Secondary glazing is often suggested when a property is cold, noisy, draughty, or stuck with original windows that cannot easily be replaced. We see it a lot in Kensington, Chelsea, Notting Hill and Westminster, especially in Victorian terraces, mansion blocks, converted flats and listed or conservation-area properties where changing the outside windows is not straightforward.
The main mistake is expecting secondary glazing to behave exactly like modern double glazing. It can make a big difference to road noise, heat loss and draughts, but only if the panels are measured properly, sealed sensibly and matched to the way the window is used. A cheap acrylic sheet stuck over a sash window may help a little, but it will not solve every winter problem.
Getting it wrong can waste a fair amount of money. A basic DIY magnetic panel might cost £80 to £250 per window, while properly fitted hinged or sliding secondary glazing commonly lands between £350 and £900 per window, and a large bay or tall period sash arrangement can run from £1,500 to £3,000 depending on access, glass type and frame condition. Before spending that, it is worth knowing where secondary glazing works well and where it does not.
Understand Why Secondary Glazing Helps Noise and Heat
Secondary glazing works by adding a separate pane inside the existing window, creating a still air gap between the original glass and the new panel. For noise, the gap matters. A deep cavity of 100mm or more, which is common with old sash window reveals, usually performs better than a tight 20mm gap because it separates the two panes more effectively.
For heat, the benefit comes from reducing draughts and slowing heat transfer through the window opening. Many London period windows lose more heat through gaps around the sash, staff beads and meeting rails than through the glass itself. Secondary glazing helps most when those gaps are controlled and the internal panel closes neatly against a continuous seal.
There are limits. If the outer frame is rotten, the sill is letting in rain, or the wall around the window is damp, secondary glazing will not cure the underlying building fault. It can even make condensation worse if moisture is trapped, so the existing window and room ventilation need checking before anything is fitted.
Check the Existing Window Before Choosing a System
Start with the window itself, not the secondary panel. Look for soft timber at the bottom rail, cracked putty, loose beads, failed paint, missing sash cord covers and gaps where daylight shows through. On many Edwardian and Victorian sashes we work on, the lower corners and sill joints tell you more than the glass does.
If the existing window is wet, rotten or moving in the frame, repair that first. A carpenter may need to splice in timber, refit beads, free a painted-shut sash or renew parting beads before secondary glazing is worth fitting. Our carpentry services are often involved on older windows for exactly this reason.
Also check how the room is ventilated. Under Part F of the Building Regulations, ventilation matters when work affects air movement, and older homes can become stuffy if every draught is sealed without thought. Secondary glazing should reduce unwanted draughts, not remove all fresh air from a bedroom or rented flat.
Choose the Right Type of Secondary Glazing
There are several types, and the right one depends on the window and how often it is opened. Magnetic panels are the cheapest option and can work for small, rarely opened windows, but they are not ideal for tall sashes or places where tenants need regular access. Expect DIY magnetic or clip-in systems to cost around £80 to £250 per average window before any making-good.
Hinged units suit casement windows, bathrooms and some sash windows where the whole panel needs to swing open for cleaning. Sliding aluminium systems are usually better for vertical sash windows because the secondary panel can move in a similar way to the original sashes. Professionally fitted hinged or sliding units are commonly £350 to £900 per window in London, more if the opening is large, out of square or needs special glass.
Lift-out panels can be tidy on small secondary windows, but we would not usually recommend them for large panes, high-level windows or awkward shopfront areas. Once glass becomes heavy, handling it becomes a safety issue. Toughened glass to BS EN 12150 or laminated safety glass to BS EN 14449 may be needed in low-level glazing, near doors or where impact risk is higher.
Measure the Reveal Accurately and Allow for Old Walls
Older London properties are rarely square. Measure the width at the top, middle and bottom, then the height on both sides and through the centre. A difference of 5mm to 12mm is not unusual in old plaster reveals, and that can be enough to make a panel bind or leave a visible gap.
Do not measure only the glass. Secondary glazing usually fixes to the internal reveal, the staff bead area or a sub-frame, so you need to know where screws can safely bite. In flats, especially mansion blocks and converted houses, there can be electrical cabling, alarm wiring or old shutter pockets near the window reveals, so drilling should not be treated casually.
For bay windows, measure each face separately and check the angles. A three-part bay in a Kensington terrace can need individual frames, trims and careful junction sealing. This is where professional measuring normally saves money, because one wrong bay panel is expensive to remake.
Prepare the Surrounding Frame and Finish First
Secondary glazing should not be fitted over flaky paint, loose plaster or damp timber. Strip back loose paint, fill obvious defects, sand rough edges and prime any exposed timber before the new frame goes in. If the room is being redecorated, do the messy repairs first and finish the final paint coats around the glazing after installation.
This is particularly important with sash windows, where decades of paint can stop the original window from working properly. If you fit secondary glazing before freeing or repairing the sash, you may lock in a problem that becomes awkward to reach later. We often combine small joinery and decorating work, and our decorating services can make the finished reveal look part of the room rather than an add-on.
In listed buildings or conservation areas, do not assume that internal work is always exempt from permission. Secondary glazing is often acceptable because it leaves the external appearance alone, but listed building consent may still be required if fixings affect historic fabric. If in doubt, check with the local authority before drilling into original joinery or stone surrounds.
Fit the Frame Square and Seal It Without Trapping Moisture
The secondary frame needs to be level, plumb and firmly fixed. Packing pieces may be needed where the reveal is uneven, and fixings should be tight enough to hold the frame without twisting it. A twisted track causes sliding panels to rattle, scrape or leave gaps, which defeats the point of the work.
Sealing should be controlled, not excessive. Use appropriate draught seals and neat internal sealant where needed, but leave the original outer window able to breathe and drain as intended. If an old sash already suffers from condensation on the inside face of the outer glass, the room may need better background ventilation as well as secondary glazing.
A proper installer will also check that panels can be removed or opened for cleaning and maintenance. This sounds obvious, but we have seen secondary units fixed so tightly that the original window cannot be painted, cleaned or repaired without dismantling half the room.
Test Noise, Draughts and Everyday Operation
Once fitted, test the window in real use. Open and close the original window, then the secondary unit, and check latches, locks, curtains, blinds and window boards. In bedrooms, make sure the arrangement is practical in the dark and that occupants can ventilate the room without a fight.
For noise, listen at the edges as well as the centre of the pane. Traffic noise often sneaks through small gaps around frames, trickle vents, poorly sealed floor voids and party walls, not only through the window glass. If a flat faces a busy road such as Cromwell Road or a bus route through Chelsea, secondary glazing can reduce the harshness, but it will not make the room completely silent.
For heat, check for draughts on a windy day using the back of your hand around the frame edges. If the wall below the sill is cold or damp, there may be a separate issue with pointing, flashing, guttering or internal condensation. Window work should not be used to hide a leak from above.
Plan Timing, Budget and Day-to-Day Use
Autumn is the sensible time to look at secondary glazing if you want the benefit before winter. Once the cold weather arrives, suppliers get busier and condensation problems become more obvious. For rental properties, it is best done between tenancies if the job involves sanding, painting or working in bedrooms.
Budget realistically. A single small bathroom window may be a few hundred pounds, but a front room with two tall sash windows and a bay can easily reach £2,000 to £4,000 once measuring, manufacturing, fitting and making-good are included. Special acoustic glass, slim frames, powder-coated finishes and awkward access all add to the price.
Think about who uses the room. A homeowner may be happy with a lift-out panel in a study, while a landlord needs something robust that tenants can operate without damaging it. Businesses with street-facing offices, clinics or shops often need easier daily ventilation and cleaning, so sliding or hinged systems are usually more practical than removable panels.
Call a professional rather than attempting DIY if the pane is large, the window is high, the frame is rotten, the property is listed, or the window may be part of a fire escape route. Also get advice if there are cables, radiators, shutters or built-in furniture tight to the reveal. Kensington Maintenance Services Ltd can measure, repair surrounds, fit secondary glazing and make good the finish, so if you want a clear price before committing you can get a quote from the team.
Frequently Asked Questions
Q1: Does secondary glazing stop traffic noise?
It can reduce traffic noise noticeably, especially where there is a decent air gap between the original glass and the new pane. It works best when the frame edges are well sealed and the noise is mainly coming through the window, not through walls, vents or floors.
Q2: Is secondary glazing cheaper than double glazing?
Usually, yes, especially in period properties where replacing windows requires bespoke timber units or planning consent. A fitted secondary unit might cost £350 to £900 per window, while full replacement sash windows can cost considerably more.
Q3: Will secondary glazing cause condensation?
It should not if the room has suitable ventilation and the original window is sound. Condensation problems often appear when warm, moist air is trapped, so bathrooms, bedrooms and kitchens need particular care.
Q4: Can secondary glazing be fitted in a listed building?
Often it can, because it is internal and usually keeps the external appearance unchanged. Listed building consent may still be needed if the work affects original fabric, so check before fixing into historic frames or surrounds.
Q5: Is DIY secondary glazing worth trying?
DIY kits can be worthwhile for small, simple windows where appearance and daily use are not critical. For large sashes, bays, rental properties or safety glass areas, professional measuring and fitting is usually the better value.
The plain answer is that secondary glazing is worth it when the existing window is basically sound, the right system is chosen, and the fitting is accurate. It is not a magic cure for rotten frames, damp walls or poor ventilation, but done properly it can make a London room warmer, quieter and easier to live with. If you want us to look at your windows, advise on the sensible options and price the work properly, contact Kensington Maintenance Services Ltd through our contact page and one of our team will talk you through the next step.