Secondary Glazing: Cost, Performance & DIY Options

Secondary glazing adds a second pane of glass or acrylic to the inside of an existing window. Cheaper and less disruptive than replacement double glazing.

Last reviewed: 2026-06-27 • Reading time: 7 minutes • Verified against: 6 sources

Secondary glazing installs a second window pane (glass or acrylic) inside your existing window, creating an insulating air gap. Professional installation costs £150–£400 per window (vs £400–£600 for replacement double glazing). DIY kits cost £30–£100 per window. It improves single-glazed windows from U-value 5.0 W/m²K to 2.5–3.0 W/m²K (vs 1.2–1.6 W/m²K for double glazing). Noise reduction is 10–15 dB, better than double glazing for sound insulation. Legal for listed buildings where replacement windows aren't permitted.

Secondary Glazing vs Double Glazing: Quick Comparison

Feature	Secondary Glazing	Replacement Double Glazing
Cost per window	£150–£400 (professional) £30–£100 (DIY) 50–70% cheaper	£400–£600 Supply + fit
U-value (thermal)	2.5–3.0 W/m²K From 5.0 W/m²K single-glazed	1.2–1.6 W/m²K Better insulation
Heat loss reduction	40–50% vs single glazing	70–80% vs single glazing
Noise reduction	10–15 dB Better than double glazing (wider gap)	6–10 dB
Planning permission	Not required Internal alteration	Required for listed buildings Often refused
Installation disruption	Minimal Fitted inside, 1–2 hours per window	High Remove old frames, brickwork, 1 day+ per window
Lifespan	10–20 years	20–30 years
Best for	Listed buildings, sash windows, noise reduction, rental properties, budget-conscious	Replacement projects, new builds, maximum thermal performance

What Is Secondary Glazing?

Secondary glazing adds a second pane of glass or acrylic to the room-facing side of your existing window. It's mounted on the window frame or reveal (the recess around the window), creating a sealed air gap of 50–200mm between the original window and the secondary pane.

The air gap acts as insulation, reducing heat loss and blocking sound. Because secondary glazing doesn't replace the original window, it's legal for listed buildings and properties in conservation areas where planning restrictions apply.

Cost Breakdown

Professional installation: £150–£400 per window, depending on size, material (glass vs acrylic), and opening mechanism (fixed, sliding, hinged).

DIY kits: £30–£100 per window for magnetic or clip-in acrylic panels. You measure, order, and fit yourself.

Whole-house cost (3-bed semi, 12 windows): £1,800–£4,800 (professional) vs £360–£1,200 (DIY)
Replacement double glazing (same house): £4,800–£7,200
Payback time (energy savings): 10–20 years (vs single glazing)

Secondary glazing is 50–70% cheaper than replacement double glazing. The energy savings are lower (because U-value is worse), but the upfront cost is also much lower.

Thermal Performance

Secondary glazing improves single-glazed windows (U-value 5.0–6.0 W/m²K) to 2.5–3.0 W/m²K. This is a 40–50% reduction in heat loss through the window. Double glazing achieves 1.2–1.6 W/m²K, a 70–80% reduction.

The air gap width affects performance:

50–100mm gap: U-value ~3.0 W/m²K
100–200mm gap: U-value ~2.5 W/m²K (wider gap = better insulation)
Over 200mm: Convection currents reduce performance (wider is not always better)

For a typical 3-bedroom house, secondary glazing saves £150–£250/year on heating bills compared to single glazing (vs £250–£400/year for double glazing). The lower saving means longer payback, but the lower cost often makes it the right choice for budget-conscious projects.

Noise Reduction

Secondary glazing is better for noise reduction than replacement double glazing. The wider air gap (50–200mm vs 12–20mm in sealed double-glazed units) blocks more low-frequency noise.

Secondary glazing noise reduction: 10–15 dB (up to 20 dB with acoustic-grade glass)
Double glazing noise reduction: 6–10 dB
Perceived loudness: 10 dB reduction = sounds half as loud

For homes near busy roads, airports, or railways, secondary glazing can deliver better noise reduction at lower cost than acoustic double glazing.

DIY vs Professional Installation

DIY secondary glazing kits (£30–£100 per window) use thin acrylic sheets that attach with magnetic strips, adhesive frames, or snap-in clips. You measure the window, order the kit, cut the acrylic to size (if needed), and clip it in place.

Pros: Very cheap, removable (good for rental properties), no tools needed

Cons: Acrylic scratches easily, poor seal (air leakage reduces thermal performance), limited lifespan (5–10 years), aesthetically obvious

Professional secondary glazing (£150–£400 per window) uses 4–6mm glass in slim aluminium or uPVC frames. The frames are screwed to the window reveal or original frame, with brush seals or compression gaskets for airtightness. You can choose sliding, hinged, or lift-out panels.

Pros: Better thermal and acoustic performance, glass looks like a proper window, longer lifespan (15–20 years), proper sealing

Cons: More expensive, permanent fixture (not easily removed)

Listed Buildings and Conservation Areas

Replacing windows in a listed building requires listed building consent, which is often refused to protect the building's character. Secondary glazing is an internal alteration that doesn't change the external appearance, so it doesn't require consent in most cases.

Check with your local planning authority. Some conservation area restrictions may still apply, but secondary glazing is generally the only viable option for improving thermal and acoustic performance without altering historic windows.

Secondary Glazing for Sash Windows

Georgian and Victorian sash windows are often single-glazed and draughty. Replacing them with double-glazed sashes is expensive (£1,000–£2,000 per window) and may not be permitted in listed buildings.

Secondary glazing designed for sash windows uses vertical sliding panels that match the sash's operation. You can open the secondary panel and the original sash independently. This preserves the window's character while adding insulation and noise reduction.

Full cost breakdown: Secondary Glazing Cost →

Is Secondary Glazing Right for You?

Secondary glazing makes sense if:

You have single-glazed windows in a listed building or conservation area
You want to keep original windows (character, planning restrictions)
You need noise reduction (secondary glazing outperforms double glazing here)
You're on a tight budget (DIY kits are very cheap)
You're renting and can't replace windows (removable DIY kits)
You want minimal disruption (fitted in a few hours, no scaffolding or brickwork)

Consider replacement double glazing if:

Your existing windows are rotten or broken beyond repair
You want the best thermal performance (1.2–1.6 W/m²K vs 2.5–3.0 W/m²K)
You're doing a whole-house renovation anyway (marginal cost is lower)
You don't have planning restrictions

Related Guides

Sources

Energy Saving Trust, "Secondary Glazing" guidance (U-value and cost data, 2026).
Historic England, "Energy Efficiency and Historic Buildings: Secondary Glazing" (listed building guidance, 2024).
BRE, "Noise Control for Homes" (acoustic performance of secondary glazing, 2023).
Which?, "Secondary Glazing: Is It Worth It?" (cost comparison vs double glazing, 2025).
BFRC (British Fenestration Rating Council), "Secondary Glazing Performance" (U-value testing, 2025).
Selectaglaze (manufacturer), "Secondary Glazing for Listed Buildings" (case studies and planning context, 2024).

Last reviewed: 2026-06-27