timber Frame Extension for London Victorian Homes: SIPS, Open-Panel, and Closed-Panel Build Systems

THE PRIMARY REASONS LONDON HOMEOWNERS AND THEIR ARCHITECTS CHOOSE TIMBER FRAME OVER TRADITIONAL MASONRY FOR EXTENSIONS ARE AS FOLLOWS.

FASTER BUILD PROGRAMME: A TIMBER FRAME REAR EXTENSION CAN BE FABRICATED OFF-SITE AND ERECTED ON SITE IN 1 TO 3 DAYS, VERSUS 3 TO 6 WEEKS OF BRICKLAYING FOR THE EQUIVALENT MASONRY STRUCTURE. THE OFF-SITE FABRICATION PERIOD (TYPICALLY 3 TO 6 WEEKS FROM ORDER PLACEMENT) CAN OFTEN RUN IN PARALLEL WITH GROUNDWORKS AND FOUNDATION CONSTRUCTION, SIGNIFICANTLY COMPRESSING THE OVERALL PROGRAMME.

BETTER INSULATION PERFORMANCE: A FULL-FILL TIMBER FRAME WALL WITH 140MM STUDS PACKED WITH ROCKWOOL FLEXI MINERAL WOOL INSULATION ACHIEVES A U-VALUE OF APPROXIMATELY 0.20 TO 0.25 W/M'K — SUPERIOR TO A STANDARD MASONRY CAVITY WALL AT 0.25 TO 0.30 W/M'K. FOR VERY LOW U-VALUES (0.15 W/M'K OR BELOW, APPROACHING PASSIVHAUS STANDARD), TIMBER FRAME IS THE PREFERRED STRUCTURAL SYSTEM AS IT AVOIDS THE THERMAL BRIDGING OF CONTINUOUS MORTAR BEDS AND WALL TIES IN MASONRY CAVITY CONSTRUCTION.

LIGHTER WEIGHT: A TIMBER FRAME WALL IS APPROXIMATELY 40 TO 60% LIGHTER THAN AN EQUIVALENT MASONRY WALL. ON LONDON CLAY SITES WHERE GROUND CONDITIONS ARE CHALLENGING OR WHERE THE EXISTING REAR WALL CANNOT SUPPORT HEAVY MASONRY LOADS, A LIGHTER TIMBER FRAME STRUCTURE CAN REDUCE FOUNDATION REQUIREMENTS AND COST.

PRECISION: FACTORY-FABRICATED TIMBER FRAME PANELS HAVE DIMENSIONAL TOLERANCES ACHIEVABLE IN A CONTROLLED FACTORY ENVIRONMENT — TYPICALLY +/- 1.5MM ON PANEL DIMENSIONS. THIS PRECISION IS VALUABLE FOR LARGE-FORMAT GLAZING SYSTEMS (BIFOLD OR SLIDING DOORS, ROOFLIGHTS) WHERE ACCURATE STRUCTURAL OPENINGS ARE CRITICAL.

THE OPEN-PANEL SYSTEM IS THE MOST WIDELY USED TIMBER FRAME METHOD FOR LONDON RESIDENTIAL EXTENSIONS. IN AN OPEN-PANEL SYSTEM, THE FACTORY-FABRICATED PANELS ARE DELIVERED TO SITE BEFORE BEING INSULATED — THE INSULATION IS INSTALLED ON SITE AFTER PANEL ERECTION.

COMPONENTS OF AN OPEN-PANEL SYSTEM: TIMBER STUDWORK FRAME — TYPICALLY 89MM X 38MM OR 140MM X 38MM C16 OR C24 STRUCTURAL GRADE WHITEWOOD STUDS AT 400MM OR 600MM CENTRES (400MM CENTRES FOR LOAD-BEARING WALLS; 600MM CENTRES FOR NON-LOAD-BEARING PARTITIONS), WITH NOGGINGS AND HERRINGBONE STRUTTING ADDED AS REQUIRED BY STRUCTURAL ENGINEER. SHEATHING BOARD — 9MM ORIENTED STRAND BOARD (OSB3, MOISTURE-RESISTANT GRADE — NORBORD SMARTPLY OSB3, KRONOSPAN) OR 12MM PLYWOOD (FOR HIGHER RACKING RESISTANCE IN AREAS OF HIGH WIND OR SEISMIC LOAD) FIXED TO THE EXTERNAL FACE OF THE STUDS. THE SHEATHING PROVIDES RACKING RESISTANCE AND A SUBSTRATE FOR THE BREATHER MEMBRANE. BREATHER MEMBRANE — INSTALLED OVER THE SHEATHING BOARD, TAPED AT ALL JOINTS AND LAPS. DUPONT TYVEK HOUSEWRAP, WRAPTITE SA (SELF-ADHESIVE MEMBRANE FROM A. PROCTOR GROUP), OR SOLITEX PLUS FROM PRO CLIMA ARE THE LEADING PRODUCTS. THE BREATHER MEMBRANE IS VAPOUR-PERMEABLE (ALLOWS WATER VAPOUR OUT) BUT WATERPROOF (EXCLUDES LIQUID WATER).

INSULATION (INSTALLED ON SITE AFTER PANEL ERECTION): THE STUD CAVITY IS FILLED WITH ROCKWOOL FLEXI 034 (34 W/MK LAMBDA) CUT-TO-FIT BATTS OR KNAUF EARTHWOOL RAFTER ROLL DUO (FOR RAFTER APPLICATIONS). FOR 140MM STUDS FULL-FILL, U-VALUE APPROXIMATELY 0.22 W/M'K. FOR ENHANCED PERFORMANCE, AN ADDITIONAL 50MM RIGID PIR BOARD (CELOTEX GA4000 OR KINGSPAN KOOLTHERM K12) CAN BE FIXED TO THE INTERNAL FACE OF THE STUDS OVER A VAPOUR CONTROL LAYER, ACHIEVING A U-VALUE BELOW 0.15 W/M'K.

CLOSED-PANEL TIMBER FRAME IS A MORE ADVANCED SYSTEM WHERE THE WALL PANELS ARRIVE FROM THE FACTORY FULLY INSULATED AND OFTEN WITH INTERNAL SERVICES CHANNELS AND VAPOUR CONTROL LAYERS ALREADY INSTALLED.

COMPONENTS OF A CLOSED-PANEL SYSTEM: PANEL INCLUDES: STRUCTURAL STUD FRAME + SHEATHING BOARD + BREATHER MEMBRANE + FULL-FILL INSULATION + VAPOUR CONTROL LAYER (PROCLIMA INTELLO PLUS, WRITEFLEX, OR EQUIVALENT INTELLIGENT VAPOUR CONTROL LAYER) + PLASTERBOARD (SOMETIMES INSTALLED AT FACTORY ON THE INTERNAL FACE). THE INTELLIGENCE IN THE VAPOUR CONTROL LAYER IS A FEATURE OF MODERN CLOSED-PANEL SYSTEMS — PRO CLIMA INTELLO PLUS IS AN INTELLIGENT VCL THAT HAS VARIABLE VAPOUR RESISTANCE (HIGH RESISTANCE IN WINTER WHEN THE WALL IS COLD AND CONDENSATION RISK IS HIGH; LOWER RESISTANCE IN SUMMER TO ALLOW DRYING). THIS REDUCES THE RISK OF INTERSTITIAL CONDENSATION IN THE WALL STRUCTURE.

AIRTIGHTNESS: THE MOST SIGNIFICANT ADVANTAGE OF CLOSED-PANEL SYSTEMS FOR ENERGY PERFORMANCE IS AIRTIGHTNESS. MODERN CLOSED-PANEL SYSTEMS FROM MANUFACTURERS SUCH AS BREATHABOARD (UK), SCOTFRAME (SCOTLAND), AND FRAME TECHNOLOGIES (IRELAND) CAN ACHIEVE AIR PERMEABILITY RATES OF BELOW 1.0 M'/HR/M' AT 50 PASCALS — NECESSARY FOR NEARLY ZERO ENERGY BUILDING (NZEB) STANDARD AND APPROACHING PASSIVHAUS. THIS COMPARED TO A STANDARD MASONRY EXTENSION WHICH TYPICALLY ACHIEVES 8 TO 15 M'/(HR.M').

COMPATIBILITY WITH LONDON VICTORIAN PROPERTIES: THE INTERFACE BETWEEN A NEW HIGH-PERFORMANCE CLOSED-PANEL EXTENSION AND AN EXISTING Victorian MASONRY HOUSE IS THE CRITICAL JUNCTION — THE EXISTING HOUSE HAS VERY POOR AIRTIGHTNESS, AND NO MATTER HOW GOOD THE NEW EXTENSION IS, THE JUNCTION BETWEEN NEW AND OLD IS A MAJOR LEAKAGE POINT. CAREFUL DETAILING OF THE AIRTIGHTNESS MEMBRANE WRAPPING AROUND THE JUNCTION BETWEEN THE NEW EXTENSION FRAME AND THE EXISTING MASONRY IS REQUIRED.

STRUCTURAL INSULATED PANELS (SIPS) COMBINE THE STRUCTURAL AND INSULATION FUNCTIONS IN A SINGLE FACTORY-MANUFACTURED PANEL. A SIP CONSISTS OF A RIGID FOAM INSULATION CORE (TYPICALLY EPS — EXPANDED POLYSTYRENE — OR POLYISOCYANURATE PIR) SANDWICHED BETWEEN TWO STRUCTURAL FACING SHEETS OF OSB3 (TYPICALLY 12MM OSB3 ON EACH FACE).

KINGSPAN TEK (PIR SIP SYSTEM): KINGSPAN TEK IS THE LEADING BRANDED SIP SYSTEM IN THE UK RESIDENTIAL MARKET. THE CORE INSULATION IS KINGSPAN KOOLTHERM (POLYISOCYANURATE PIR FOAM — LAMBDA 0.022 W/MK), ACHIEVING SIGNIFICANTLY HIGHER INSULATION PERFORMANCE PER MILLIMETRE THAN EPS-CORE SIP SYSTEMS. KINGSPAN TEK PANELS ARE AVAILABLE IN THICKNESSES FROM 100MM TO 200MM TOTAL. A 142MM KINGSPAN TEK PANEL (12MM OSB3 + 118MM PIR + 12MM OSB3) ACHIEVES A U-VALUE OF APPROXIMATELY 0.17 W/M'K — WITHOUT ANY ADDITIONAL INSULATION LAYERS.

EPS-CORE SIPS (SIGMA SIPS, INSULTHERM, SIPS ECO): EPS-CORE SIPS ARE MANUFACTURED BY SIGMA SIPS (YORK), INSULTHERM, AND OTHERS. EPS HAS A LOWER THERMAL PERFORMANCE PER MILLIMETRE (LAMBDA APPROXIMATELY 0.037 W/MK) THAN PIR, SO EPS SIP PANELS MUST BE THICKER TO ACHIEVE THE SAME U-VALUE. A 200MM EPS SIP (12MM OSB3 + 176MM EPS + 12MM OSB3) ACHIEVES APPROXIMATELY 0.19 W/M'K. EPS-CORE SIPS ARE LOWER COST THAN KINGSPAN TEK BUT REQUIRE GREATER PANEL THICKNESS FOR EQUIVALENT PERFORMANCE.

SIPS FOR ROOFLIGHTS AND ROOFS: SIPS ARE PARTICULARLY WELL-SUITED FOR FLAT ROOF CONSTRUCTION ON LONDON EXTENSIONS — KINGSPAN TEK ROOF PANELS SPAN BETWEEN STRUCTURAL BEAMS OR TIMBER PORTAL FRAMES, PROVIDING THE COMBINED STRUCTURAL DECK, INSULATION, AND VAPOUR CONTROL IN A SINGLE ELEMENT. THE ROOF COVERING (SARNAFIL G410 OR FIRESTONE EPDM) IS APPLIED DIRECTLY TO THE TOP OSB FACE OVER A SEPARATION LAYER.

A TIMBER FRAME STRUCTURE REQUIRES A WEATHER-RESISTANT EXTERNAL CLADDING SYSTEM — THE TIMBER FRAME ITSELF IS NOT WEATHERPROOF AND MUST BE PROTECTED FROM DIRECT RAIN AND UV EXPOSURE. THE EXTERNAL CLADDING CHOICE ALSO DETERMINES THE AESTHETIC OF THE EXTENSION.

BRICK SLIP CLADDING (MOST COMMON FOR LONDON): WHERE THE EXTENSION MUST MATCH THE EXISTING VICTORIAN BRICK FACADE IN APPEARANCE, BRICK SLIPS (12 TO 20MM THICK REAL BRICK SLIVERS BONDED TO THE EXTERNAL OSB SHEATHING OR TO A CARRIER BOARD) PROVIDE AN AUTHENTIC BRICK APPEARANCE FROM A LIGHTWEIGHT SYSTEM. SUPPLIERS: EUROBRICK K-SYSTEM (EWI-TYPE ADHESIVE BRICK SLIP SYSTEM), NIDA BRICK (PANEL SYSTEM — PRE-LAID SLIPS ON GRP PANEL CARRIER, FIXED TO SUBFRAME), AND TRADITIONAL HAND-LAID BRICK SLIPS ON ADHESIVE. BRICK SLIPS ALLOW THE EXTENSION TO READ VISUALLY AS BRICK-BUILT WHILE BENEFITING FROM THE THERMAL PERFORMANCE AND SPEED OF TIMBER FRAME.

RAINSCREEN CLADDING (CONTEMPORARY FINISH): FOR CONTEMPORARY-STYLE EXTENSIONS, OPEN-JOINTED RAINSCREEN CLADDING IS INSTALLED OVER THE BREATHER MEMBRANE ON A TIMBER BATTEN SUBFRAME, CREATING A 25MM VENTILATED CAVITY BETWEEN THE CLADDING AND THE STRUCTURAL PANEL. CLADDING MATERIALS AS USED WITH MASONRY: CEDRAL CLICK FIBRE CEMENT, ACCOYA HARDWOOD, WESTERN RED CEDAR, KEBONY. THE SAME FIRE SAFETY RULES APPLY FOR TIMBER FRAME AS FOR MASONRY EXTENSIONS — PROPERTIES BELOW 11M ALLOW COMBUSTIBLE RAINSCREEN WITH CAVITY FIRE BARRIERS; ABOVE 11M, CLASS A2 NON-COMBUSTIBLE CLADDING ONLY.

RENDER ON TIMBER FRAME: K-REND SILICONE TC15 AND WEBER SILICONE ONE-COAT RENDERS CAN BE APPLIED TO TIMBER FRAME STRUCTURES USING A CARRIER BOARD SUBSTRATE (CEMBRIT MASONITE, OR KNAUF AQUAPANEL OUTDOOR) FIXED OVER THE BREATHER MEMBRANE ON A MECHANICAL FIXING SYSTEM WITH A VENTILATED CAVITY — NOT DIRECTLY APPLIED TO OSB SHEATHING, WHICH WOULD BRIDGE THE CAVITY AND RISK MOISTURE TRAPPING.

THE COMPARATIVE COST OF TIMBER FRAME VERSUS MASONRY FOR A LONDON REAR EXTENSION DEPENDS ON SPECIFICATION LEVEL AND FINISH.

TIMBER FRAME STRUCTURE SUPPLY AND ERECT (OPEN-PANEL SYSTEM, WALLS AND FLAT ROOF STRUCTURE, 4M X 4M EXTENSION): £8,000 TO £15,000 FOR STRUCTURE AND ERECTION ONLY — THIS IS THE FRAME ONLY, EXCLUDING FOUNDATIONS, EXTERNAL CLADDING, INTERNAL FIT-OUT, GLAZING, UTILITIES.

KINGSPAN TEK SIPS STRUCTURE SUPPLY AND ERECT (4M X 4M EXTENSION): £12,000 TO £20,000 FOR PANELS AND ERECTION ONLY.

MASONRY EQUIVALENT STRUCTURE (4M X 4M EXTENSION, BLOCKWORK INTERNAL FACE + BRICK OUTER LEAF + CAVITY INSULATION): £9,000 TO £16,000 IN MATERIALS AND LABOUR FOR THE STRUCTURAL SHELL ONLY.

ON A LIKE-FOR-LIKE STRUCTURE COST BASIS, TIMBER FRAME AND MASONRY ARE BROADLY COMPARABLE — THE ADVANTAGES OF TIMBER FRAME ARE IN PROGRAMME SPEED AND THERMAL PERFORMANCE RATHER THAN COST SAVINGS. FOR COMPLEX DETAILS (JUNCTIONS, AIRTIGHTNESS MEMBRANES, SPECIALIST CAVITY BARRIERS) TIMBER FRAME MAY BE MORE EXPENSIVE IN DETAILING COSTS.

TOTAL PROJECT COST FOR A COMPLETED 4M X 4M SINGLE-STOREY REAR EXTENSION WITH FLAT ROOF AND BIFOLD DOORS (EITHER MASONRY OR TIMBER FRAME): £60,000 TO £90,000 — THE STRUCTURAL SYSTEM IS A RELATIVELY SMALL COMPONENT OF THE TOTAL COST; GLAZING, KITCHEN FIT-OUT, AND FINISHING DOMINATE.