Secondary GlazingThe Complete Guide
Secondary glazing is the most effective way to make single-glazed and period windows quieter and warmer — without replacing them. This guide explains what it is, how it works, what it costs, and how to choose the right system for your home.
What Is Secondary Glazing?
Secondary glazing is a slim, independent pane of glazing installed on the room side of your existing window. It does not replace your original glass — it works alongside it. The two panes trap a wide pocket of still air, and that air gap is what delivers the performance.
You may also see it called secondary double glazing, interior storm windows or simply inner windows. They all describe the same idea: a second, independently framed window on the inside, rather than a sealed double-glazed unit that replaces the original glass.
Because it is entirely internal, secondary glazing makes no change to the outside of your building. That single fact is why it is the preferred upgrade for listed buildings and homes in conservation areas, where replacing original windows is restricted.
The Key Benefits
- Dramatic noise reduction. The air gap absorbs sound energy, cutting traffic, aircraft and railway noise by up to 80%. With 10.8mm acoustic laminate glass you can achieve up to 54dB reduction.
- Warmer rooms, lower bills. Secondary glazing reduces heat loss through single-glazed windows by up to 65%, improving comfort and your U-values.
- No draughts or condensation. Sealed brush gaskets stop cold air and keep the inner pane warm, reducing condensation.
- Heritage-friendly and reversible. It preserves irreplaceable original windows while delivering modern performance.
Types of Secondary Glazing
The right style depends on how your existing window opens. The main options are:
- Vertical sliders — mirror traditional sash windows
- Horizontal sliders — ideal for wide openings
- Hinged casements — swing open for full access
- Lift-out units — removable, for seasonal use
- Fixed panels — slimmest and most affordable
For a side-by-side breakdown of every option, see our guide to the types of secondary glazing.
How Much Does It Cost?
Prices depend on window size, glass specification and how the unit opens. As a 2026 guide, standard units run £300–£800 per window, acoustic laminate £600–£1,200, and heritage grid-matched units £1,000–£2,200. For full per-window and whole-home pricing, see our secondary glazing cost guide — and see whether it is worth it for your home.
Secondary Glazing vs Double Glazing
The two are often confused, but they work very differently. Secondary glazing keeps your original window and adds a second internal pane, giving it the edge on noise and heritage suitability, while double glazing replaces the window entirely. See the full secondary glazing vs double glazing comparison.
Installation and Planning
Most installations are completed in 1–2 days with no external works. In the vast majority of cases no planning permission is required — read our guide to planning permission for secondary glazing. We serve period and listed homes across London, including Kensington, Westminster and Islington.
What Your EPC Rating Means
An Energy Performance Certificate rating is a standardised grade from A (most efficient) to G (least efficient) that measures a property's energy use and carbon emissions. Select a band to see what it means in practice.
Most unmodernised period homes with original single glazing sit in bands D, E, F.
Band E
SAP 39-54 · Below average
Noticeably inefficient, with high heating bills and rooms that are difficult to keep warm. Single glazing is a frequent cause.
- Typically
- Period homes with original single-glazed windows.
- Letting a property
- The current legal minimum for letting a property.
Windows are one of the largest sources of heat loss in a period home, and EPC assessors record glazing type directly. Secondary glazing is recognised as an energy efficiency improvement, though the effect on any individual rating depends on the property as a whole.
Band boundaries shown are the published SAP score ranges. Descriptions are indicative guidance only and are not a prediction of any property's rating. Only an accredited assessor can determine an official EPC.
Specialist Secondary Glazing Solutions
Explore our dedicated guides for the windows and problems we're asked about most.
Sash Window Secondary Glazing
Discreet vertical-sliding units that mirror your original sashes — the specialist choice for Georgian and Victorian homes.
Learn more →Bay Window Secondary Glazing
Bespoke multi-panel systems individually made to fit angled, square and curved bays without spoiling the sightlines.
Learn more →Noise Reduction Windows
Acoustic laminate glass and wide air gaps that cut traffic, rail and aircraft noise by up to 80%.
Learn more →Secondary Glazing for Listed Buildings
Fully reversible, consent-friendly installations that satisfy conservation officers while keeping historic fabric intact.
Learn more →Commercial Secondary Glazing
Acoustic and thermal upgrades for schools, hotels, offices and churches, phased around your hours in occupied buildings.
Learn more →Secondary Glazing: Frequently Asked Questions
The most common questions about how secondary glazing works, what it costs, and where it can be fitted.
Get a Free Secondary Glazing Survey
Every home is different. The simplest way to get an accurate, no-obligation quote is a free survey, where we measure your windows and recommend the right specification.
From the Journal
Start Here
The questions every first-time buyer of secondary glazing asks, answered in depth: cost, whether it is worth it, how long it lasts, and the mistakes period-property owners make.
- Technical GuideIs Secondary Glazing Worth It? Honest Pros, Cons and ROIAn honest look at whether secondary glazing is worth the money—covering noise reduction, energy savings, comfort, drawbacks, and return on investment.8 min read
- Technical GuideHow Long Does Secondary Glazing Last? Lifespan and MaintenanceHow long secondary glazing lasts, what affects its lifespan, and the simple maintenance that keeps it performing for decades.7 min read
- Thermal Performance5 Energy-Saving Mistakes Period Property Owners Make (And How to Avoid Them)Five common mistakes cost heritage homeowners comfort, money and character — and how to fix them the right way.5 min read
- Thermal PerformanceEPC Rating and Windows: What Landlords Need to Know for 2026Proposed EPC band C rules are reshaping how landlords prepare period rentals. How secondary glazing helps without replacing original windows.10 min read