Insulated Plasterboard (PIR Laminate Board) for London Victorian Terrace Walls and Ceilings

THE THREE MOST WIDELY SPECIFIED PIR LAMINATE BOARD PRODUCTS IN UK RESIDENTIAL INTERNAL WALL INSULATION APPLICATIONS ARE CELOTEX PL4000, KNAUF AIRTEC, AND ECOTHERM ECO-VERSAL — ALL ARE PIR FOAM (POLYISOCYANURATE — LAMBDA/THERMAL CONDUCTIVITY APPROXIMATELY 0.022-0.023 W/MK) BONDED TO A 12.5MM PLASTERBOARD FACING.

CELOTEX PL4000 (CELOTEX LTD — ST IVES, CAMBRIDGESHIRE — PART OF THE RECTICEL GROUP — THE MOST WIDELY USED PIR INSULATION BRAND IN UK RESIDENTIAL RENOVATION): AVAILABLE IN PIR THICKNESS: 22.5MM (PL4022 — TOTAL BOARD THICKNESS 35MM), 32.5MM (PL4032 — TOTAL 45MM), 47.5MM (PL4047 — TOTAL 60MM), 62.5MM (PL4062 — TOTAL 75MM). LAMBDA VALUE: 0.022 W/MK. U-VALUE OF BOARD ALONE (NOT INCLUDING WALL): PL4022 (22.5MM PIR): R-VALUE = 22.5/0.022 = 1.02 M²K/W; PL4032 (32.5MM PIR): 1.48 M²K/W; PL4047 (47.5MM PIR): 2.16 M²K/W; PL4062 (62.5MM PIR): 2.84 M²K/W. BOARD SIZE: 1200MM X 2400MM (STANDARD). FACING: 12.5MM TAPERED EDGE PLASTERBOARD (SUITABLE FOR SKIM COAT FINISH OR TAPE AND JOINT). PRICE (SUPPLY ONLY — UK BUILDERS MERCHANT — 2025): PL4022 (35MM TOTAL): APPROXIMATELY £18-25 PER BOARD (2.88M² PER BOARD = £6.25-8.70/M²); PL4047 (60MM TOTAL): APPROXIMATELY £25-35 PER BOARD (£8.70-12.15/M²); PL4062 (75MM TOTAL): APPROXIMATELY £30-42 PER BOARD (£10.42-14.58/M²).

KNAUF AIRTEC (KNAUF UK — SITTINGBOURNE, KENT): VERY SIMILAR SPECIFICATION TO CELOTEX PL4000 — PIR CORE WITH 12.5MM TAPERED EDGE PLASTERBOARD. AVAILABLE IN: 30MM (17.5MM PIR + 12.5MM PB), 37.5MM, 52.5MM, 67.5MM TOTAL THICKNESSES. LAMBDA: 0.022 W/MK. PRICE RANGE: SIMILAR TO CELOTEX PL4000 (WITHIN 5-10%).

ECOTHERM ECO-VERSAL (ECOTHERM INSULATION LTD — KILKENNY, IRELAND — WIDELY AVAILABLE IN UK BUILDERS MERCHANTS): SIMILAR SPECIFICATION TO CELOTEX AND KNAUF — PIR + 12.5MM PLASTERBOARD — AVAILABLE IN THICKNESSES 30-90MM TOTAL. LAMBDA: 0.022 W/MK. PRICE: TYPICALLY 5-15% LOWER THAN CELOTEX PL4000 FOR EQUIVALENT THICKNESS. NOTE: ISOVER MULTICOMFORT WALL BOARD (MINERAL WOOL — LAMBDA 0.033-0.035 W/MK): THE MINERAL WOOL ALTERNATIVE TO PIR LAMINATE BOARD — LOWER FIRE RISK, NO OFF-GASSING IN FIRE, BETTER ACOUSTIC PERFORMANCE — BUT REQUIRES SIGNIFICANTLY GREATER THICKNESS TO ACHIEVE THE SAME R-VALUE (MINERAL WOOL 75MM VS PIR 32MM FOR SIMILAR THERMAL PERFORMANCE) — TAKING MORE FLOOR AREA FROM THE ROOM.

INSULATED PLASTERBOARD (PIR LAMINATE BOARD) CAN BE FIXED TO THE INTERNAL FACE OF A SOLID BRICK WALL USING TWO METHODS: ADHESIVE DABS (DOT AND DAB) OR MECHANICAL FIXING (BATTENS OR DIRECT SCREW THROUGH BOARD TO WALL). THE CHOICE DEPENDS ON THE BOARD THICKNESS AND WEIGHT, THE CONDITION AND FLATNESS OF THE EXISTING WALL, AND THE STRUCTURAL AND FIRE REQUIREMENTS OF THE APPLICATION.

ADHESIVE DAB METHOD (DOT AND DAB — BRITISH GYPSUM'S REGISTERED TERM FOR THEIR DRY-LINING ADHESIVE DAB SYSTEM — THE MOST COMMON FIXING METHOD FOR INSULATED PLASTERBOARD UP TO 60MM TOTAL THICKNESS ON SOLID BRICK WALLS): THE BOARD IS FIXED TO THE WALL USING DABS OF BRITISH GYPSUM BONDING COMPOUND (GYPROC DRYWALL ADHESIVE — OR BG LITE ADHESIVE FOR LIGHTER BOARDS) — APPLIED IN COLUMNS OF DABS (THREE DABS PER COLUMN — ONE COLUMN AT EACH BOARD EDGE AND ONE IN THE CENTRE — TOTAL 9 DABS PER 1200X2400MM BOARD). THE BOARD IS PRESSED AGAINST THE ADHESIVE DABS AND PLUMBED VERTICALLY — SPACERS HOLD THE BOARD OFF THE WALL WHILE THE ADHESIVE SETS (TYPICALLY 12-24 HOURS). THE 10-20MM AIR GAP BETWEEN THE BACK OF THE BOARD AND THE WALL FACE (THE DABS CREATE A STANDOFF) IS AN INHERENT FEATURE OF THE DOT AND DAB SYSTEM — THIS AIR GAP PROVIDES SOME ADDITIONAL THERMAL RESISTANCE BUT ALSO CREATES AN AIR CIRCULATION PATH BEHIND THE BOARD — WHICH CAN ALLOW MOISTURE TO ACCUMULATE BETWEEN THE BOARD AND THE WALL IN CERTAIN CONDITIONS. LIMITATION OF DOT AND DAB FOR INSULATED PLASTERBOARD: THE AIR GAP BEHIND THE BOARD IS A THERMAL BYPASS PATH — COLD AIR CAN CIRCULATE FROM THE PERIMETER OF THE BOARD ARRAY (AT FLOOR AND CEILING LEVEL AND AT WINDOW/DOOR REVEALS) BEHIND THE BOARDS AND REDUCE THE EFFECTIVE THERMAL PERFORMANCE — THE PERIMETER OF THE DOT AND DAB ARRAY MUST BE SEALED WITH A CONTINUOUS BEAD OF BONDING COMPOUND (THE PERIMETER SEAL — BRITISH GYPSUM SPECIFIES THIS AS MANDATORY IN THEIR DOT AND DAB APPLICATION GUIDANCE).

MECHANICAL FIXING (TIMBER BATTEN FRAME OR DIRECT SCREW FIX FOR THICKER/HEAVIER BOARDS): FOR BOARDS ABOVE 60MM TOTAL THICKNESS (WHERE ADHESIVE DABS ALONE MAY NOT GIVE ADEQUATE FIXING STRENGTH IN A DAMP OR CONTAMINATED SUBSTRATE): A TIMBER BATTEN FRAME (47X50MM C16 TIMBER — AT 600MM CENTRES TO SUIT 1200MM BOARD WIDTHS) IS FIXED TO THE WALL THROUGH THE INSULATION LAYER — THE BOARD IS THEN SCREWED TO THE BATTEN FRAME. DISADVANTAGE: THE TIMBER BATTENS CREATE DIRECT THERMAL BRIDGES THROUGH THE INSULATION — EACH BATTEN IS A PATH FOR HEAT TO BYPASS THE PIR LAYER — THE LINEAR THERMAL TRANSMITTANCE OF THE BATTEN BRIDGES MUST BE CALCULATED IN THE U-VALUE ASSESSMENT TO GIVE THE CORRECT COMPOSITE U-VALUE. DIRECT SCREW FIX (WITHOUT BATTENS): FOR MEDIUM THICKNESS PIR LAMINATE BOARDS (35-60MM) ON A FLAT, SOUND, DRY SUBSTRATE — THE BOARD IS SCREWED DIRECTLY TO THE WALL THROUGH THE PIR LAYER WITH LONG SELF-TAPPING MASONRY SCREWS (TYPICALLY 100-150MM LONG — RECESSING INTO THE BRICK/BLOCK SUBSTRATE AFTER PENETRATING THE PIR LAYER). LOWER THERMAL BRIDGE THAN BATTEN FRAME BUT HIGHER THAN PURE ADHESIVE DAB.

THERMAL BRIDGING (THE BYPASS OF INSULATION BY HEAT CONDUCTING THROUGH HIGHER-CONDUCTIVITY ELEMENTS — STRUCTURAL ELEMENTS, WINDOW FRAMES, FLOOR JUNCTIONS, AND WALL PERIMETERS) IS THE MOST IMPORTANT PERFORMANCE ISSUE TO ADDRESS WHEN INSTALLING PIR LAMINATE BOARD INTERNAL WALL INSULATION IN A LONDON VICTORIAN TERRACE.

PERIMETER COLD BRIDGE (THE MOST SIGNIFICANT THERMAL BRIDGING ISSUE IN DOT AND DAB PIR LAMINATE BOARD INSTALLATIONS): AT THE FLOOR-WALL JUNCTION: THE FLOOR JOISTS OF A TIMBER GROUND FLOOR (OR A CONCRETE SUSPENDED FLOOR) TYPICALLY RUN PARALLEL TO THE FRONT AND REAR WALLS OF THE VICTORIAN TERRACE — PERPENDICULAR TO THE PARTY WALL — AND BEAR INTO THE SOLID BRICK EXTERNAL WALL AT THE FRONT AND REAR. THE SOLID BRICK OF THE EXTERNAL WALL CONTINUES FROM BELOW THE FLOOR TO ABOVE IT — THE INSULATION BOARD ON THE WALL IS TYPICALLY INTERRUPTED AT THE FLOOR LEVEL (THE BOARD SITS ON THE FLOOR) — AND THE SECTION OF EXTERNAL BRICK WALL EXPOSED BELOW THE BOARD (BEHIND THE SKIRTING BOARD) IS UNINSULATED — CREATING A PERIMETER COLD BRIDGE AT FLOOR LEVEL. AT THE CEILING JUNCTION: THE SOLID BRICK WALL CONTINUES ABOVE THE INSULATED BOARD AREA (ABOVE THE PLASTERBOARD/CEILING JOINT) — UNLESS THE CEILING IS ALSO UPGRADED WITH INSULATION — ANOTHER PERIMETER COLD BRIDGE. AT WINDOW AND DOOR REVEALS: THE BRICK JAMB (SIDE REVEAL) OF EACH WINDOW IS TYPICALLY UNINSULATED WHEN PIR LAMINATE BOARD IS INSTALLED ON THE ADJACENT MAIN WALL — CREATING A COLD BRIDGE AT EACH WINDOW OPENING.

MITIGATING THE PERIMETER COLD BRIDGE: FLOOR-LEVEL COLD BRIDGE MITIGATION: CUT THE PIR LAMINATE BOARD TO FIT DOWN TO THE SUB-FLOOR LEVEL (NOT JUST TO FLOOR SURFACE LEVEL) — INSULATING THE LOWER SECTION OF THE WALL AS FAR AS POSSIBLE. FIT A THIN PIR INSULATION STRIP (25-50MM) BEHIND THE SKIRTING BOARD WHERE THE BOARD CANNOT BE EXTENDED. WINDOW REVEAL INSULATION: FIT INSULATED PLASTERBOARD TO THE WINDOW REVEALS (JAMBS, HEAD, CILL) — 25-35MM TOTAL THICKNESS IN A REVEAL — CUT AND FIT WITH THE SAME DOT AND DAB ADHESIVE METHOD.

CONDENSATION RISK AND VAPOUR CONTROL: PIR LAMINATE BOARD HAS A MUCH LOWER VAPOUR PERMEABILITY THAN THE SOLID BRICK WALL BEHIND IT (PIR FOAM IS ESSENTIALLY VAPOUR-CLOSED — SDValue/MU FACTOR APPROXIMATELY 60-100). THE PIR LAYER ACT AS AN INTEGRAL VAPOUR CONTROL LAYER — REDUCING THE AMOUNT OF WATER VAPOUR DIFFUSING THROUGH THE WALL FROM INSIDE TO OUTSIDE — WHICH IN A DRY WELL-VENTILATED VICTORIAN TERRACE IS BENEFICIAL (REDUCES INTERSTITIAL CONDENSATION RISK IN THE INSULATION). HOWEVER: IF THE WALL IS NOT ADEQUATELY VENTILATED AND THE PIR LAYER IS INTERRUPTED AT PERIMETERS (ALLOWING AIR CIRCULATION BEHIND THE BOARD IN THE DOT AND DAB CAVITY), INTERSTITIAL CONDENSATION CAN FORM ON THE BACK OF THE PIR BOARD — PARTICULARLY IN POORLY VENTILATED GROUND FLOOR ROOMS. MITIGATION: SEAL ALL PERIMETERS OF THE BOARD ARRAY WITH CONTINUOUS BONDING COMPOUND BEAD (ELIMINATE AIR CIRCULATION BEHIND BOARDS).

THE PART L 2021 APPROVED DOCUMENT (CONSERVATION OF FUEL AND POWER — VOLUME 1: DWELLINGS) SETS U-VALUE TARGETS FOR WALLS IN EXISTING DWELLINGS UNDERGOING RENOVATION. THE RELEVANT TARGETS FOR A LONDON VICTORIAN TERRACE SOLID BRICK EXTERNAL WALL (U-VALUE APPROXIMATELY 1.7-2.1 W/M²K UNINSULATED) ARE: WHERE THE WALL IS BEING RENOVATED (ANY CLADDING, PLASTERING, OR SUBSTANTIAL WORK TO THE EXTERNAL OR INTERNAL FACE OF THE WALL IS BEING CARRIED OUT): THE THERMAL ELEMENT (WALL) SHOULD BE UPGRADED TO AT LEAST THE BETTER OF: (A) 0.55 W/M²K (THE REASONABLE WORST-CASE THRESHOLD ELEMENT U-VALUE FOR AN INSULATED EXTERNAL WALL IN AN EXISTING DWELLING), OR (B) THE THRESHOLD VALUE THAT GIVES A REASONABLE PAYBACK PERIOD ON THE COST OF THE INSULATION UPGRADE — TYPICALLY 0.3 W/M²K FOR A SOLID WALL.

U-VALUE CALCULATION FOR A VICTORIAN SOLID BRICK WALL WITH PIR LAMINATE BOARD INTERNAL INSULATION (SIMPLIFIED — CENTRE-OF-INSULATION — IGNORING LINEAR THERMAL BRIDGES): EXTERNAL BRICK WALL (225MM SOLID BRICK — LONDON STOCK BRICK — LAMBDA 0.77 W/MK): R = 0.225/0.77 = 0.29 M²K/W. INTERNAL PLASTER (15MM LIME PLASTER — LAMBDA 0.80 W/MK — OR EXISTING SKIM): 0.019 M²K/W (NEGLIGIBLE). PIR LAMINATE BOARD (CELOTEX PL4000 SERIES): PL4022 (22.5MM PIR, LAMBDA 0.022): R = 1.02 M²K/W → COMPOSITE U-VALUE ≈ 1/(0.13+0.29+1.02+0.13) = 1/1.57 = 0.64 W/M²K (DOES NOT MEET 0.55 W/M²K). PL4032 (32.5MM PIR): R = 1.48 M²K/W → U ≈ 1/(0.13+0.29+1.48+0.13) = 1/2.03 = 0.49 W/M²K (MEETS 0.55 W/M²K — REASONABLE THRESHOLD). PL4047 (47.5MM PIR): R = 2.16 M²K/W → U ≈ 1/(0.13+0.29+2.16+0.13) = 1/2.71 = 0.37 W/M²K (APPROACHING 0.3 W/M²K — GOOD PERFORMANCE). PL4062 (62.5MM PIR): R = 2.84 M²K/W → U ≈ 1/(0.13+0.29+2.84+0.13) = 1/3.39 = 0.30 W/M²K (MEETS 0.3 W/M²K TARGET — EXCELLENT). PRACTICAL RECOMMENDATION: FOR A LONDON VICTORIAN TERRACE WALL RENOVATION: PL4032 (45MM TOTAL — 32.5MM PIR) IS THE MINIMUM TO MEET THE PART L 0.55 W/M²K THRESHOLD. PL4047 (60MM TOTAL) IS THE RECOMMENDED SPECIFICATION FOR GOOD THERMAL PERFORMANCE WITHOUT EXCESSIVE ROOM WIDTH LOSS (60MM IS MANAGEABLE IN MOST VICTORIAN TERRACE ROOMS). PL4062 (75MM TOTAL) IS RECOMMENDED WHERE THE ROOM WIDTH ALLOWS (TYPICALLY FRONT RECEPTION ROOMS — 3.5-4.5M WIDTH — CAN ACCOMMODATE 75MM BETTER THAN SMALLER SECONDARY ROOMS).

THE CORRECT INSTALLATION SEQUENCE FOR PIR LAMINATE BOARD (CELOTEX PL4000 OR EQUIVALENT) ON A SOLID BRICK WALL IN A LONDON VICTORIAN TERRACE IS AS FOLLOWS.

PREPARATION: STEP 1 — CHECK THE EXISTING WALL FOR DAMP (RISING DAMP OR PENETRATING DAMP): ANY ACTIVE DAMP MUST BE TREATED BEFORE INSULATED PLASTERBOARD IS APPLIED — APPLYING INSULATED PLASTERBOARD OVER A DAMP WALL TRAPS THE MOISTURE AND ACCELERATES DETERIORATION. IN A LONDON VICTORIAN TERRACE: CHECK THE GROUND FLOOR WALLS FOR RISING DAMP (USING A TRAMEX DEEP WALL MOISTURE METER OR CAPACITANCE METER — NOT A SURFACE METER — TO DETERMINE THE DEPTH OF MOISTURE IN THE WALL) — IF ACTIVE RISING DAMP IS PRESENT, ADDRESS THE CAUSE (DPC, DRAINAGE, GROUND LEVELS) BEFORE APPLYING PIR LAMINATE BOARD. STEP 2 — CHECK THE EXISTING PLASTER: IF EXISTING PLASTER IS SOUND (WELL-BONDED TO THE BRICK — NOT HOLLOW — NOT CRUMBLING): LEAVE IN PLACE (IT ADDS SOME THERMAL MASS AND PROVIDES A FLAT BACKGROUND FOR THE ADHESIVE DABS). IF EXISTING PLASTER IS HOLLOW OR CRUMBLING: HACK OFF AND ALLOW THE BRICK TO DRY BEFORE APPLYING BOARDS.

BOARD APPLICATION: STEP 3 — APPLY PERIMETER SEAL: APPLY A CONTINUOUS BEAD OF GYPROC DRYWALL ADHESIVE AROUND THE FULL PERIMETER OF THE AREA TO BE BOARDED (AT FLOOR LEVEL, CEILING LEVEL, WINDOW AND DOOR REVEALS, AND AT THE JUNCTION WITH ADJACENT WALLS) — THIS PERIMETER SEAL IS MANDATORY TO PREVENT AIR CIRCULATION IN THE DOT AND DAB CAVITY. STEP 4 — APPLY ADHESIVE DABS: APPLY DABS OF GYPROC DRYWALL ADHESIVE (OR ECOPHIT ADHESIVE FOR LIGHTER BOARDS) IN THREE COLUMNS ON THE BACK OF EACH BOARD — ONE COLUMN AT EACH VERTICAL EDGE AND ONE IN THE CENTRE — THREE DABS PER COLUMN — NINE DABS TOTAL PER BOARD. STEP 5 — FIX BOARDS: PRESS THE BOARD AGAINST THE WALL — PLUMB VERTICALLY AND CHECK LEVEL — WEDGE SPACERS AT THE BOTTOM TO HOLD THE BOARD OFF THE FLOOR WHILE ADHESIVE SETS. STEP 6 — ELECTRICAL SERVICES: WHERE ELECTRICAL SOCKETS AND LIGHT SWITCHES ARE ON THE EXTERNAL WALL, THE BACK BOX MUST BE MOVED TO THE FACE OF THE NEW PLASTERBOARD — IN A PIR LAMINATE BOARD INSTALLATION, THIS TYPICALLY REQUIRES SURFACE-MOUNTED BACK BOXES (MK K189 OR SIMILAR SURFACE BACK BOX) MOUNTED ON THE FACE OF THE INSULATED PLASTERBOARD — OR RECESSED BACK BOXES SET INTO THE PIR LAYER (CUTTING THROUGH THE PIR WITH A HOT WIRE CUTTER OR ROUTER TO THE REQUIRED BACK BOX DEPTH).

FINISHING: STEP 7 — JOINTING AND SKIMMING: THE TAPERED EDGE PLASTERBOARD FACE OF THE PIR LAMINATE BOARD IS FINISHED WITH EITHER TAPE AND JOINT (GYPROC JOINTING COMPOUND + SCRIM TAPE — FOR A PAINT-READY SURFACE WITHOUT SKIM) OR THIN COAT SKIM (BRITISH GYPSUM THISTLE MULTIFINISH — 3-5MM TWO-COAT SKIM — FOR A TRADITIONAL SMOOTH PLASTER FINISH). STEP 8 — SKIRTING BOARDS: THE NEW SKIRTING BOARD IS FIXED TO THE FACE OF THE PLASTERBOARD (NOT BACK TO THE ORIGINAL WALL — THE PLASTERBOARD FACE IS NOW THE REFERENCE SURFACE). IN A LISTED BUILDING OR CONSERVATION AREA: ENSURE THE NEW SKIRTING BOARD MATCHES THE ORIGINAL VICTORIAN PROFILE (TORUS, OGEE, OR FLAT TOP WITH CHAMFER — DEPENDING ON THE ORIGINAL PROPERTY SPECIFICATION) — THE SKIRTING BOARD HEIGHT MAY NEED TO BE INCREASED SLIGHTLY TO COVER THE PIR LAMINATE BOARD EDGE AT FLOOR LEVEL.

TWO KEY PRACTICAL CONSTRAINTS ON PIR LAMINATE BOARD INTERNAL WALL INSULATION IN A LONDON VICTORIAN TERRACE: THE COST PER M² AND THE FLOOR AREA LOST AS EACH WALL IS THICKENED.

ROOM AREA LOSS: EVERY 10MM OF BOARD THICKNESS ON A WALL REDUCES THE ROOM WIDTH BY 10MM ON THAT SIDE. IN A TYPICAL LONDON VICTORIAN TERRACE FRONT RECEPTION ROOM (ROOM WIDTH APPROXIMATELY 3.5-4.0M — TYPICALLY ONLY THE FRONT EXTERNAL WALL IS INSULATED — NOT THE PARTY WALLS OR INTERNAL WALLS): 35MM BOARD (PL4022): ROOM WIDTH REDUCED BY 35MM — NEGLIGIBLE. 60MM BOARD (PL4047): ROOM WIDTH REDUCED BY 60MM — STILL ACCEPTABLE IN MOST ROOMS. 75MM BOARD (PL4062): ROOM WIDTH REDUCED BY 75MM — ACCEPTABLE IN LARGER ROOMS — MAY FEEL TIGHT IN SMALLER SECONDARY ROOMS (2.5-3.0M WIDTH). FOR A COMPLETE WRAP (ALL EXTERNAL WALLS INSULATED — BOTH FRONT AND SIDE WALLS OF A BACK BEDROOM ADJACENT TO TWO EXTERNAL WALLS): 60MM BOARDS ON BOTH WALLS = ROOM SHRINKS BY 60MM IN BOTH LENGTH AND WIDTH — APPROXIMATELY 6-8% AREA REDUCTION IN A 3.5M X 3.5M SECONDARY BEDROOM — SIGNIFICANT BUT USUALLY ACCEPTABLE WHERE THERMAL COMFORT AND ENERGY SAVING ARE THE PRIORITY.

INSTALLATION COSTS (LONDON 2025 — SUPPLY AND FIT — CONTRACTOR — INCLUDING BOARD SUPPLY, ADHESIVE, PERIMETER SEAL, TAPE AND JOINT FINISH OR SKIM, AND SKIRTING BOARD REPLACEMENT): PIR LAMINATE BOARD SUPPLY (CELOTEX PL4047 — 60MM TOTAL): APPROXIMATELY £8.70-12.15/M² (1200X2400MM BOARD). INSTALLATION LABOUR (SPECIALIST DRY-LINING CONTRACTOR — DOT AND DAB FIX, PERIMETER SEAL, TAPE AND JOINT FINISH): £18-30/M² (SUPPLY AND FIT INCLUDING MATERIAL = £26-42/M²). SKIMMING (THIN-COAT THISTLE MULTIFINISH OVER BOARD — IF REQUIRED): ADDITIONAL £8-15/M². ELECTRICAL BACK BOX RELOCATION (PER SOCKET OR SWITCH): £40-80. TYPICAL VICTORIAN TERRACE GROUND FLOOR RECEPTION ROOM (FRONT EXTERNAL WALL — 12M² WALL AREA — CELOTEX PL4047 60MM TOTAL): BOARD SUPPLY: £100-145. LABOUR AND MATERIALS (DOT AND DAB, PERIMETER SEAL, TAPE AND JOINT): £215-360. SKIM FINISH: £95-180. TOTAL SUPPLY AND FIT: £410-685 FOR A SINGLE EXTERNAL WALL OF ONE ROOM (NOT INCLUDING ELECTRICAL WORK). FULL GROUND FLOOR INTERNAL WALL INSULATION (ALL EXTERNAL WALLS OF GROUND FLOOR — APPROXIMATELY 40M² WALL AREA): APPROXIMATELY £1,600-2,800 SUPPLY AND FIT (TAPE AND JOINT FINISH) OR £2,000-3,400 WITH SKIM FINISH.