Secondary Glazing for London Victorian Terraces: Noise Reduction, Thermal Performance, and Heritage

SECONDARY GLAZING SYSTEMS FOR LONDON VICTORIAN TERRACES ARE AVAILABLE IN SEVERAL CONFIGURATIONS — THE CHOICE DEPENDS ON THE WINDOW TYPE (SASH, CASEMENT), THE SIZE OF THE WINDOW, AND THE INTENDED OPENING PATTERN (HORIZONTAL SLIDING, VERTICAL SLIDING, HINGED, OR LIFT-OUT FOR VENTILATION ACCESS).

HORIZONTAL SLIDING SECONDARY GLAZING (THE MOST COMMON SECONDARY GLAZING TYPE FOR LONDON VICTORIAN TERRACE SASH WINDOWS): A HORIZONTAL SLIDING SECONDARY GLAZING UNIT CONSISTS OF ONE OR MORE ALUMINIUM-FRAMED GLASS PANELS THAT SLIDE HORIZONTALLY IN A TRACK MOUNTED TO THE WINDOW REVEAL OR WALL. FOR A STANDARD LONDON VICTORIAN TERRACE SASH WINDOW (APPROXIMATELY 800MM WIDE X 1200MM TALL): A TWO-PANEL HORIZONTAL SLIDER ALLOWS THE PANELS TO BE SLID OPEN FOR VENTILATION WITHOUT RAISING THE EXISTING SASH WINDOW. SLIMLITE SECONDARY GLAZING (THE MOST WIDELY SPECIFIED SECONDARY GLAZING BRAND BY LONDON CONSERVATION AND LISTED BUILDING SPECIALISTS): SLIMLITE HORIZONTAL SLIDER (6.35MM TOUGHENED GLASS OR 6.8MM ACOUSTIC LAMINATE GLASS — 6.4MM SGS): FRAME IN POWDER-COATED ALUMINIUM (WHITE OR GREY — MATCHING THE EXISTING WINDOW FRAME COLOUR). SELECTAGLAZE SERIES 20 (HORIZONTAL SLIDER — PREFERRED SECONDARY GLAZING BRAND FOR LONDON LISTED BUILDING AND CONSERVATION AREA INSTALLATIONS): SELECTAGLAZE IS THE UK MARKET LEADER IN SECONDARY GLAZING AND THE BRAND MOST WIDELY ACCEPTED BY ENGLISH HERITAGE AND LONDON BOROUGH CONSERVATION OFFICERS AS APPROPRIATE FOR LISTED BUILDINGS.

VERTICAL SLIDING SECONDARY GLAZING (TECHNICALLY SUPERIOR FOR SASH WINDOWS — MATCHES THE OPERATION OF THE PRIMARY SASH WINDOW): A VERTICAL SLIDING SECONDARY GLAZING UNIT MIMICS THE OPERATION OF THE ORIGINAL SASH WINDOW — THE INNER PANEL SLIDES UP AND DOWN. THIS ALLOWS VENTILATION FROM THE TOP OR BOTTOM OF THE WINDOW (MATCHING THE NATURAL VENTILATION PATTERN OF THE ORIGINAL SASH). REDDISEALS VERTICAL SLIDING SECONDARY GLAZING (THE LEADING CHOICE FOR SYMPATHETIC SASH WINDOW SECONDARY GLAZING IN LONDON): FRAME: SLIM-PROFILE ALUMINIUM (AVAILABLE IN WHITE, GREY, OR DARK GREY — POWDER-COATED). GLASS: 6.4MM ACOUSTIC LAMINATE OR 6.35MM TOUGHENED. REDDISEALS PANELS TRAVEL ON SPRING-LOADED SLIDERS AND CAN BE TILTED INWARD FOR CLEANING — AN IMPORTANT PRACTICAL FEATURE FOR UPPER-FLOOR LONDON VICTORIAN TERRACE WINDOWS WHERE EXTERNAL CLEANING IS DIFFICULT.

HINGED (SIDE-HUNG) SECONDARY GLAZING — FOR CASEMENT WINDOWS OR WHERE ACCESS TO THE PRIMARY WINDOW IS REQUIRED: A HINGED SECONDARY GLAZING UNIT IS HINGED TO THE WINDOW REVEAL AND SWINGS OPEN LIKE A CASEMENT WINDOW — ALLOWING FULL ACCESS TO THE ORIGINAL WINDOW. SUITABLE FOR CASEMENT WINDOWS AND FOR SASH WINDOWS WHERE OCCASIONAL ACCESS (CLEANING, MAINTENANCE) IS REQUIRED MORE EASILY THAN WITH A SLIDING SYSTEM.

ACOUSTIC NOISE REDUCTION IS THE PRIMARY DRIVER FOR SECONDARY GLAZING INSTALLATION IN MANY LONDON VICTORIAN TERRACES — PARTICULARLY THOSE NEAR BUSY ROADS (THE A4, A3, A23, NORTH CIRCULAR, SOUTH CIRCULAR), RAILWAY LINES (OVERGROUND, UNDERGROUND, ELIZABETH LINE), AND HIGH STREETS WHERE EXTERNAL NOISE LEVELS CAN REACH 65-80 DB(A) DAYTIME.

HOW SECONDARY GLAZING REDUCES NOISE IN LONDON TERRACE PROPERTIES: NOISE REDUCTION THROUGH A GLAZING UNIT IS ACHIEVED BY: THE MASS OF THE GLASS (HEAVIER GLASS = MORE MASS = BETTER SOUND REDUCTION). THE AIR GAP BETWEEN THE PRIMARY WINDOW AND THE SECONDARY GLAZING UNIT — THE WIDER THE AIR GAP, THE BETTER THE LOW-FREQUENCY NOISE REDUCTION (TRAFFIC, RAILWAY RUMBLE, BASS). THE ACOUSTIC PERFORMANCE OF THE SEALS AROUND THE SECONDARY GLAZING UNIT — SOUND LEAKS THROUGH GAPS IN THE SEAL.

ACOUSTIC GLASS OPTIONS FOR SECONDARY GLAZING: STANDARD TOUGHENED GLASS (6.35MM — STANDARD SECONDARY GLAZING GLASS): RW = APPROXIMATELY 32 DB (SOUND REDUCTION INDEX — ISO 140-3). LAMINATED ACOUSTIC GLASS (6.4MM — PILKINGTON OPTIPHON 6.4MM OR SAINT-GOBAIN SGS 6.4MM ACOUSTIC LAMINATE): ACOUSTIC PVVB INTERLAYER BETWEEN TWO 3MM GLASS PANES. RW = APPROXIMATELY 38-40 DB — APPROXIMATELY 6-8 DB IMPROVEMENT OVER STANDARD TOUGHENED. 8.8MM ACOUSTIC LAMINATE (PILKINGTON OPTIPHON 8.8 OR SAINT-GOBAIN STADIP SILENCE 8.8MM — 2 X 4MM + 0.8MM PVB): RW ≈ 42-44 DB — THE PREFERRED CHOICE FOR LONDON PROPERTIES ON VERY NOISY ROADS OR NEAR RAILWAY LINES.

TYPICAL COMBINED ACOUSTIC PERFORMANCE (SECONDARY GLAZING + EXISTING SINGLE-GLAZED SASH): EXISTING SINGLE-GLAZED SASH WINDOW (4MM GLASS, UNWEATHERSTRIPPED): RW ≈ 25-28 DB. ADDING SLIMLITE OR SELECTAGLAZE HORIZONTAL SLIDER (6.4MM ACOUSTIC LAMINATE, 100MM AIR GAP): COMBINED RW ≈ 40-45 DB — REDUCTION IN PERCEIVED NOISE OF APPROXIMATELY 50-60% (3 DB = HALVING OF ENERGY, 10 DB = HALVING OF PERCEIVED LOUDNESS). AIR GAP IMPORTANCE: A 50-100MM AIR GAP BETWEEN THE PRIMARY WINDOW AND SECONDARY GLAZING IS THE OPTIMUM FOR ACOUSTIC PERFORMANCE — LARGER GAPS (ABOVE 150MM) PROVIDE MINIMAL ADDITIONAL BENEFIT FOR TRAFFIC AND RAIL NOISE. SMALLER GAPS (BELOW 30MM) SIGNIFICANTLY REDUCE LOW-FREQUENCY NOISE ATTENUATION.

SECONDARY GLAZING ALSO SIGNIFICANTLY IMPROVES THE THERMAL PERFORMANCE OF SINGLE-GLAZED ORIGINAL WINDOWS IN LONDON VICTORIAN TERRACES — REDUCING HEAT LOSS THROUGH THE WINDOW AND ELIMINATING THE COLD DOWNDRAUGHTS THAT MAKE SINGLE-GLAZED SASH WINDOWS UNCOMFORTABLE IN WINTER.

U-VALUE IMPROVEMENT FROM SECONDARY GLAZING: SINGLE GLAZED SASH WINDOW (4MM GLASS, DRAUGHT-FREE SASH): U-VALUE APPROXIMATELY 5.0 W/M²K (HEAT LOSS THROUGH THE WINDOW IS HIGH). ADDING SECONDARY GLAZING (6.4MM ACOUSTIC LAMINATE, 50-100MM AIR GAP): COMBINED U-VALUE APPROXIMATELY 1.8-2.5 W/M²K — A REDUCTION OF 50-65% IN HEAT LOSS THROUGH THE WINDOW. FOR COMPARISON: A MODERN PART L 2021-COMPLIANT DOUBLE-GLAZED WINDOW (LOW-E COATING, ARGON-FILLED): U-VALUE APPROXIMATELY 1.2-1.4 W/M²K. SECONDARY GLAZING ON AN EXISTING SINGLE GLAZED WINDOW APPROACHES (THOUGH DOES NOT REACH) THE THERMAL PERFORMANCE OF A NEW DOUBLE-GLAZED REPLACEMENT WINDOW — WHILE RETAINING THE ORIGINAL WINDOW FRAME AND GLASS.

DRAUGHT AND INFILTRATION REDUCTION: LONDON VICTORIAN TERRACE SASH WINDOWS ARE NOTORIOUS FOR DRAUGHTS — THE GAP BETWEEN MEETING RAIL, THE GAPS BETWEEN SASH AND FRAME, AND THE GAP AT THE PULLEY HORNS ALL ADMIT COLD AIR. SECONDARY GLAZING WITH GOOD PERIMETER SEALS (PILE SEALS OR SILICON-RUBBER COMPRESSION SEALS AROUND THE SECONDARY GLAZING PERIMETER) EFFECTIVELY CREATES AN AIR-SEALED LAYER BEHIND THE EXISTING WINDOW — ELIMINATING MOST DRAUGHT INFILTRATION. PART L 2021 AND SECONDARY GLAZING: SECONDARY GLAZING IN LISTED BUILDINGS AND CONSERVATION AREAS IS OFTEN THE ONLY THERMALLY IMPROVING INTERVENTION THAT IS ACCEPTABLE TO THE PLANNING AUTHORITY — PART L 2021 ACKNOWLEDGES THIS AND DOES NOT REQUIRE LISTED BUILDING OWNERS TO REPLACE ORIGINAL WINDOWS (WHERE RETENTION IS REQUIRED BY LISTED BUILDING CONSENT OR CONSERVATION AREA GUIDANCE) — INSTEAD, THE BEST PRACTICAL THERMAL IMPROVEMENT SHOULD BE ACHIEVED, WHICH TYPICALLY MEANS SECONDARY GLAZING + DRAUGHT-PROOFING OF THE ORIGINAL WINDOW.

SECONDARY GLAZING IS GENERALLY CONSIDERED A NON-PLANNING-RELEVANT INTERNAL ALTERATION — SINCE IT IS INSTALLED ON THE INTERIOR FACE OF THE EXISTING WINDOW AND DOES NOT CHANGE THE EXTERNAL APPEARANCE OF THE BUILDING.

PLANNING STATUS OF SECONDARY GLAZING IN LONDON: SECONDARY GLAZING DOES NOT REQUIRE PLANNING PERMISSION IN: RESIDENTIAL PROPERTIES OUTSIDE CONSERVATION AREAS (PD RIGHTS APPLY — INTERNAL ALTERATIONS ARE NEVER PLANNING RELEVANT). CONSERVATION AREAS: SECONDARY GLAZING IS INSTALLED ON THE INTERIOR OF THE EXISTING WINDOW — IT IS NOT VISIBLE FROM THE EXTERIOR AND DOES NOT AFFECT THE EXTERNAL APPEARANCE OF THE BUILDING. THEREFORE, SECONDARY GLAZING IN A CONSERVATION AREA DOES NOT REQUIRE PLANNING PERMISSION.

LISTED BUILDINGS AND SECONDARY GLAZING: FOR A LISTED LONDON VICTORIAN TERRACE (GRADE II OR GRADE II* LISTED), ANY ALTERATION THAT AFFECTS THE CHARACTER OF THE BUILDING (INCLUDING INTERNAL ALTERATIONS THAT AFFECT ORIGINAL FABRIC) REQUIRES LISTED BUILDING CONSENT (LBC). SECONDARY GLAZING AND LISTED BUILDING CONSENT: IN MOST CASES, SECONDARY GLAZING IN A LISTED BUILDING IS CONSIDERED REVERSIBLE AND NON-DESTRUCTIVE TO THE EXISTING FABRIC — THE SECONDARY GLAZING FRAME IS TYPICALLY FIXED TO THE WINDOW REVEAL (NOT INTO THE ORIGINAL SASH BOX OR FRAME) AND CAN BE REMOVED WITHOUT DAMAGE. HOWEVER, SOME LISTED BUILDING CONSERVATION OFFICERS REQUIRE LBC FOR SECONDARY GLAZING IN A GRADE II* OR GRADE I LISTED BUILDING — ALWAYS SEEK PRE-APPLICATION ADVICE FROM THE LOCAL LONDON BOROUGH CONSERVATION OFFICER BEFORE INSTALLING SECONDARY GLAZING IN A LISTED BUILDING. ENGLISH HERITAGE (NOW HISTORIC ENGLAND) GUIDANCE: HISTORIC ENGLAND SUPPORTS SECONDARY GLAZING AS A SYMPATHETIC MEANS OF IMPROVING THERMAL AND ACOUSTIC PERFORMANCE OF LISTED BUILDINGS WITHOUT REPLACING ORIGINAL WINDOWS. SELECTAGLAZE AND SLIMLITE SECONDARY GLAZING HAVE BEEN USED IN NUMEROUS LONDON LISTED BUILDING PROJECTS WITH LISTED BUILDING CONSENT.

SECONDARY GLAZING INSTALLATION IN A LONDON VICTORIAN TERRACE SASH WINDOW IS TYPICALLY A HALF-DAY TASK PER WINDOW — WITH NO MAJOR DISRUPTION TO THE PROPERTY.

INSTALLATION METHOD: FRAME FIXING: THE SECONDARY GLAZING FRAME IS FIXED TO THE WINDOW REVEAL (THE INTERNAL FACE OF THE WINDOW OPENING — PLASTER OR TIMBER) USING STAINLESS STEEL WOOD SCREWS (REVEAL MUST BE SOUND — LOOSE OR CRUMBLING PLASTER REVEALS SHOULD BE MADE GOOD BEFORE SECONDARY GLAZING INSTALLATION). SEALING: THE PERIMETER OF THE SECONDARY GLAZING FRAME IS SEALED WITH A LOW-MODULUS SILICONE SEALANT (SOUDAL SILIRUB OR MAPEI MAPESEAL SILICONE) TO PREVENT AIR INFILTRATION AROUND THE FRAME — THIS IS CRITICAL FOR BOTH THERMAL AND ACOUSTIC PERFORMANCE. MEASURE AND ORDER: ALL REPUTABLE SECONDARY GLAZING SUPPLIERS (SLIMLITE, SELECTAGLAZE, REDDISEALS) PROVIDE A MEASURE AND QUOTE VISIT — THE WINDOWS ARE MEASURED IN SITU AND SECONDARY GLAZING UNITS MANUFACTURED TO ORDER (TYPICALLY 3-6 WEEK LEAD TIME).

SECONDARY GLAZING COSTS FOR LONDON VICTORIAN TERRACE SASH WINDOWS: SUPPLY AND INSTALL PER WINDOW (HORIZONTAL SLIDER, 6.4MM ACOUSTIC LAMINATE — SLIMLITE OR SELECTAGLAZE): SMALL WINDOW (UP TO 600MM X 900MM — TYPICAL BEDROOM): £350 TO £600. MEDIUM WINDOW (600MM X 1200MM — TYPICAL GROUND FLOOR RECEPTION): £500 TO £900. LARGE BAY WINDOW (3-PANEL — FRONT RECEPTION, APPROX 2500MM WIDE): £1,200 TO £2,500. FULL HOUSE (12-15 WINDOWS — LONDON 3-BED TERRACE): £6,000 TO £12,000 SUPPLY AND INSTALL. BUDGET ALTERNATIVES (POLYCARBONATE SECONDARY GLAZING — MAGNETGLAZE MAGNETIC SECONDARY GLAZING): MAGNETIC SECONDARY GLAZING (PLEXIGLASS OR POLYCARBONATE SHEET WITH MAGNETIC PERIMETER SEALS — DIY INSTALLATION): SUPPLY ONLY: £80 TO £200 PER WINDOW. ACOUSTIC PERFORMANCE: INFERIOR TO GLASS-BASED SYSTEMS (LOWER MASS, LESS EFFECTIVE ABOVE 250HZ). THERMAL PERFORMANCE: ADEQUATE FOR DRAUGHT REDUCTION AND BASIC THERMAL IMPROVEMENT — NOT SUITABLE FOR LISTED BUILDINGS.

THE FUNDAMENTAL CHOICE FOR IMPROVING WINDOW PERFORMANCE IN A LONDON VICTORIAN TERRACE IS BETWEEN: SECONDARY GLAZING (RETAIN ORIGINAL WINDOWS, ADD INNER SECONDARY UNIT) AND DOUBLE GLAZING REPLACEMENT (REPLACE ENTIRE ORIGINAL WINDOW WITH A NEW DOUBLE-GLAZED UNIT). FOR MOST LONDON VICTORIAN TERRACES OUTSIDE CONSERVATION AREAS OR LISTED BUILDING DESIGNATIONS, THIS IS A PRACTICAL AND COMMERCIAL DECISION. FOR PROPERTIES IN CONSERVATION AREAS OR WITH LISTED BUILDING STATUS, THE PLANNING AUTHORITY MAY RESTRICT OR REFUSE REPLACEMENT DOUBLE GLAZING — MAKING SECONDARY GLAZING THE ONLY VIABLE OPTION.

ACOUSTIC COMPARISON: SECONDARY GLAZING (6.4MM ACOUSTIC LAMINATE, 100MM AIR GAP): COMBINED RW WITH EXISTING SINGLE GLASS ≈ 40-45 DB — EXCELLENT ACOUSTIC PERFORMANCE. REPLACEMENT DOUBLE GLAZING (4-16-4 ARGON, LOW-E): RW ≈ 30-33 DB — SIGNIFICANTLY WORSE ACOUSTICALLY THAN SECONDARY GLAZING (THE NARROW CAVITY OF A STANDARD DOUBLE-GLAZED UNIT IS POOR AT ATTENUATING TRAFFIC AND RAIL NOISE IN THE FREQUENCY RANGE DOMINANT IN LONDON STREET ENVIRONMENTS). ACOUSTIC LAMINATE TRIPLE GLAZING (4-12-3.3.2-12-4 — PILKINGTON OPTIPHON COMBINATION): RW ≈ 40-45 DB — COMPARABLE TO SECONDARY GLAZING BUT AT SIGNIFICANTLY HIGHER COST (£600-1,500 PER WINDOW SUPPLY ONLY).

THERMAL COMPARISON: REPLACEMENT DOUBLE GLAZING (1.4 W/M²K — PART L 2021 TARGET): BETTER THERMAL PERFORMANCE THAN SECONDARY GLAZING ON EXISTING SINGLE GLAZING (1.8-2.5 W/M²K COMBINED). SECONDARY GLAZING ADVANTAGE: ALSO SEALS DRAUGHTS THROUGH THE EXISTING WINDOW — ENERGY SAVINGS IN PRACTICE ARE OFTEN CLOSE TO NEW DOUBLE-GLAZED REPLACEMENT WINDOWS (PARTICULARLY IN DRAUGHT-PRONE VICTORIAN SASH WINDOWS WHERE INFILTRATION IS SIGNIFICANT).

HERITAGE VALUE: THE ORIGINAL SASH WINDOWS OF A LONDON VICTORIAN TERRACE — PARTICULARLY THE ORIGINAL GLASS (WITH ITS CHARACTERISTIC SLIGHT DISTORTIONS) — HAVE IRREPLACEABLE HERITAGE VALUE. SECONDARY GLAZING PRESERVES THESE WINDOWS. REPLACEMENT DOUBLE GLAZING DESTROYS THEM. FOR A NON-LISTED LONDON VICTORIAN TERRACE, THE HOMEOWNER CAN MAKE THIS CHOICE FREELY — BUT MANY LONDON HOMEOWNERS REGRET REPLACING ORIGINAL SASHES WITH UPVC OR ALUMINIUM DOUBLE GLAZING ONCE THE HERITAGE LOSS IS APPARENT.