Double glazed and vacuum glazed sash windows: your options and what to expect
Last updated: October 2026
Yes, sash windows can be double glazed, but not usually by putting double glazing into the sashes you already have. Original sashes are too slim to hold a standard double glazed unit. Your realistic options are new double glazed sashes made to fit your existing box frames, a complete new window, or vacuum glazing, which is thin enough to give single-glazed proportions with better-than-double-glazed warmth.
If your Victorian or Georgian sashes rattle in the wind, leave the rooms cold all winter and let in every bus and passing plane, this guide explains each option, how they compare, what is involved, and how to decide which suits your home.
Can sash windows be double glazed?
Most original London sash windows were made for a single sheet of glass a few millimetres thick, held in a shallow rebate by putty. Standard double glazed units vary from 14mm to 24mm thick, so they simply will not fit into an original sash, and the slender glazing bars on Georgian and Victorian windows are not deep enough to hold them.
Double glazed sashes are also heavier than single glazed ones. A sash window works because the sash is balanced by weights hidden inside the box frame, so the weights have to match the new sashes for the window to open and close smoothly.
That leaves three realistic routes to warmer, quieter windows:
- New double glazed sashes in your existing box frames (sash replacement)
- A complete new double glazed window, frame and sashes
- Vacuum glazing, a much thinner insulating glass that keeps traditional proportions, in either new sashes or a complete new window
Not sure whether your windows need repairing or replacing? Our repair or replace guide explains how to decide.
Your options at a glance
| Option | What we replace | Glass | Keeps your box frames? | Best for | Typical price per window (ex VAT) |
|---|---|---|---|---|---|
| Draught seal and overhaul | Nothing, we repair and seal | Your existing single glazing | Yes | Sound windows where budget is the priority | £400 to £700 |
| New double glazed sashes | The sashes only | 14mm to 24mm double glazing | Yes | Sound box frames, awkward surrounds | £1,000 to £1,600 |
| Complete replacement | Frame and sashes | 24mm double glazing | No | Rotten frames, or a change of design | £1,800 to £2,500 |
| Vacuum glazed sashes | The sashes, or the whole window | Vacuum glazing, around 8.3mm | Either | Slim glazing bars and the best thermal performance | Around £500 more than sash replacement or complete replacement |
Prices vary with size, glazing bars, access and finish. Our guide to double glazed sash window costs breaks this down in detail.
Option 1: new double glazed sashes in your existing box frames
For many London homes this is the best value route. We make new double glazed sashes by hand to fit your existing box frames, then remove the old sashes and fit the new ones on site.
Why your box frames can usually stay
The box frame is the first thing our surveyor checks, and it is usually in good condition. Around 85% of a typical box frame sits behind the brickwork: the frame is roughly 101mm deep, with only 15 to 16mm on show outside. That protects it from the weather, so frames often outlast their sashes by decades.
What is involved
- New sashes made in our East Sussex workshop with 14mm to 24mm toughened double glazed units: two 4mm panes either side of an argon-filled cavity. The thickness of the unit is determined by the thickness of the existing sashes and box frame.
- Full draught sealing, new cords and a complete set of ironmongery
- Your old sashes removed and taken away
- The existing frames repainted if you would like them to match
Keeping the frames also avoids disturbing what surrounds them. Deep window linings, window boards, a kitchen worktop running up to the cill or tiles in a bathroom can all add cost and risk to a complete replacement, but none of them needs touching when only the sashes are changed.
Case study: In a Victorian mansion block flat on Prince of Wales Drive, Battersea, we fitted 15 new double glazed sashes into the existing box frames. Deep linings, a kitchen worktop and marble tiles meant a complete replacement would have caused far more disruption. Read the Prince of Wales Drive case study.
Find out more about our sash window replacement service.
Option 2: complete replacement windows
If the box frame has rotted, or you want to change the design, a complete replacement is the right choice. We remove the whole window and fit a new handmade frame and double glazed sashes.
It involves more work on site than sash replacement, particularly around the linings and window boards, but you end up with an entirely new window throughout. Every complete replacement we fit is registered with FENSA, so you receive a certificate confirming it meets building regulations. Your solicitor will ask for it when you sell.
Vacuum glazing: what it is and when it makes sense
Vacuum glazing, sometimes sold as vacuum double glazing, is a different product from conventional double glazing. It uses two panes of glass, but instead of filling the gap between them with argon, the air is removed to create a vacuum less than 1mm wide. Heat struggles to cross a vacuum, so the glass insulates exceptionally well while staying around 8.3mm thick.
The benefits
- Slim, traditional proportions. Close to the thickness of single glazing, so sashes and glazing bars can keep their original slender look.
- Outstanding warmth. The vacuum glass we use is around eight times more efficient than single glazing and outperforms a 36mm triple glazed unit.
- More natural light than double or triple glazing.
- Lighter than triple glazing, at around half the weight.
- A 15-year guarantee.
The trade-offs
Vacuum glazing costs more than standard double glazing. Look very closely and you will also see a fine grid of tiny support pillars between the panes, which most people stop noticing within days.
When to choose it
Vacuum glazing is worth considering where slim glazing matters to the look of the house, on streets where the council expects windows to look as close to the originals as possible, or simply where you want the best thermal performance available.
Learn more about vacuum insulated glass, or see it in practice in our Wandsworth vacuum glass sash replacement and Maida Vale vacuum glazed sashes projects.
Other glazing choices for sash windows
Acoustic glass
Acoustic glass is laminated glass with an interlayer designed to reduce how much sound passes through it. It is popular in Putney, under the Heathrow flight path, and on busy roads such as those around New Cross. A typical acoustic unit combines a 6.8mm acoustic pane, a 14mm argon-filled cavity and a 4mm pane. See our Putney acoustic glass project.
Toughened and laminated safety glass
All of our double glazed units use toughened glass as standard, which is up to five times stronger than ordinary glass and breaks into small, blunt pieces rather than sharp shards. Laminated glass adds an interlayer that holds the glass together if it is broken.
Stained glass
Original stained glass can be restored and sealed inside a new double glazed unit, so you keep the feature and gain the warmth. Find out about stained glass encapsulation.
What about triple glazing?
We do not recommend triple glazing for traditional timber sash windows. A triple glazed unit is around 36mm thick and much heavier, which means bulkier sashes, heavier weights and a look that rarely suits a period home. Vacuum glazing beats a 36mm triple glazed unit thermally at a fraction of the thickness.
What about secondary glazing?
Secondary glazing is a separate inner pane fitted behind your existing window. It is a reversible way to cut heat loss and noise, which makes it a common choice where the windows themselves cannot be changed. The downsides are two sets of windows to open and clean, and a less traditional look from inside.
Will double glazed sashes change how my windows look?
Very little. We make every sash by hand to our standard profiles, which are designed with traditional stiles in mind, so the frames stay the same and the sash profiles are very similar. You may notice slight differences in the moulding on the inside and in the outside bead, because the sash has to hold a thicker unit.
Glazing bars need the most thought. How they are made with double glazing affects how authentic the window looks, and it is one of the main reasons people choose vacuum glazing. We will show you the options at your survey. Read about the difference between astragal and Georgian bars.
Planning permission and building regulations
- Houses outside a conservation area: replacing windows does not usually need planning permission.
- Flats and maisonettes: these do not have the same permitted development rights as houses, so check with your council. In a mansion block you will usually need your freeholder’s or managing agent’s consent under your lease too.
- Conservation areas: some councils restrict window changes, particularly where an Article 4 direction applies. Many accept double glazed timber sashes that match the originals, but always check with your local planning authority first. Our planning page explains how we can help.
- Listed buildings: almost any change to the windows needs listed building consent. We do not work on listed buildings.
What to expect: from survey to installation
- Enquiry and survey. Our surveyor visits, checks the condition of your box frames and sashes, measures up and talks through the glass options.
- Quote and choices. You receive a written quote. You are welcome to visit our showroom to see glass, glazing bars, paint finishes and ironmongery before deciding.
- Order and final measurements. Once you place an order, we take precise final measurements on site.
- Manufacture. Your sashes or windows are made by hand in our East Sussex workshop and spray finished. Manufacture typically takes 12 to 14 weeks.
- Installation. Our installers protect your home, remove the old sashes or windows, fit the new ones, draught seal and test every window. Depending on the service, we can usually complete one to three windows per day.
- Aftercare. You receive guidance on looking after your new windows, backed by our guarantee. Read our aftercare guidelines.
What does it cost?
As a guide, excluding VAT:
- Draught seal and overhaul (single glazing kept): £400 to £700 per window
- New double glazed sashes in existing frames: £1,000 to £1,600 per window
- Complete replacement with double glazing: £1,800 to £2,500 per window
- Vacuum glazing: around £500 extra per window
Extras such as decorating the existing frames, removing shutters or replacing pulley wheels add to the price. On the Battersea project, those extras brought the cost to around £1,700 to £1,800 per window.
For a full breakdown, read our double glazed sash window costs guide, or get an instant guide price with our online quote builder.
Which option is right for your home?
- Your box frames are sound and you want a warmer, quieter home: new double glazed sashes.
- Your frames are rotten, or you want a new design: complete replacement.
- Slim glazing matters, or you want the best thermal performance: vacuum glazing.
- You live on a busy road or under a flight path: add acoustic glass.
- You want to keep your single glazing for now: a draught seal and overhaul. Read our draught proofing guide.
FAQ
Can you double glaze existing sash windows?
Not usually. Original sashes are too slim to hold a 14mm to 24mm double glazed unit. The practical solution is new double glazed sashes made to fit your existing box frames, which keeps the frames and surrounds untouched. Vacuum glazing is thin enough to suit traditional sash proportions.
Can sash windows be double glazed in a conservation area?
Often, yes. Many London councils accept double glazed timber sashes that closely match the originals, but rules vary, especially where an Article 4 direction applies. Check with your local planning authority before ordering.
Is vacuum glazing better than double glazing?
Thermally, yes. The vacuum glass we use is around eight times more efficient than single glazing and outperforms 36mm triple glazing, while staying close to the thickness of single glazing. It costs more than standard double glazing, so it suits homes where slim proportions or maximum warmth matter most.
Are triple glazed sash windows worth it?
For traditional timber sash windows, we do not recommend them. Triple glazed units are around 36mm thick and much heavier, which means bulkier sashes and a look that rarely suits a period home. Vacuum glazing gives better thermal performance in a much slimmer unit.
Do double glazed sash windows look different?
Very little. Handmade sashes keep similar proportions and profiles to your originals, with only slight differences. The biggest visual decision is how the glazing bars are made, which we go through at survey.
Do double glazed sash windows add value to a property?
Well-made timber double glazed sashes can add to a period home’s appeal, because buyers value warmth, quiet and original character. A FENSA certificate for complete replacements also makes the sale more straightforward.
How long do double glazed sash windows last?
With routine care, including cleaning, touching up the paint and occasional servicing, modern double glazed timber sash windows can last well beyond 20 years.
Book a free survey
Our surveyor will check your windows, explain which option suits your home and give you a written quote with no obligation. Call us on 020 7924 7303, contact us online or get a guide price with our online quote builder.
