Double Glazing for London Victorian Terraces: Replacing Windows and Meeting Conservation Requirements

STANDARD UPVC DOUBLE-GLAZED WINDOWS — NOT RECOMMENDED FOR MOST LONDON VICTORIAN TERRACES: STANDARD UPVC CASEMENT OR SASH REPLACEMENT WINDOWS ACHIEVE GOOD THERMAL PERFORMANCE (A-RATED DOUBLE GLAZING — U-VALUE TYPICALLY 1.2-1.6 W/M²K FOR THE WHOLE UNIT) BUT ARE ALMOST UNIVERSALLY INAPPROPRIATE FOR A LONDON VICTORIAN TERRACE: - PROFILE WIDTH: UPVC FRAMES HAVE A FAR WIDER VISIBLE SECTION PROFILE THAN TRADITIONAL TIMBER SASH FRAMES. ORIGINAL VICTORIAN SASH WINDOWS HAVE A 32-50MM VISIBLE STILE AND RAIL WIDTH; A STANDARD UPVC REPLACEMENT WINDOW HAS A 70-80MM PROFILE — VISUALLY COMPLETELY DIFFERENT AND OBVIOUSLY ANACHRONISTIC. - SASH MOVEMENT: UPVC "SASH WINDOWS" USE A SPRING BALANCE MECHANISM RATHER THAN TRADITIONAL WEIGHTS AND PULLEYS — THE AESTHETIC AND TACTILE EXPERIENCE IS COMPLETELY DIFFERENT FROM A TRADITIONAL WEIGHTED SASH. - PLANNING: REPLACEMENT OF WINDOWS IN A CONSERVATION AREA (THE MAJORITY OF LONDON VICTORIAN TERRACES ARE IN CONSERVATION AREAS) WITH UPVC IS ALMOST ALWAYS REFUSED PLANNING PERMISSION. EVEN OUTSIDE CONSERVATION AREAS, ON A LISTED BUILDING THE CHANGE IS NOT PERMITTED WITHOUT LISTED BUILDING CONSENT.

TIMBER SLIM-LINE DOUBLE GLAZING (THE PREFERRED PERIOD-APPROPRIATE APPROACH): SEVERAL MANUFACTURERS PRODUCE SLIM-LINE DOUBLE GLAZED UNITS DESIGNED TO FIT INTO ORIGINAL OR REPLICA TIMBER SASH WINDOW FRAMES: - SLIM UNIT REPLACEMENT (RETAINING ORIGINAL SASHES): THE ORIGINAL HARDWOOD OR SOFTWOOD SASH IS MODIFIED — THE ORIGINAL SINGLE PANE IS REMOVED; THE REBATE DEPTH IS INCREASED (BY CUTTING A DEEPER REBATE — ROUTING THE EXISTING REBATE OR REPLACING THE GLAZING BEAD WITH A DEEPER PROFILE); A SLIM-LINE DOUBLE GLAZED UNIT IS FITTED (6-10MM OVERALL THICKNESS — E.G., 4MM GLASS + 2-4MM ARGON GAP + 4MM GLASS). THE SLIM UNIT HAS A U-VALUE OF APPROXIMATELY 2.4-3.0 W/M²K (BETTER THAN SINGLE GLASS AT 5.0, BUT NOT AS GOOD AS A STANDARD 28MM DOUBLE GLAZED UNIT AT 1.2). - NEW PERIOD-STYLE DOUBLE GLAZED TIMBER SASH WINDOWS (REPLACEMENT WINDOWS): FULL REPLACEMENT WITH A SLIMLINE DOUBLE GLAZED TIMBER SASH WINDOW (SLIMLITE, FINEO, HISTOGLASS — VARIOUS MANUFACTURER OPTIONS). THESE WINDOWS ARE MADE TO MATCH THE ORIGINAL PROFILE (THINNED FRAME WITH 28MM OR SLIM UNIT DOUBLE GLAZING) AND SATISFY MOST CONSERVATION OFFICERS. - VACUUM GLAZING (PILKINGTON SPACIA, FINEO): THE MOST THERMALLY ADVANCED SLIM OPTION — A VACUUM BETWEEN TWO 4MM GLASS PANES (TOTAL THICKNESS 8-9MM); U-VALUE APPROXIMATELY 0.7-1.1 W/M²K — EQUIVALENT TO MODERN TRIPLE GLAZING IN A UNIT THAT FITS A TRADITIONAL SINGLE-GLAZING REBATE DEPTH. HIGHLY EXPENSIVE (VACUUM UNITS ARE APPROXIMATELY £300-500 PER M² — APPROXIMATELY 3-5× THE COST OF A STANDARD IGU). BUT THEY ELIMINATE THE REBATE DEPTH MODIFICATION AND ARE THE MOST THERMALLY EFFECTIVE SLIM OPTION.

SECONDARY GLAZING (THE MOST CONSERVATION-AREA-FRIENDLY OPTION): SECONDARY GLAZING IS A SECOND LAYER OF GLAZING FITTED INSIDE THE ORIGINAL WINDOW REVEAL — AN INNER PANE OF GLASS OR POLYCARBONATE IS FITTED IN A SLIM ALUMINIUM FRAME WITHIN THE ROOM SIDE OF THE REVEAL, LEAVING THE EXTERNAL WINDOW VISIBLE AND UNALTERED: - CONSERVATION BENEFIT: SECONDARY GLAZING DOES NOT ALTER THE EXTERNAL APPEARANCE OF THE BUILDING — THE ORIGINAL SASH IS RETAINED AND UNCHANGED. PLANNING PERMISSION IS NOT REQUIRED IN MOST CASES (INCLUDING CONSERVATION AREAS). - THERMAL BENEFIT: SECONDARY GLAZING CREATES A LARGE AIR CAVITY (100-200MM) BETWEEN THE ORIGINAL WINDOW AND THE INNER PANE — THE LARGE AIR CAVITY HAS A HIGH THERMAL RESISTANCE (APPROXIMATELY 0.5 M²K/W). COMBINED WITH THE ORIGINAL SINGLE PANE AND THE SECONDARY PANE, THE WHOLE UNIT U-VALUE IS APPROXIMATELY 1.8-2.2 W/M²K — SIMILAR TO STANDARD DOUBLE GLAZING. - ACOUSTIC BENEFIT: THE LARGE AIR CAVITY OF SECONDARY GLAZING IS SIGNIFICANTLY BETTER AT ATTENUATING SOUND THAN STANDARD DOUBLE GLAZING (WHOSE 16-20MM CAVITY IS TOO NARROW FOR MAXIMUM ACOUSTIC ATTENUATION). SECONDARY GLAZING WITH A 100MM+ CAVITY ACHIEVES APPROXIMATELY 39-44DB OF ACOUSTIC ATTENUATION — USEFUL FOR LONDON VICTORIAN TERRACES NEAR BUSY ROADS, RAILWAYS, OR FLIGHT PATHS. - SYSTEMS: SELECTAGLAZE (THE LEADING UK SECONDARY GLAZING SPECIALIST — USED ON HISTORIC BUILDINGS INCLUDING THE PALACE OF WESTMINSTER), SECONDARY GLAZING COMPANY, RENSON EZEE. FRAMES IN SLIM ALUMINIUM — WHITE OR CLEAR ANODISED.

DRAUGHT-PROOFING SASH WINDOWS — OFTEN OVERLOOKED BUT VERY EFFECTIVE: BEFORE INVESTING IN ANY GLAZING UPGRADE, DRAUGHT-PROOFING THE EXISTING SASH WINDOWS IS THE MOST COST-EFFECTIVE IMPROVEMENT: - THE PROBLEM: ORIGINAL VICTORIAN SASH WINDOWS HAVE NO DRAUGHT SEALS — AIR PASSES FREELY AROUND THE SASHES (TOP RAIL, MEETING RAIL, BOTTOM RAIL, SIDE STILES). THE DRAUGHT ACCOUNTS FOR APPROXIMATELY 20-40% OF THE WINDOW\'S TOTAL HEAT LOSS IN A LONDON WINTER (THE REST IS CONDUCTIVE HEAT LOSS THROUGH THE GLASS — WHICH ONLY GLAZING REPLACEMENT ADDRESSES). - DRAUGHT-PROOFING SYSTEMS: SASH WINDOW DRAUGHT-PROOFING INVOLVES ROUTING GROOVES INTO THE PARTING BEAD AND THE STAFF BEAD AND INSERTING BRUSH PILE SEALS (POLYPILE WOOLPILE, SCHLEGEL Q-LON) — A SPECIALIST SASH WINDOW COMPANY (SASH WINDOWS LONDON, VENTROLLA, SASH HERITAGE) DOES THIS AS A STANDARD SERVICE. A FULL DRAUGHT-PROOFING OF A FOUR-WINDOW VICTORIAN TERRACE (FRONT ELEVATION + REAR) TYPICALLY COSTS £600-1,500. - BENEFIT: DRAUGHT-PROOFING REDUCES AIR INFILTRATION TO NEAR ZERO AND CAN REDUCE THE WINDOW\'S EFFECTIVE U-VALUE (FACTORING IN INFILTRATION LOSSES) BY 10-20%. IT IS ALSO THE MINIMUM SPEC MOST CONSERVATION OFFICERS REQUIRE BEFORE THEY WILL CONSIDER GLAZING REPLACEMENT AS A JUSTIFIED INTERVENTION.

PLANNING FOR DOUBLE GLAZING IN LONDON VICTORIAN TERRACES: - CONSERVATION AREA: REPLACING WINDOWS IN A CONSERVATION AREA REQUIRES PLANNING PERMISSION. CONSERVATION OFFICERS TYPICALLY ACCEPT: SLIM-LINE DOUBLE GLAZED TIMBER SASH (MATCHING ORIGINAL PROFILE AND DIMENSIONS), VACUUM GLAZING IN THE ORIGINAL FRAME, AND SECONDARY GLAZING (NO PLANNING REQUIRED). THEY TYPICALLY REFUSE: UPVC REPLACEMENT, WIDE-FRAME DOUBLE GLAZED UNITS, AND CASEMENTS REPLACING SASHES. - ARTICLE 4 DIRECTION: SOME LONDON CONSERVATION AREAS HAVE AN ARTICLE 4 DIRECTION THAT REMOVES PERMITTED DEVELOPMENT RIGHTS FOR WINDOW REPLACEMENT — EVEN LIKE-FOR-LIKE REPLACEMENT MAY REQUIRE PLANNING PERMISSION. CHECK WITH THE LOCAL PLANNING AUTHORITY.

COSTS (SUPPLY AND FIT, LONDON 2025-2026): DRAUGHT-PROOFING (PER SASH WINDOW — BRUSH PILE SEALS, SUPPLY AND FIT): £150-350 SLIM-LINE DOUBLE GLAZING UNIT IN EXISTING SASH FRAME (PER WINDOW, SUPPLY AND FIT): £400-900 NEW PERIOD-STYLE SLIM DOUBLE GLAZED TIMBER SASH WINDOW (PER WINDOW, SUPPLY AND FIT): £900-2,500 VACUUM GLAZING UNIT (PILKINGTON SPACIA — PER M² OF GLASS): £300-500/M² SECONDARY GLAZING (PER WINDOW, SUPPLY AND FIT — SELECTAGLAZE OR EQUIVALENT): £500-1,200