Sash Window Secondary Glazing
Vertical sliders engineered for traditional timber sashes
The definitive guide to secondary glazing for traditional timber sash windows. We specialize in Vertical Sliders that mirror your original window's operation. This allows for easy cleaning and ventilation while adding a massive layer of soundproofing. Our 2026 cost guide factors in the 'Balanced Vertical Slider'—the premium choice for heavy acoustic glass in large sash windows.
Standard vertical slider
Balanced slider + acoustic glass
Large bay sash windows
Why Vertical Sliders Are the Gold Standard for Sash Windows
A sash window slides vertically — so the secondary glazing should too. Horizontal sliders, hinged panels, or fixed units all compromise the character and functionality of the original window. Our vertical sliding secondary glazing replicates the exact motion of your sashes, maintaining the period aesthetic while creating a sealed air cavity that dramatically improves both thermal and acoustic performance.
The key advantage is operational harmony. Both the original sash and the secondary panel can be opened independently, allowing full ventilation in summer and airtight insulation in winter. For cleaning, the secondary panel tilts inward or lifts out entirely, giving unobstructed access to the original glass.
The Balanced Vertical Slider
Standard vertical sliders use simple friction stays to hold the panel in position. This works well for lightweight 4mm glass, but when you upgrade to 6.4mm acoustic laminate — which can weigh 16kg/m² — friction alone isn't enough. The Balanced Vertical Slider uses a concealed spring-balance mechanism (identical in principle to the original sash weights) that counterbalances the panel's weight, allowing effortless one-finger operation even with the heaviest acoustic glass.
Standard Slider
- Friction-stay mechanism
- Suitable for 4mm float glass
- Windows up to 1.5m tall
Balanced Slider
- Spring-balance counterweight
- Handles 6.4mm+ acoustic laminate
- Windows up to 3m tall
Key Benefits
Soundproofing
Up to 50dB noise reduction with acoustic glass and a 100mm+ air cavity. Transforms busy London streets into peaceful interiors.
Ventilation
Both sashes slide independently. Open the secondary panel for ventilation while the original sash stays closed, or vice versa — total flexibility.
Easy Cleaning
Tilt-in or lift-out access to the cavity between panels. Clean both the inside of the original glass and the secondary panel without scaffolding.
Find out what a vertical slider system would cost for your sash windows.