Published 22 September 2026 · Sash Window Draught Proofing
Secondary glazing is the single most effective upgrade available to owners of original sash windows who cannot, or do not want to, replace the historic joinery. It is an independent glazed unit fitted to the inside face of the existing window, creating a sealed second barrier against heat loss, noise and draughts while leaving the original timber, glass and external appearance completely untouched.
This guide explains how to choose a system: which glass, which frame, which opening style, what it costs in London in 2026, and when draught proofing alone is the smarter purchase.
How secondary glazing actually works
A sash window loses heat and admits noise through two separate routes. Air leaks through the gaps around the sliding sashes, meeting rails and beads. Heat and sound also pass straight through single-pane glass, which offers almost no resistance on its own.
Secondary glazing addresses both at once. The new internal pane stops the air path, and the large air gap between the two panes — typically 100mm to 150mm — decouples the panes acoustically. That gap is the important number. Sealed double glazing works with a 16mm to 20mm cavity, which is efficient thermally but poor at low frequencies. The much wider cavity in a secondary system is why it outperforms replacement double glazing on traffic noise, as explained in our piece on the science of the 150mm air gap.
Choosing your glass
The glass specification determines what problem you solve.
4mm float glass. The budget option. Adequate if your only concern is warmth and draughts in a quiet street. Adds very little acoustic benefit.
6.4mm laminated. A sensible middle specification. Better sound performance than float, plus improved security because laminated glass holds together when struck.
6.8mm to 10.8mm acoustic laminated (Stadip Silence or equivalent). The specification to choose on a busy road, a bus route or a flight path. The specialist interlayer damps vibration at exactly the frequencies where ordinary glass fails. Paired with a wide air gap, a properly specified system achieves 45dB to 54dB of noise reduction — the difference between a main road and a quiet library indoors. See our Chelsea townhouse case study for measured results.
Low-E coated glass. A microscopically thin metallic coating reflects radiant heat back into the room. Worth the modest extra cost wherever thermal performance is the priority, and it can be combined with laminated or acoustic glass.
Choosing your frame and opening style
Frames should follow the lines of the window behind them so the system disappears visually.
Vertical sliders mirror the action of the sash window itself and are the natural choice for the great majority of period properties. Both sashes remain usable for ventilation and cleaning.
Horizontal sliders suit wide windows and bays, where a vertical unit would be unreasonably heavy.
Hinged casements are best for smaller windows and for anywhere you need full, unobstructed access — often used behind shutters.
Lift-out and fixed panels are the cheapest and the most thermally efficient, but they are only appropriate for windows that are never opened.
Slimline aluminium is the standard frame material. Powder-coated to match existing joinery (RAL 9010 or 9016 in most London homes), the sightlines are narrow enough that the system reads as part of the window rather than an addition to it.
Secondary glazing vs draught proofing: which do you need?
These are not competing products; they are steps on the same ladder, and a large proportion of homeowners only need the first.
| Draught proofing | Secondary glazing | |
|---|---|---|
| Cost per window (London, 2026) | £150–£300 | £400–£800 |
| Heat-loss reduction | Up to 30% | Up to 70% |
| Noise reduction | Modest, removes rattle | Up to 50dB with acoustic glass |
| Typical payback | About 2 years | 5–8 years |
| Appearance | Invisible | Visible internally, unchanged outside |
| Reversible | Yes | Yes |
The practical rule: if your windows rattle, whistle and feel cold to sit beside, start with draught proofing. If you are still woken by traffic, or a north-facing room stays cold after sealing, add secondary glazing over the top. Our full comparison walks through the decision in more detail, and the cost guide gives room-by-room pricing.
Listed buildings and Conservation Areas
Secondary glazing is the conservation-approved route to thermal and acoustic improvement in heritage properties, and it is why so many Grade II listed owners choose it after being refused double glazing.
Three characteristics explain that acceptance. It is internal, so the external elevation is unchanged. It is reversible, so it can be removed without any loss of historic fabric. And it leaves the original glass — including irreplaceable crown and cylinder glass — in place. Historic England's own guidance treats reversible internal measures as preferable to the replacement of historic windows, and conservation officers across London follow the same logic.
Listed building consent is sometimes required for internal alterations, so confirm your position with the local authority before ordering. In practice, approval for slimline internal secondary glazing is routine. Our Grade II listed draught proofing guide covers the consent process, and Part L 2026 explains how the current regulations treat heritage assets.
What it costs in London in 2026
Prices reflect size, glass specification, frame configuration and access.
- Small casement or single-panel unit, 4mm glass: £350–£450
- Standard vertical slider, 6.4mm laminated: £450–£600
- Large vertical slider, 6.8mm acoustic laminated: £600–£750
- Bay window, three to five panels, acoustic glass: £1,800–£3,200
- Whole-house installation, eight to ten windows: £4,000–£7,000
Pair that against the likely saving. A typical London terrace with ten single-glazed sashes loses a substantial share of its heating through the windows; sealing and glazing them commonly reduces heating costs by 20% to 35%, on top of the comfort and noise benefits that do not show up on a bill. Our article on cutting heating bills without replacing windows sets out the arithmetic.
Installation: what to expect
A survey establishes the opening sizes, out-of-square tolerances, reveal depth and the position of shutters, architraves and window furniture. Period openings are rarely square, so every unit is made to measure.
Manufacture typically takes two to four weeks. Installation is quick and clean: one to two windows per hour for straightforward units, a full day for a bay. There is no wet trade, no scaffolding and no disturbance to the external elevation.
The sequence matters. Where both are being carried out, draught proofing and any sash cord replacement or timber repair should be completed first — the original window needs to be sound and correctly balanced before a second layer goes in front of it. Our restoration cost guide explains what that preparatory work involves.
Common mistakes to avoid
Specifying float glass on a noisy road. The most frequent and most expensive error. The frame and the fitting are identical; only the glass changes. Upgrading later means replacing the panes.
Narrowing the air gap to save space. Mounting the unit tight to the sash destroys the acoustic advantage. Fit the system on the reveal face wherever the depth allows.
Glazing over an unrepaired window. Rot, failed cords and split beads do not stop deteriorating behind a secondary unit; they simply become harder to reach.
Losing the shutters. In panelled London houses the secondary system must be planned around working shutters. A competent installer designs for them from the survey stage.
Sealing everything and ignoring ventilation. A well-sealed period room needs trickle ventilation or humidity control, otherwise moisture migrates to the coldest surface. Our Victorian repair mistakes article covers condensation management.
Is secondary glazing right for your property?
Choose secondary glazing if you live on a busy road, own a listed or Conservation Area property, have already draught proofed and still feel cold, or want the thermal performance of double glazing without altering a single external detail.
Start with draught proofing instead if your windows are simply draughty and rattling, noise is not a problem, or your budget is limited and you want the fastest return.
For most London period homes the best answer is both, in that order: restore and seal the original window, then add secondary glazing where noise or exposure demands it.
Frequently asked questions
Does secondary glazing work as well as double glazing?
For noise, it works better — the 100mm to 150mm air gap outperforms a 16mm sealed unit at the low frequencies produced by traffic. Thermally, a good secondary system with Low-E glass approaches the performance of modern double glazing while keeping the original window intact.
Can I still open my sash window with secondary glazing fitted?
Yes. Vertical sliding units are designed to align with your existing sashes, so you open the secondary panel and the primary window together for ventilation and cleaning.
Does secondary glazing need planning permission?
Not usually. It is an internal, reversible alteration, so planning permission is not normally required. Listed buildings may require listed building consent for internal work, so check with your local conservation officer first.
How much noise reduction can I realistically expect?
A well-specified acoustic system delivers 45dB to 54dB of reduction. On a road producing 75dB to 85dB, that brings indoor levels down to roughly 30dB — comparable to a quiet library.
Will secondary glazing cause condensation?
Correctly installed, it reduces condensation on the original glass because the inner pane stays warmer. Problems arise only when a unit is poorly sealed or a room has no ventilation strategy.
How long does secondary glazing last?
Powder-coated aluminium frames and laminated glass have a service life measured in decades. There are no sealed units to fail, which is a common failure mode of retrofitted double glazing in period frames.
Can secondary glazing be removed if I sell the house?
Yes. It is fixed to the reveal and lifts out, leaving only small fixing points. That reversibility is precisely why conservation officers accept it.
Arrange a free survey
We have specified and installed secondary glazing in London period properties for 15 years, from single sash windows to whole Grade II listed terraces. Contact us for a free survey, acoustic assessment and fixed quotation.