Secondary glazing is an additional window panel installed on the room side of an existing window. Unlike double glazing, which replaces your windows entirely with a sealed two-pane unit, secondary glazing leaves your original window untouched and adds a separate layer of glass or acrylic inside the room. Think of it as a second window fitted over the first, with an air gap between them that acts as insulation against heat loss and noise.
How It Differs From Double Glazing
The distinction comes down to construction. A double-glazed unit bonds two panes of glass together in a single frame, with the gap between them either vacuum-sealed or filled with an inert gas like argon. That sealed gap is what makes double glazing effective. Secondary glazing, by contrast, has nothing between the two windows except ordinary air. The original window stays in place, and the secondary panel sits separately on the interior side of the window reveal.
This air gap is actually much wider than what you get in a double-glazed unit. Standard double glazing has a gap of 6 to 20mm between panes. Secondary glazing typically creates a gap of 100 to 200mm, because the secondary panel sits at the depth of the window reveal or further into the room. That wider gap matters more than you might expect, particularly for soundproofing.
One practical trade-off: opening the window means dealing with two layers. You need to open the secondary panel first, then the original window behind it, to let fresh air in.
Thermal Performance
A single-pane window has a U-value (a measure of how much heat escapes through it) of roughly 5.5 W/(m²K). Adding a secondary glazing panel with its air layer cuts that to about 2.8 W/(m²K), halving the heat loss. That’s a significant improvement, especially in older homes where replacing windows isn’t practical or desirable.
The insulating effect comes almost entirely from the trapped air layer, not from the panel material itself. Whether you choose glass or acrylic for the secondary panel, the thermal difference is negligible. The air gap does the work by slowing the transfer of heat between the warm interior and the cold outer pane.
Noise Reduction
Soundproofing is where secondary glazing genuinely outperforms standard double glazing. Properly installed secondary glazing with acoustic-grade panels can reduce external noise by up to 54 decibels, equivalent to blocking about 80% of outside sound. Standard replacement double glazing typically achieves 28 to 36 decibels of reduction.
The reason is that wider air gap. Sound waves lose energy crossing larger air spaces, and the 100 to 200mm gap in secondary glazing is far more effective at dampening noise than the narrow 16mm gap inside a double-glazed unit. For homes near busy roads, motorways, or flight paths, secondary glazing is often the better acoustic solution, even compared to brand-new double-glazed windows.
Types of Secondary Glazing
Secondary glazing comes in several styles, each suited to different window types and budgets:
- Horizontal sliders use panels that slide left and right, similar to a sliding patio door. They work well on wide windows and allow easy access to the original window for cleaning or ventilation.
- Vertical sliders mimic the movement of a traditional sash window, sliding up and down. They’re the natural fit for period properties with existing sash windows.
- Hinged casement panels swing open on hinges like a small door. These give full access to the original window and suit casement-style frames.
- Lift-out panels are fixed in place and simply lifted out when you need access. They’re the most affordable option and work well for windows you rarely open.
- Magnetic panels attach using magnetic strips stuck to both the secondary panel and the window surround. You click the panel into place when needed and remove it in warmer months. This is the most flexible and least permanent option.
Glass vs. Acrylic Panels
You can choose either glass or acrylic (sometimes sold as plexiglass or perspex) for the secondary panel. Glass is heavier and more expensive but stays optically clear over time and resists scratching. Acrylic is lighter, cheaper, and easier to handle during installation, but it scratches and dulls more quickly. For permanent installations in living spaces where appearance matters, glass is the better long-term choice. Acrylic works well for temporary or seasonal setups, spare rooms, or magnetic lift-out systems where the panels are regularly handled.
Dealing With Condensation
Condensation between the original window and the secondary panel is the most common problem with secondary glazing, and it’s almost always caused by sealing the gap too tightly. If warm, moist room air gets trapped between the two layers with no way to escape, it hits the cold outer pane and forms water droplets.
The key is controlled ventilation. The original window should not be fully draught-sealed, because that creates a condensation trap. Instead, many secondary glazing systems include small trickle vents built into the frame. These allow a low level of background airflow within the window cavity, keeping the air between the panes dry enough to prevent fogging. A well-fitted secondary panel with an unsealed or vented primary window strikes the right balance: the inner panel provides the thermal and acoustic seal, while gentle airflow in the cavity prevents moisture from building up.
Cost Compared to Replacement Windows
Secondary glazing is significantly cheaper than ripping out existing windows and fitting new double-glazed units. Typical costs run 30 to 60% less than full replacement. As a rough benchmark, fitting secondary glazing across 10 windows might cost around £4,000, while replacing those same 10 windows with new double glazing could reach £10,000.
The savings go beyond the panels themselves. Installation is faster, causes less disruption, and doesn’t require exterior scaffolding or changes to the building’s facade. For renters or homeowners on a budget, magnetic or lift-out panels can bring costs down further, since they can be self-installed without specialist tools.
Listed Buildings and Heritage Properties
Secondary glazing is often the only realistic option for improving thermal and acoustic performance in listed or heritage buildings. Replacing original windows in a listed property typically requires listed building consent, and approval is frequently refused because it would alter the building’s historic character. Secondary glazing sidesteps this problem. It’s installed internally, preserves all original glazing bars, frames, and architectural details, and can be removed without causing any damage to the building fabric.
Listed building consent is still technically required for any alteration to a listed property, so you’ll need to check with your local planning authority. But because secondary glazing is reversible and has minimal visual impact (especially with slim-profile frames matched to the window color), it’s far more likely to be approved. It also offers a side benefit: by shielding the original windows from temperature extremes and condensation, it can actually extend the life of historic glass and timber frames.