Two ways to get double glazing into a sash window
The first route keeps the existing timber box frame and changes only the glass inside it.
That means fitting slimline double glazed units into the original sashes, or adding a second window on the inside known as secondary glazing.
The second route replaces the whole sash window, frame and all, with one manufactured to be double glazed from the start.
We only carry out this second route, since it removes the guesswork of opening up decades-old timber.
Both end up being called double glazed sashes, though building one is nothing like building the other.
What decides which route is right for you
The same two routes apply to sash bay windows and other bay windows built with sliding sashes, though the extra corners make retrofitting more fiddly than on a single flat sash.
Both routes solve the same underlying problem: a single pane of glass doesn’t hold heat the way two panes with a sealed air gap between them do.
Which route makes sense for you depends on whether keeping the exact original timber frames matters more than the lower U-value and lower maintenance a full replacement gives you.
Retrofitting double glazing into timber sash windows
Traditional wooden sashes were built around a single pane of glass.
The glazing bar and the sash itself are often too narrow for a modern double glazed unit without alteration.
Most wooden windows built before the 1980s fall into this category.
Some joiners rebate the existing timber to install double glazed units as slimline as 10-12mm, against the 24-28mm of a standard pane.
Others recommend secondary glazing instead. This is a discreet second window fixed to the inside of the reveal that leaves the original sash untouched.
Retrofitting suits you if keeping the exact original glazing bars and old sash weights matters more than anything else.
Whether the original frame can take it
Retrofitting an original frame is specialist joinery work rather than a standard glazing job, closer to cabinet-making than fitting double glazed units into casement windows, which open on a simple hinge rather than a weighted box frame.
Box sash windows are named for that boxed frame, which conceals the counterweights on each side.
Not every original wooden box has enough depth to take even a slimline unit without the top sash and bottom sash being built up.
How the original timber holds up
Pine, the most common of the older sash window materials, moves and swells with damp more than the oak or cedar used in some grander properties.
That affects how well a rebated unit seals over time.
A heavier double glazed pane also asks more of the sash weights than the single pane they were sized for. The counterbalance sometimes needs adjusting or replacing alongside the glass, not just the glass on its own.
Retrofit double glazing needs checking every decade or so, since the rebate holding a slimline double glazing unit sits proud of where the single pane used to sit.
The paint or putty around it wears faster than the frame itself, so it needs ongoing maintenance to ensure it is functioning as it should.
Replacing with a new uPVC sash window
The alternative, and the one we install, is a new sash window manufactured in modern uPVC with double glazing built in as standard rather than added afterwards.
Instead of rebating old timber to squeeze in a unit it was never designed for, the window arrives already engineered around the glass.
A spiral balance mechanism does the job the old sash weights and cords used to do.
That standard double glazing meets Scottish Building Regulations at 1.4 W/m²K, and it can be tightened to 1.2 W/m²K if you’d rather have increased protection.
Installing the replacement is a straightforward swap, since the old timber frame simply comes out.
The new window is built to measure, so the depth of the old box, uPVC or timber, stops being a factor.
Will the windows look different afterwards?
Not if it’s specified properly.
A well-made uPVC sash window replicates the sash horns, the glazing bars (sometimes called muntins), and the slim sightlines of the original window.
Modern sash windows built this way can stand next to traditional sash windows on the same street without looking out of place.
Georgian grids and the plainer proportions of an Edwardian sash both get replicated too, whichever period the property was originally built in.
Listed buildings and conservation areas
A property in a conservation area or on the listed building register faces tighter rules on window replacement.
The answer changes street by street rather than following one national standard.
Full replacement windows in a conservation area sometimes need planning permission, depending on the local authority’s Article 4 direction.
A listed building typically needs listed building consent for any change to the window at all, including reglazing.
We can guide you on reputable timber window specialists if uPVC replacements are not permitted.
Energy efficiency and noise once it’s done
A single glazed sash loses heat through the glass constantly.
It lets draughts through wherever the meeting rail and the old sash cords have worn loose over the decades.
Either double glazing route closes that gap.
A full uPVC replacement generally reaches a lower U-value than a retrofitted slimline unit can, simply because it’s built as one sealed system rather than fitted around an existing frame’s limitations.
Triple glazing goes further again.
Our standard specification reaches 0.8 W/m²K across the whole window using a Soft Coat Xtra coating with argon gas between the panes of glass, which matters most on a north-facing sash or one that catches road noise as well as cold.
Noise reduction and insulation both come from the same sealed air gap doing its job, so you get a quieter room alongside the lower heating bill if that’s what you’re after.
Sash window costs: what affects the price
Retrofitting double glazing, secondary glazing, and full uPVC replacement each call for their own materials and their own labour, so pricing them the same way would be misleading.
Size, the number of damaged windows needing full replacement versus simple repair, and the finish colour all move the final figure too.
A property under listed building consent sometimes adds cost as well. This is because the conditions can specify a particular glazing bar profile or paint finish that takes longer to source and fit.
Rather than quote a number that won’t match your property, we survey first and follow up with a written quote.
Our team ensures that you know the real cost of replacing your windows before committing to anything.
0% finance is available with no deposit required, and every uPVC sash window installation carries a 15-year guarantee.
Double glazed sash windows: your questions answered
Can I convert my original sash windows to double glazed?
Yes, either by retrofitting slimline units or secondary glazing into the existing timber frame, or by replacing the window entirely with a new uPVC sash window built double glazed from the start. We only carry out the second option.
Do I need planning permission to replace sash windows?
It depends on whether your property is listed and whether the area has an Article 4 direction removing permitted development rights. We check this at survey rather than assume, since the position varies house to house.
How long do double glazed sash windows last?
A uPVC sash window with a sealed double glazed unit typically performs well for decades. The spiral balance mechanism is sealed inside the frame rather than exposed to weather, the way old sash cords were. Retrofitted units in a timber frame depend more on how well the original wood has been maintained.
Is it worth double glazing sash windows?
If your home is losing heat through single glazing, yes. The saving on energy bills and the reduction in draughts and outside noise add up over the years the window is in place, and a full uPVC replacement gets there with the least ongoing maintenance.
Will double glazing make my sash windows look different?
Not noticeably, provided the sash horns, glazing bars and proportions are specified to match the original. A poorly specified job, in either timber or uPVC, is what causes the visible mismatch you might be worried about.
Book your free survey for double glazed sash windows
Whether you want to install new double glazed sash windows across a whole house, or a single original frame just needs assessed, our free home survey covers the property and the planning position together.
Call us on 0800 050 2502 or request a free quote online.
We manufacture every uPVC sash window to the exact opening it’s replacing, backed by a 15-year guarantee and flexible 0% finance with nothing to pay upfront.
