Sash Window Restoration Guide for London Homes

BEFORE DECIDING ON A RESTORATION ROUTE, A THOROUGH ASSESSMENT OF EACH SASH WINDOW IS NEEDED. THE MAIN DEFECTS TO IDENTIFY: (1) BROKEN SASH CORDS — THE MOST COMMON DEFECT. THE UPPER OR LOWER SASH SASH CORD PASSES OVER A PULLEY IN THE PULLEY STILE AND IS ATTACHED TO A CAST IRON WEIGHT HIDDEN IN THE BOX FRAME. BROKEN CORDS CAUSE THE SASH TO FALL, BECOME IMPOSSIBLE TO HOLD OPEN, OR SLAM SHUT. CORD REPLACEMENT REQUIRES OPENING THE POCKET PIECE (A REMOVABLE SECTION OF LINING AT THE BOTTOM OF THE BOX FRAME) TO ACCESS THE WEIGHTS AND RETHREAD THE CORD. WAXED COTTON SASH CORD (SOLID BRAID, SIZE 8 = 8MM DIAMETER) IS THE CORRECT HISTORIC SPECIFICATION; POLYPROPYLENE CORD IS ADEQUATE BUT LOOKS DIFFERENT. (2) ROT IN CILLS, BOTTOM RAILS, AND MEETING RAILS — THESE TIMBER ELEMENTS ARE MOST EXPOSED TO WEATHER. EARLY-STAGE ROT (WOOD STILL FIRM BUT DISCOLOURED): TREAT WITH RONSEAL WOOD HARDENER THEN FILL WITH RONSEAL HIGH PERFORMANCE WOOD FILLER OR EPOXY FILLER (LTI WOODFLEX). MORE ADVANCED ROT (WOOD SOFT, PUNKY, OR MISSING): SPLICE REPAIR WITH NEW TIMBER (ACCOYA MODIFIED WOOD IS IDEAL FOR CILL REPLACEMENTS DUE TO 50-YEAR DURABILITY GUARANTEE) OR FULL MEMBER REPLACEMENT. (3) STICKING, PAINTED-SHUT, OR SWOLLEN SASHES — DECADES OF PAINT LAYERS CAN FIX SASHES SHUT. CAREFUL SCORING OF PAINT JUNCTIONS WITH A SASH PAINT TOOL OR STANLEY KNIFE AND PLANING OF SASH EDGES RESTORES MOVEMENT. (4) RATTLE AND DRAUGHT — THE PRIMARY FUNCTIONAL COMPLAINT. CAUSED BY WORN PARTING BEADS (THE SLIM TIMBER STRIP SEPARATING UPPER AND LOWER SASHES), WORN STAFF BEADS (INNER BEAD), AND GAPS BETWEEN SASH AND FRAME.

DRAUGHT-PROOFING IS THE HIGHEST-VALUE INTERVENTION FOR MOST SASH WINDOWS. THE ENERGY TRUST AND HISTORIC ENGLAND BOTH CITE SASH WINDOW DRAUGHT-PROOFING AS AMONG THE BEST VALUE HEAT-RETENTION MEASURES FOR OLDER LONDON PROPERTIES — TYPICALLY REDUCING AIR INFILTRATION THROUGH THE WINDOW BY 85-90%. MAIN SPECIALIST SYSTEMS: (1) VENTROLLA — THE MARKET LEADER IN SASH WINDOW RESTORATION AND DRAUGHT-PROOFING. THE VENTROLLA SYSTEM REPLACES STAFF BEADS AND PARTING BEADS WITH NEW PRECISION-ROUTED TIMBER BEADS INCORPORATING A PILE WEATHERSTRIP (POLYPROPYLENE PILE, SIMILAR TO THAT USED IN SLIDING DOOR TRACKS). THE SASH BOX IS CLEANED OUT, WEIGHTS CHECKED, CORDS REPLACED IF NECESSARY, SASHES PLANED TO FIT PRECISELY, AND THEN REASSEMBLED WITH WEATHERSTRIP THROUGHOUT. COST: £200-£400 PER WINDOW PROFESSIONALLY INSTALLED BY VENTROLLA (THEY OFFER A NATIONWIDE SERVICE WITH LONDON COVERAGE). (2) SPRING SASH (SASH WINDOW SPECIALIST COMPANY) — SIMILAR PILE WEATHERSTRIP SYSTEM WITH A FOCUS ON TIMBER CONDITION AND CORD REPLACEMENT. (3) RENNICK SYSTEM — PROPRIETARY BRUSH SEAL SYSTEM INSTALLED BY TIMBER WINDOW SPECIALISTS. (4) DIY DRAUGHT-PROOFING — FOR BUDGET PROJECTS, RONSEAL OR STORMGUARD V-SEAL WEATHERSTRIP SELF-ADHESIVE FOAM CAN BE APPLIED TO THE MEETING RAIL AND SILL LINE — BUT THIS IS LESS DURABLE AND LESS EFFECTIVE THAN PILE SYSTEMS. A BASIC PROFESSIONAL DRAUGHT-PROOF AND CORD REPLACEMENT PER WINDOW ADDS THE FUNCTIONAL EQUIVALENT OF SECONDARY GLAZING IN TERMS OF HEAT RETENTION, AT A FRACTION OF THE COST.

THE TRADITIONAL OBJECTION TO RETROFITTING DOUBLE GLAZING INTO ORIGINAL SASH FRAMES WAS THAT THE STANDARD 24-28MM DGU UNIT WAS TOO THICK FOR THE ORIGINAL PUTTY GLAZING REBATE (TYPICALLY 8-12MM). MODERN SLIM-LINE DGU UNITS (10MM-16MM TOTAL UNIT THICKNESS) NOW MAKE IT POSSIBLE TO RETROFIT DOUBLE GLAZING INTO MANY ORIGINAL SASH WINDOWS WITHOUT CHANGING THE FRAME. KEY PRODUCTS: (1) HISTOGLASS SLIM-LINE DGU (12MM UNIT: 4-4 WITH 4MM AIR GAP) — U-VALUE APPROXIMATELY 2.6 W/M2K COMPARED WITH 5.4 W/M2K FOR SINGLE GLAZING. SUPPLY: £80-£140 PER M2 BESPOKE UNIT. (2) FINEO VACUUM GLAZING (6MM TOTAL UNIT: 4MM GLASS + VACUUM + 4MM GLASS) — U-VALUE 0.7-1.0 W/M2K; THE THINNEST HIGH-PERFORMANCE GLAZING AVAILABLE. SUPPLY: £300-£500 PER M2 BESPOKE. IDEAL FOR LISTED BUILDINGS AND CONSERVATION AREAS WHERE VISUAL IMPACT MUST BE MINIMISED. (3) GUARDIAN SUPER SPACER UNI-FORM HERITAGE DGU (12MM): WARM-EDGE SPACER, 4-4 CONFIG, U-VALUE 2.8 W/M2K. SUPPLY: £75-£120 PER M2. FOR LISTED BUILDING AND CONSERVATION AREA PROPERTIES: ALWAYS CONFIRM WITH YOUR LONDON BOROUGH LPA BEFORE REPLACING GLAZING. SOME CONSERVATION AREA DIRECTIONS REQUIRE THAT REPLACEMENT GLAZING REPLICATES SINGLE-PANE APPEARANCE (ASTRAGAL BAR DETAILING APPLIED TO DGU TO SIMULATE GEORGIAN PANES). LISTED BUILDING CONSENT IS REQUIRED FOR ANY CHANGE TO WINDOWS IN A LISTED BUILDING — THIS APPLIES EVEN TO LIKE-FOR-LIKE GLAZING REPLACEMENT.

SECONDARY GLAZING — A SECOND PANE OF GLASS OR ACRYLIC FITTED TO THE INTERIOR FACE OF AN EXISTING WINDOW — IS THE PREFERRED HEAT-RETENTION MEASURE FOR LISTED AND CONSERVATION AREA PROPERTIES WHERE ALTERING ORIGINAL SASHES IS RESTRICTED. SECONDARY GLAZING IS REVERSIBLE, REQUIRES NO PLANNING PERMISSION OR LISTED BUILDING CONSENT IN MOST CASES, AND CAN ACHIEVE U-VALUES AS LOW AS 1.1 W/M2K WHEN COMBINED WITH ORIGINAL SINGLE GLAZING (COMPARED WITH 5.4 W/M2K FOR SINGLE GLAZING ALONE). MAIN SYSTEMS AND SUPPLIERS FOR LONDON: (1) SELECTAGLAZE SERIES 20 HINGED CASEMENT SECONDARY GLAZING (6.4MM LAMINATED GLASS, SLIM ALUMINIUM FRAME IN CLEAR ANODISED OR PAINTED FINISHES): £400-£800 PER WINDOW SUPPLY AND FIT. (2) SECONDARY GLAZING SPECIALISTS REGALEAD SECONDARY GLAZING (LONDON COVERAGE). (3) MAGNETITE SECONDARY GLAZING (MAGNETIC FRAME SYSTEM — REMOVABLE FOR SUMMER MONTHS WITHOUT TOOLS): £150-£350 PER WINDOW SUPPLY AND FIT. (4) HEATKEEPER SECONDARY GLAZING (ACRYLIC SHEET WITH MAGNETIC SEAL): £100-£250 PER WINDOW SUPPLY AND FIT — LIGHTER BUT LESS DURABLE THAN GLASS. NOTE: SECONDARY GLAZING ALSO PROVIDES SIGNIFICANT SOUND ATTENUATION — 40-45DBA REDUCTION IS ACHIEVABLE WITH A 100MM+ AIR GAP BETWEEN PRIMARY AND SECONDARY PANE — MAKING IT THE BEST ACOUSTIC SOLUTION FOR LONDON PROPERTIES ON BUSY ROADS.

WHERE ORIGINAL SASH WINDOWS ARE BEYOND ECONOMIC REPAIR (ROT EXCEEDING 30-40% OF TIMBER MEMBERS; FRAMES NO LONGER SQUARE; REPEATED PREVIOUS REPAIRS HAVE EXHAUSTED THE TIMBER SECTION), FULL REPLACEMENT BECOMES THE ONLY OPTION. FOR CONSERVATION AREA AND LISTED BUILDING PROPERTIES, REPLACEMENT MUST REPLICATE THE ORIGINAL DESIGN: MATCHING PROFILE DIMENSIONS, GLAZING BAR SIZE, HORNS, AND CILL PROFILE. THE TWO BEST MATERIALS FOR CONSERVATION-GRADE SASH WINDOW REPLACEMENTS: (1) ACCOYA MODIFIED WOOD — MODIFIED ACETYLATED RADIATA PINE. DIMENSIONALLY STABLE IN MOISTURE CYCLING; 50-YEAR DURABILITY WARRANTY AGAINST ROT IN EXPOSED APPLICATIONS. FINISHES WITH DULUX TRADE WEATHERSHIELD OR OTEX PRIMER + TOPCOAT. MUMFORD & WOOD' TOWN HOUSE ACCOYA SASH (THEIR FLAGSHIP LONDON CONSERVATION PRODUCT) — A COMPLETE DOUBLE-SASH UNIT IN ANY SIZE WITH SLIM 14MM DGU: £900-£1,600 SUPPLY PER WINDOW PLUS £300-£500 INSTALLATION. (2) HARDWOOD (SAPELE, IROKO, OR FSC-CERTIFIED OAK) — HISTORICALLY MORE COMMONLY SPECIFIED THAN SOFTWOOD FOR EXPOSED LONDON SETTINGS. EXCELLENT DURABILITY; HIGHER COST. £1,200-£2,200 PER WINDOW SUPPLY. UPVC SASH WINDOWS ARE NOT APPROPRIATE FOR CONSERVATION AREAS AND ARE INCREASINGLY REFUSED BY LONDON BOROUGHS ON CHARACTER GROUNDS EVEN OUTSIDE CONSERVATION AREAS — ALWAYS CHECK BEFORE SPECIFYING UPVC. IF A FULL REPLACEMENT IS REQUIRED IN A CONSERVATION AREA, THE LPA WILL USUALLY REQUIRE A "LIKE-FOR-LIKE" SPECIFICATION — TIMBER WITH MATCHING PROFILE, HORN DETAIL, AND MEETING RAIL PROFILE.

TYPICAL LONDON MARKET COSTS IN 2025: ASSESSMENT AND REPORT (SPECIALIST SASH WINDOW SURVEYOR OR TIMBER WINDOW SPECIALIST): £100-£200 FOR FIRST VISIT ADVICE. BROKEN CORD REPLACEMENT (BOTH SASHES, ONE WINDOW): £80-£150 LABOUR. COMPLETE PROFESSIONAL DRAUGHT-PROOFING AND CORD REPLACEMENT (VENTROLLA OR EQUIVALENT, ONE WINDOW): £200-£400. SLIM DGU RETROFIT INTO EXISTING SASH (GLAZIER REMOVING AND REPLACING GLASS WITH 12-14MM SLIM DGU): £180-£320 PER WINDOW (PLUS SLIM DGU SUPPLY £80-£140 PER M2). SECONDARY GLAZING INSTALLATION (SELECTAGLAZE OR SIMILAR, HINGED CASEMENT): £400-£800 PER WINDOW. ROT REPAIR (CILL SPLICE OR RAIL REPAIR, EPOXY CONSOLIDATION): £150-£450 PER WINDOW DEPENDING ON EXTENT. FULL SASH WINDOW REPLACEMENT SUPPLY AND FIT (ACCOYA, STANDARD 450X900MM LOWER AND UPPER SASH, SLIM 14MM DGU): £1,200-£2,100 PER WINDOW INSTALLED. FULL FRONT ELEVATION RESTORATION PROGRAMME (LONDON 3-STOREY TERRACE, 6 WINDOWS — DRAUGHT-PROOF, CORD REPLACE, SLIM DGU, DECORATION): £4,000-£8,000 ALL-IN. NOTE: GRANT FUNDING — THE GREAT BRITISH INSULATION SCHEME (GBIS) AND SOME LONDON BOROUGH SCHEMES PROVIDE FUNDING FOR DRAUGHT-PROOFING AND SECONDARY GLAZING IN ELIGIBLE PROPERTIES (PARTICULARLY SOCIAL HOUSING AND LOW-INCOME OWNER-OCCUPIERS).