Secondary Glazing For Sash Windows

Sash windows are one of the defining features of period homes across the UK and a much-prized feature when they have been maintained in good condition. However, as good as they look, original single-glazed sash windows can be draughty, cold, and rattle in the frames if they haven't been taken care of, or reconditioned.

Replacing sash windows with uPVC casement windows could have a negative impact on the aesthetic and potential value of a property and is only advised where care and attention to detail is taken to replicate the original window. In most cases it's not an option for anyone living in a heritage home, or in a conservation area which most period properties fall under.

To improve energy efficiency of these beautiful but cold windows, secondary glazing is an effective way to warm up your rooms without any invasive measures. For most listed buildings that can't change their single-glazed windows, secondary glazing is the only option, but there is a vast range in the quality of these units that can also have a negative impact on the aesthetic of the property.

When fitted right, quality secondary glazing sits into its surrounding without drawing too much attention and acts as thermal and acoustic barrier that makes a genuine, measurable difference to comfort.

Too often customers overlook usability when buying secondary glazing and this can have a significant impact on the usability of your old windows if it is not considered.

How secondary glazing works with sash windows

Secondary glazing is installed on the inside reveal, behind the original window. The unit can be fitted against the existing window or on the edge of the reveal creating a wider gap. High thermal and acoustic performance of secondary glazing is achieved through the creating an optimal gap between the unit and the original window.

The frame for Everest secondary glazing has a slim aluminium profile in white and fitted with annealed glass. These details have a significant impact on the aesthetic in the room and are far superior to a cheaper plastic unit. From the inside, the frame should be discreet and unobtrusive in the room.

When fitted, a secondary glazed unit should echo the sightlines of the original window frame so that from the outside, the unit should be invisible. This means the opening frame of the secondary glazing should echo the horizontal bars of the sash window.

To clean the windows, the secondary glazed unit should open to allow access to the window behind.

To practically ventilate the room, the secondary unit sash slides open first and then the original window behind is opened. Both sashes can be opened at different positions to achieve either limited background ventilation or increased airflow.

Why standard secondary glazing doesn't work on sash windows

There is a detail with secondary glazing specifically for sash windows that is often overlooked and that is how each frame opens.

Standard casement windows open vertically outwards, hinged on the side. They also have vertical sightlines as the dominant format. Standard secondary glazing echoes this and slides horizontally, or can open as a hinged casement. 

To fit a standard configuration to a sash window means operationally and practically it is very awkward to use.

There's also the aesthetic. A horizontal sliding panel in front of a vertical sash window cuts across the natural sightline of the original design. This looks like a generic system fitted to a window it wasn't really designed for. Sash window secondary glazing slides vertically up and down to mirror the sash window and allow ease of use.

It's a common mistake and many cheaper secondary glazed units are fitted with a side sliding unit to a vertical sliding sash.  At Everest, we always match our secondary glazing systems to the design and style of the window for minimal visual intrusion and maximum practical use.

The benefits of secondary glazing for sash windows

Improved thermal efficiency

Old single-glazed sash windows can have a U-value of 4.8-5.8 W/m²K making them considerably poor at keeping heat in a room. Current building regulations require replacement windows to achieve a U-value of 1.4 W/m²K or better.

Secondary glazing, can reduce a single-glazed window's U-value to around 1.8 to 2.5 W/m²K depending on the specification. 

Significant noise reduction

This is where secondary glazing consistently outperforms standard double glazing. A double-glazed unit has a sealed air gap of 12mm to 28mm between the panes. Secondary glazing creates a much larger gap, and sound transmits poorly across a large air gap.

Quality secondary glazing, well fitted, is one of the best ways to soundproof your windows and for homes on busy roads, or in urban locations, this is often the most immediately noticeable benefit after installation.

Draught elimination

Timber sash windows expand and contract with temperature and humidity changes over many years. Gaps develop at the meeting rail, along the sides, and at the sill.

Secondary glazing seals the inside of the window, stopping cold air from entering the room regardless of what's happening in the original frame. The result is a noticeably warmer and more comfortable room in cold weather.

The original windows stay untouched

Secondary glazing doesn't require any changes to the existing window frame, glass, or external appearance. The installation is made entirely on the inside, using fixings into the window reveal rather than the frame itself. This makes it a fully reversible solution and can be removed without trace if required.

Because it can be removed and it doesn't impact the original windows is why it's an accepted solution for listed buildings. Where thermal efficiency needs to be improved, secondary glazing is recommended for conservation areas and heritage buildings.

What to look for in secondary glazing for sash windows

There is a lot of variance in the quality and construction of secondary glazing where the specification matters. These are the things worth checking before you make an order.

Vertical sliding operation. This is essential to retain the integrity of your sash window. Horizontal secondary glazing for a vertically sliding window has many issues (see above). Make sure you look for a unit that slides open in the same way as the sash window.

Slim, discreet frames. A secondary glazing frame sits inside your window reveal and will be visible from inside the room. Everest systems uses the narrowest aluminium frame profile that still provides a good seal. Thicker frames are cheaper to make, but they look more obvious than a quality system.

Quality of the seal. The seal between the secondary glazing frame and the window reveal determines how well the system performs in practice and a poor seal defeats most of the purpose. Made-to-measure systems, fitted by experienced installers like Everest, seal properly. Off-the-shelf panels fitted without a survey won't compete on energy performance.

Professional installation. Sash windows in period properties vary considerably with reveals and frames that have dropped over time and aren't square. A system that's cut to measure on site, by someone who has surveyed the window properly, will perform and look significantly better than one that isn't.

Why precision matters with sash windows

Secondary glazing on any window is only as good as the installation. But on a sash window, in a period property, where the original frame may have moved and settled over generations, the margin for error is much greater.

A poorly fitted system shows where the gap between the secondary frame and the reveal isn't even. The panel doesn't slide smoothly because the track isn't quite true and the seal isn't complete, so the thermal performance doesn't materialise as expected. In a room with original architectural detail, a frame that sits slightly off can have a significant negative impact on the overall aesthetic.

A full professional survey is essential to measure the opening, check the reveal depth, assess the condition of the original frame and confirm the operation of the existing sash. All of that informs how the system is designed and what will be needed to make it work properly.

Sash windows are precise units crafted to close tolerances, balanced carefully and the secondary glazing fitted to them should be treated with the same care.

Frequently Asked Questions

What is secondary glazing?

Secondary glazing is a separate, independently-operated pane of glass fitted inside your existing window. It doesn't replace the original, it works alongside it. The air gap between the two panes is what delivers the performance improvement as it slows the movement of heat, and interrupts the path sound takes to travel into the room.

For sash windows, secondary glazing needs to be designed around how those windows actually work. A sash window slides vertically. The lower sash raises, the upper sash lowers. The secondary glazing fitted inside it needs to do the same thing, sliding on its own vertical track so both the original window and the inner panel can be opened independently.

That's what sets secondary glazing for sash windows apart from a standard system. It's built around the window it's fitted to, not adapted from something designed for a different window type entirely.

Can you fit secondary glazing yourself?

Technically yes, DIY secondary glazing kits are widely available, typically consisting of a thin acrylic or polycarbonate sheet in a magnetic or clip-in frame. They're inexpensive and can be fitted in an afternoon with basic tools, but on a sash window, DIY secondary glazing tends to fall short where it matters.

Sash windows in period properties are rarely perfectly square where frames have moved, reveals have shifted after years of seasonal expansion and contraction. A secondary glazed kit made to basic measurements will not get a precise fit and won't seal fully at the edges. This will compromise thermal performance and still allow drafts around the edges.

A DIY panel is usually fixed or lift-out, not a true sliding unit. On a sash window, that means every time you want to open the window, you're removing the secondary panel entirely, not simply sliding it open.

For a listed building or a conservation area property, DIY systems raise an additional concern. The frame profile on a basic kit is rarely slim enough to look right inside a period window, and the overall finish won't meet the standard that a conservation officer would consider sympathetic.

Professionally fitted, made-to-measure secondary glazing costs more upfront. But it seals properly, operates as it should, and looks like it belongs. On a sash window in a period home, that difference is visible every time you walk into the room.