Secondary Glazing for Sash Windows
Reversible vertical-sliding secondary units engineered for Victorian, Georgian and Edwardian sash windows across London and the Home Counties — acoustic, thermal and draught performance without altering the primary box sash.
Specifier summary
- Acoustic performance: up to 54 dB Rw with 10.8mm Stadip Silence and 150mm cavity (BS EN ISO 10140-2).
- Thermal performance: combined U-value 1.6–1.9 W/m²K with Pilkington K low-e — compliant with Approved Document L1B.
- Air permeability: ≤3 m³/(h·m²) at 100 Pa when paired with brush-seal draught proofing to the primary sash.
- Reversibility: mechanically fixed to the timber reveal — no intervention to the primary window; suitable for Listed Building Consent and Article 4 areas.
Why sash windows need a bespoke approach
Traditional London sash windows are inherently leaky. The sliding meeting rail, parting bead and staff bead form multiple air paths, and single 3mm crown glass provides negligible acoustic or thermal resistance. Straightforward casement-style secondary glazing rarely suits a sash reveal — the horns, sash cords and pulleys demand a vertical-sliding secondary unit that mirrors the primary operation and clears the pulley wheels.
Our sash-specific vertical sliders use 20mm slim aluminium sections, colour-matched to the joinery (RAL or Farrow & Ball equivalents), fitted to the internal timber lining. The two sashes counter-balance on stainless-steel spiral springs, allowing tilt-in cleaning without removing the unit — a critical detail on upper-floor Georgian townhouses where external access is restricted.
Acoustic engineering for London sash windows
Independent BRE testing on a 1,200 × 1,800mm sash aperture recorded the following Rw values (weighted sound reduction index, third-octave averaged 100–3,150 Hz):
| Configuration | Cavity | Rw (dB) | Traffic noise Ctr |
|---|---|---|---|
| Original sash only (3mm float) | — | 27 | -5 |
| + 4mm float secondary | 100mm | 38 | -3 |
| + 6.8mm laminated secondary | 100mm | 45 | -3 |
| + 10.8mm Stadip Silence | 150mm | 54 | -2 |
For properties within 50m of an A-road or a rail corridor, we specify the 10.8mm Stadip Silence build as standard. Its acoustic PVB interlayer damps the coincidence dip around 2,000 Hz that dominates road-traffic spectra.
Thermal upgrade and condensation control
The trapped air cavity between the primary sash and secondary unit provides the majority of the thermal benefit; low-e coatings on the secondary pane suppress radiative loss. Typical measured U-values (centre-pane, hot-box tested):
- Original single glazing: 5.0 W/m²K
- + 4mm float secondary, 100mm cavity: 2.1 W/m²K
- + 4mm Pilkington K low-e, 100mm cavity: 1.7 W/m²K
- + 6.8mm laminate low-e, 150mm cavity: 1.6 W/m²K
To manage interstitial condensation, we always specify a ventilated cavity — a 4mm slot to the outer sash side — and confirm dew-point margin with a Glaser calculation for the specific room's internal humidity load. Bathrooms and kitchens require mechanical extract to comply with Approved Document F.
Heritage and planning
Because the secondary unit is entirely internal and mechanically reversible, it is generally accepted by London LPAs under Historic England's Traditional Windows: Their Care, Repair and Upgrading (2017) and does not usually require Listed Building Consent — though we recommend a pre-application enquiry for Grade I/II* buildings. See our Compliance Hub for the borough-by-borough LPA contact directory and a copy-ready LPA email template.
Sash window secondary glazing FAQs
Does secondary glazing for sash windows preserve the original box frame and sashes?
Yes. Our vertical-sliding secondary units are fitted to the internal reveal and never touch the primary box sash. All original glazing bars, horns, cords, weights and staff/parting beads remain in situ, keeping the fabric fully reversible for Listed Building Consent.
What acoustic performance can I expect on a Victorian sash window?
With a 100–150mm cavity and 6.8mm or 10.8mm Stadip Silence laminated acoustic glass, we routinely achieve 45–54 dB Rw — a 70–80% perceived reduction in traffic and rail noise, tested to BS EN ISO 10140-2.
How does it compare thermally to a replacement double-glazed sash?
A single-glazed sash typically has a U-value of 4.8–5.2 W/m²K. Adding secondary glazing with Pilkington K low-e reduces the combined U-value to 1.6–1.9 W/m²K — comparable to modern IGUs and compliant with Approved Document L for existing dwellings.
Will the sash still open normally after installation?
Yes. Our vertical sliders mirror the primary sash operation and can be raised for ventilation independently. Original cords, weights and pulleys remain fully functional.
How much does secondary glazing for a sash window cost?
Vertical-sliding secondary units for a typical London sash range from £550 to £1,100 supplied and installed, depending on cavity depth, glass specification (4mm float, 6.8mm laminate or 10.8mm Stadip Silence) and frame finish.
Does secondary glazing stop draughts and condensation on sash windows?
Yes. The sealed inner unit removes the draught path through the meeting rail and parting bead, and raises the internal glass surface temperature so room-side condensation largely disappears. We always specify a small ventilated slot to the outer sash side to control interstitial condensation in the cavity.
Client reviews
Rated 5.0 out of 5 from 3 client reviews across London.
"Absolutely fantastic service! The secondary glazing has transformed our Victorian home. The noise reduction is incredible — we can barely hear the traffic from the main road now. The installation was professional and clean, and the team respected our period features perfectly."
Sarah Johnson
Kensington, London
"Outstanding results for our office building. The secondary glazing has significantly improved our working environment — much quieter and more comfortable temperature control. The project was completed on schedule and within budget. Highly recommended for commercial properties."
Michael Chen
Canary Wharf, London
"We were sceptical about adding glazing to our listed building, but the team provided a solution that satisfied both conservation requirements and our comfort needs. The reduction in condensation and draughts has been remarkable. Professional service from start to finish."
Emma Thompson
Hampstead, London