Sash Window Secondary Glazing

Why Choose Vertical Sliding Secondary Glazing?

Sash windows are designed to slide vertically, and our secondary glazing replicates this movement perfectly. Unlike fixed panels or hinged units, vertical sliding secondary glazing allows you to open both your original window and the secondary unit independently, maintaining natural ventilation exactly as intended by the original design.

Our systems use precision spiral balances (the same mechanism used in quality replacement sash windows) to ensure smooth, effortless operation year after year. The sashes can be tilted inward for easy cleaning of both the secondary glazing and your original windows - ideal for Victorian and Georgian properties.

Secondary Glazing Unit Types for Sash Windows

Glass Options for Sash Secondary Glazing

The unit type sets how the secondary glazing operates; the glass specification sets how it performs. Sash units are typically glazed with one of the following, selected against the dominant problem — traffic and rail noise, heat loss, or solar and UV gain.

Heavier acoustic laminates increase sash weight, so balance and interlock hardware are specified to suit. Compare the two most-requested acoustic build-ups in our 10.8mm vs 6.4mm performance comparison.

Performance Benefits

The Sash Window Challenge

Traditional sash windows present specific challenges for energy efficiency. The sliding mechanism requires gaps between the sashes and the frame, and over time these gaps can increase as the timber shrinks and the window settles. Additionally, original sash windows typically feature single glazing, providing minimal insulation - a common issue in Victorian and Georgian properties.

While draught-proofing can help seal the gaps around the sashes, it cannot address the fundamental problem of single glazing. Secondary glazing provides a comprehensive solution, creating an insulating barrier that addresses both draughts and thermal performance.

Planning Permission, Councils and Leasehold Buildings

Because secondary glazing is fitted entirely inside your property, it sits in a uniquely favourable position when it comes to approvals. In most cases you can proceed without any formal consent at all - but the rules differ depending on whether your home is listed, in a Conservation Area, or held on a lease.

Do I Need Planning Permission?

For the vast majority of houses and flats, no planning permission is required. Secondary glazing makes no change to the external appearance of the building, so it falls outside the scope of planning control - even where an Article 4 Direction has removed permitted development rights for replacement windows. This makes it the simplest upgrade route for period sash windows in tightly controlled streets.

Listed Buildings and Conservation Areas

If your property is listed, you will normally need Listed Building Consent (LBC) before any work affecting its character - although many councils now treat slim, reversible secondary glazing as a low-risk application that conservation officers routinely support. In a Conservation Area, secondary glazing is usually permitted precisely because the council's concern is the external street scene, which remains untouched. Where a sash window replacement would almost certainly be refused, secondary glazing is the solution officers recommend. Our Compliance Hub includes a borough-by-borough directory of London planning departments and a ready-to-send pre-application email template you can use to confirm the position with your local planning authority in writing before committing.

Leasehold Flats and Mansion Blocks

Leaseholders in London's mansion blocks and converted terraces face an additional layer of approval: the freeholder or managing agent. Most leases contain alteration clauses requiring licence to alter consent for works affecting windows - even internal ones. The good news is that secondary glazing is far easier to get approved than window replacement because it:

• Does not touch the external facade - the freeholder's primary concern in blocks with a uniform appearance covenant
• Is fully reversible - units can be removed without trace, satisfying reinstatement clauses
• Requires no structural work - no scaffold, no external access, minimal disruption to neighbours

We regularly supply drawings, method statements and product specifications directly to managing agents to support licence applications, and we can coordinate installation around building management requirements such as working hours and insurance certification. If your block is also listed or in a Conservation Area, we handle the combined consent process as a single package.

Installation Process

Our installation process is designed to be quick and non-disruptive. Most sash windows can be fitted with secondary glazing in under an hour, and there is no mess or decoration damage. The frames are fixed to your existing timber surrounds using discreet fixings that can be removed in the future if required - essential for listed buildings.

We always survey each window individually, taking precise measurements to ensure a perfect fit. Our vertical sliding units are manufactured to order and delivered ready for installation.