What is Secondary Glazing & How Does it Work? Our Period Property Expert Explains

What are the material options for secondary glazing?

The frames of secondary glazing can be made from PVCu, aluminium or timber. Plastic frames are generally the most affordable option but tend to have a much shorter service life. Timber windows and higher-quality aluminium versions are typically bespoke, carefully designed to complement the style and character of the original windows.

The panes themselves can be made of either glass or polycarbonate. Polycarbonate is significantly cheaper and much lighter, which can be an important consideration, especially for larger or more awkwardly shaped windows. Glass secondary units, on the other hand, can be double glazed to provide very high levels of thermal performance, making them ideal for situations where maximising energy efficiency is the key priority.

How does a single pane installation work?

The most discreet secondary glazing comprises a single pane that covers the whole window. The aluminium frame has a magnetic strip that fixes to a matching band on the window frame. These can be powder coated so that they are indistinguishable from the window frame.

This solution can be ideal for historic windows, where minimal visual impact is a priority, as well as for large or awkwardly shaped windows. The pane can be removed for cleaning, but isn’t particularly convenient if you want to open the window. A full secondary glazing panel works particularly well if noise reduction is a key concern.

A close-up of a sliding and tilting secondary panel from the Roseview Incarnation collection

If the priority is opening the window in warm weather, a common approach is to fit the panes for the winter, leaving the windows closed, then remove them in the summer so they can then be opened at will.

If the panes need to be lifted relatively frequently, the reduced weight of polycarbonate can be a major benefit. Cheap DIY versions of this system are widely available using polycarbonate in plastic frames and less discreet fixing methods. They will give reasonable performance at significantly lower cost than bespoke systems.

What are the benefits of a secondary sliding window pane?

The budget end of secondary glazing features sliding panes with plastic or aluminium frames, available as off-the-shelf kits for DIY or fitting by a general builder. These can feature thin section frames because structural rigidity is provided by the outer frame in which they slide. This option is, however, usually the most visually intrusive because the panes often don’t align with the original window and the additional outer frame can be quite bulky.

The Incarnation secondary glazing system from Roseview. This is a full-height sliding unit

On the plus side, they can be effective at both improving thermal performance and reducing outside noise, and they allow relatively easy opening of windows. However, you would be unlikely to gain heritage consent for this type of glazing.

Good quality sliding panes can be made to mirror the opening function of the original window. The best bespoke versions will therefore complement the style and appearance of the existing fenestration, working particularly well with sliding sash windows.

How does casement-style secondary glazing work?

Casement-style secondary glazing opens on side mounted hinges, much like a casement window, with the significant difference that they must open inwards. Many old buildings have deep window reveals, where they can work particularly well. However, with standard reveals the open panels can project some way into the room.

They can be made of plastic, aluminium or timber. The best quality, and most likely to be suitable in a heritage situation, are bespoke timber frames that are designed to complement the original windows. Fitted within casement units, these can be the most discreet option.

John and Melanie Stott have transformed their Victorian terrace into a family home thanks to a renovation and extension that now offers open-plan living. Photo: David Barbour

A variation is to fit glazed shutters, which are very similar but have no central mullion. They reduce the visual impact of the glazing but increase the size of the units and therefore will project further into the room. There are a couple of potential solutions to this issue. The shutters can be bifold, though this both increases the cost and makes them more visually intrusive when closed. Alternatively, they can be fitted with demountable hinges, allowing the to be removed completely in the summer when opening is more frequent and heat loss no longer a concern.

How much does secondary glazing cost?

Costs start at about £100 per window for DIY single pane kits or £170 for sliding kits. Professionally fitted magnetic panes are about £500 per window. Bespoke units in timber or aluminium start from approximately £500 – but costs can rise considerably higher for complex or finely detailed heritage installations.