What double glazing existing sash windows can do to save money on replacement?

It’s been a while since I wrote on the site – this time I’ve taken a look at double glazing existing sash windows and how it can benefit over replacement sashes to save money – not just the price either, but also, it’s a waste to throw away decent joinery that is still useable. We’ve been double glazing existing sash windows for 24 years now and have hundreds of happy customers that have retained their original sash windows!

Not to mention the originals are beautifully handcrafted in most cases, over a hundred and twenty years ago! The timber quality in many cases supersedes that available today! Here’s a look at a window that we have used the double glazing existing sash window process on with bare timber:

Double glazing the existing sash that is bare timber

And a look at the same window from the outside:

Double glazing the existing sash window – a view from the outside of our installation that we made

In most cases from a few metres it is almost impossible to identify a window that has has the existing sashes double glazed. You need to look in at an angle to see the double glazed unit. Here’s a clearer example so you can see the double glazed unit:

A closer look into the sash so you can see the double glazing existing sash process

Not that you want to be able to see the double glazing – being well hidden improves the aesthetics considerably!

I have documented retrofitting sash windows with double glazing which is is pretty similar and covered the topic broadly in my double glazed sash window page as well as compare the performance and aesthetic differences of retrofitting double glazed sash windows vs replacement sash. However, I’ve never actually shown you the full process and benefits – I hope you can learn from this article and fully understand what you’ll get from double glazing existing sash windows, outlined in this write up! Let’s start with the benefits.

What is double glazing existing sash windows?

Simply put, double glazing existing sash windows is where we use your original, lovely sash windows that in most cases have been crafted over a hundred years ago to considerably improve the thermal efficiency of your home as well as reduce the noise pollution too.

We install the very same double glazed units into your original sashes that we use on new sashes – replacement sash windows have a higher efficiency but then cost considerably more and don’t often represent good value.

We use exactly the same units when double glazing existing sash windows as replacement sash

In any case, it’s painful to me throwing away the original sashes. And don’t get me wrong. Our new sash windows look far superior, no flaked paint, with a perfect sprayed finish, and all shiny, but it comes at a real cost, and I’m not just talking about the price. If you wanted a perfect house you would not have bought one with elegant, traditional period features. Our windows work great after we’ve completed the double glazing existing process. Here’s a quick example from Jo 🙂

A look at the difference between double glazing existing and new sashes

So why go to the lengths of entirely replacing sash windows, only to have to then make the rest of the house new in order to match that standard – there’s plenty of charm and character you can bring from sash window repair and refurbishment. In fact one job I did middle of last year for a nice chap, from roadside the difference is so marginal when you look at double glazing original windows vs replacing sashes (as outlined perfectly in the linked resource), it’s hard to stomach the additional cost financially as well as ripping out your old windows! Here’s a look at new sash sat right next to old sash with the existing sash windows double glazed:

New double glazed sash on the right vs double glazing the existing sashes on the left highlighted red and blue

I tell you what happened on that install – it was quite interesting. He was a bit of a perfectionist and if I’m honest, I looked up and I said, I reckon your originals look more charming than the new sashes. The new sashes feel a bit too sharp.

He said “nah he prefers the new ones but there wasn’t much in it”. I offered to get him a price for decorating those original sashes but he wanted to have a go himself and paint them – fair play, I love it when customers get stuck in, it’s not like we need the work (we have a good painter we recommend with 0 financial incentive our end), but he’s always well booked up in advance. Plus we don’t leave them in bad nick:

A sash window that has had the double glazing existing process complete and is ready for painting

But at the same time, we won’t change our friends and family only policy. There’s only a dozen of us (I list us off at the bottom and our relationships) but we always try to respond to every enquiry. So let’s look at the full process of double glazing existing sash windows.

How to double glaze existing sash windows

So with a little bit of background information as to what double glazing existing sash windows is, Here’s a complete rundown of the entire process from arriving to your property and what we need you to help us with, through to your windows installed with slimline or standard, 4-6-4 low-E, Argon filled, soft coat, Pilkington K double glazed units.

In some cases we can fit larger cavities as this will improve the acoustic performance a bit – note, heat wise there’s practically zero real term difference between a 4-6-4 and 4-12-4 unit. You won’t practically notice the difference, the only way to tell is a meter that literally shows the heat transfer/retention!

We arrive to your property early

We don’t mess around. Normally by 6:45 a.m we will be at your property and begin setting up quietly around 7 a.m. This is in preparation for a prompt 8 a.m start. By law we are not allowed to make work noise before. We will try to work with you to secure two parking spaces outside your property. The mobile workshop to perform double glazing existing sash windows effectively needs to be large enough to machine inside to avoid the rain. So our Lutons are about two car lengths long and very wide. Here’s a look at how big we are talking for the job:

Our van custom designed to double glaze existing sash windows

We need parking right outside for a few reasons. Firstly running power to the van is a problem at distance. Secondly walking heavy sashes with glass is a no go from a health and safety perspective. The other thing is we can keep a far better eye on your property between us (remember the windows are out). It’s sad I have to mention that but if you take the time to read my sash window security article you’ll realise I fully understand the implications of a break in! Both during and after the event. And on the security front (I’ll go into more details in the benefits section below) but for now I’ll say our double glazed sashes process, once complete, will improve security massively.

Laying dust sheets shortly after 7 a.m

We always try to gain access as early as possible. If you read our reviews you’ll see we don’t muck about we lay your dust sheets fast:

Laying dust sheets all the way through the house really doesn’t hurt

We ask you to remove your curtains and blinds – if we help, it’s at your risk – sorry to sound difficult. However, the word efficient and quick will pop up a lot. There’s some work we can do that doesn’t involve making noise. So we do as much of the prep that makes no noise pre 8 a.m as we can. We hope you can be up and ready to go anytime 7 a.m onward. It’s interesting actually, pre-lockdown people were asking us in at 6 a.m in the Summer 🙂 – now everyone is laying in. So we understand if you can’t do 7 but it is very much appreciated to get your double glazing existing process done as efficiently as possible for you.

Removing the original sash from the frame

At 8 a.m we take out your sashes, sometimes earlier if it can be done quietly. The sashes are designed to remove easily and quickly. If you read my draught proofing sash window page you’ll see just how easy it is. We will talk more about draught proofing below. So the staff bead comes off, lower sash removed, parting bead removed, and top sash cords cut. If windows aren’t painted shut terribly the process of removal is a couple of minutes. Here’s a look at sash removal:

Removing the sashes ready for double glazing the existing

Remove the old glass

Next we remove the old glass. This is a simply case of breaking it out as if we were going to re-glaze. Here’s a look at the glass being broken out. We also remove as much of the putty and nails as possible to protect our router cutter – the sharper the cutter, the cleaner the cut and the better the finish. In fact if you head into the van when we are machining (yes we welcome this and like you to see what’s going on), you’ll notice we have two routers, one for a rough sweep that takes the blows, and then a second cut that is for clear timber.

As you can see this is probably the most violent part of the process, maybe on par with the rebating, but that improves the aesthetics. More about that now.

Rebating the existing sash

So I mentioned above we use a two routers – this is because we want a sharp second cut. We use something called a rebate cutter. This cuts a square channel into your sash as you can see below. The before and after clearly show you what’s happening:

Sash before rebated:

Rebating the original sash to allow for a double glazed unit

Sash after rebated:

What your sash looks like once we’ve rebated the existing

At this point you might be thinking why on Earth are we giving away all our secrets to our competitors 😀 Well you’re right, it is kind of madness. But there’s more things going on here than first meet the eye. Firstly this is a very difficult job and most avoid it, Many companies that want to stay in a comfortable workshop making new joinery claim ‘it’s no good’ or ‘doesn’t work’. This is simply untruthful. My last 24 years experience have proved that!

I can take you to a hundred installs where people have saved a fortune and they look great. I think I’ve even linked to three installs already in this article alone explaining snippets of the process 🙂 We have the same unit failure rate on double glazed existing as we do new double glazed sash windows, which is 0.01%. And no I don’t mean 1%. I mean 1 in 10,000 fails before warranty of five years is up!

By showing our competitors the right way of doing this ensures even if you don’t get to use our service, you stand a better chance of a decent job elsewhere as a result of this information sharing. We aren’t the only people offering this service now – I was 24 years ago! Since I started it and have been documenting it (yes I am the first person on the internet to ever document the double glazing existing sash window process), many have now adopted it. I wrote about this nearly fifteen years ago when I still worked with my dad and the internet had 1 page sash window sites! That is the beauty of the internet and I hope it never changes! Sorry, back on track. Next up is unit installation.

Installing the double glazed units to the original sash

We install the units with an external grade sealant or low Butyl putty. This depends on the unit we install. As you can see the unit is bedded in sealant to the frame, creating a complete perimeter seal that completely stops water ingress. And this is part of why our installs are so successful. Water and air can’t reach the edge of the unit so it’s very hard for them to fail :). Here’s a look at the glazing being installed int the original sash:

Double glazed unit being installed into the original sash with glazing bead

You may have seen windows with misted up units. It happens more in plastic because the edges are exposed and the same applies to wooden windows where sealant isn’t used. They fail quicker.

Here’s a look at the sash fully glazed ready to go:

Here’s a sash that has been double glazed and painted ready to refit and complete the double glaze existing process

As you can see we finish them with a glazing bead. The very same Sapele hardwood we use on new sashes, if you decide this process isn’t for you. This helps to keep the install neat and tidy. Firstly putty takes ages to cure and is a security risk. Secondly when putting the sashes back fingers always end up in putty so need neatening again. Hard to do in situ when compared. The only time we finish in putty is when there’s a planning requirement.

Hanging your sashes and additional weight when cording up

We essentially double, and in some cases triple (depends on glazing spec) the weight of your original sash windows. So we need to install some lead additionally to counterbalance this. If we don’t, when you open your sash, it will just guillotine down. You may have this problem anyway 🙂

So we install the extra weight to stop this:

You’ll need extra lead to counterbalance the weight of the new heavier double glazed unit

Building and draught proofing your existing double glazed sash windows.

Now that we have corded up and the sashes ready. We install our sash window draught proofing system This is a series of brush piles. Don’t be tricked on this – it’s the same draught proofing system all reputable companies offer, they may jazz it up as their own, and that’s fine because it is their own, and they may have differences. But ultimately what you want from a draught proofing is a complete perimeter seal – without it, double glazing doesn’t work properly on sash windows. The number of times I get a call saying they’ve had new double glazed windows but it’s still noisy, given how readily available draught proofing is, still shocks me! Here’s a picture taken from my draught proofing page:

Series of brush piles are installed in the sashes and these are hidden when window refitted

Tidying up your window

Once the window is built and we’ve installed furniture, your window is now caulked and primed to cover bare patches. This is no way constitutes decoration, and not part of the quote, but we always try to leave windows looking sensible – you’ll need to decorate them yourself or engage our chap – we don’t earn from this, we just recommend him because he does sensible work at sensible money. Here’s a look at how a window typically looks after install and before final painting:

Sash windows that have had the existing double glazed and are ready for a final paint

As you can see from the picture above arches are no problem at all. It’s exactly the same process except we template the glass. Here’s a more standard example of what we leave behind:

Here’s another example of how we leave the windows after the double glazing existing process

The benefits of double glazing existing sash windows.

Now you’ve seen and understand the entire process, I just want to look at the benefits of double glazing the existing sash windows closely. Before we get into this, I just want to point out how nice it is to use windows skilfully handcrafted over a hundred years ago where possible. Please don’t forget that when considering throwing them away. Beyond sentiment lets look at the practical benefits:

Noise reduction

Firstly the noise reduction. It is genuinely noticeable. If you have well sealed single glazing the jump isn’t quite as much but still noticeable. The next question is always quantify this 😀 To which I say get your decibel app out and you’ll see the clear drop. But that means nothing. In real terms little cars that you could once feel in the room, will now be in the background and you’ll need to listen for them. You won’t stop the chug of a diesel engine or ambulance siren. But you do drone it out a fair bit.

You also need to factor I could make it so someone could do a running charge at your glass, smack it with a sledge hammer, and you still wouldn’t be able to hear it. Firstly, that’s pointless because you’ll sell a kidney, liver, and everything else to cover it, secondly. You’re limited by the insulation of your walls and roof too. So double glazing the existing sash has a nice and noticeable improvement on noise. That’s the on the ground, experienced truth.


In terms of security we always install toughened glass so far harder to break in. This has another benefit – you won’t wake up to a stone cracking your glass. So it’ll reduce the maintenance and glass breakage issues over the years, after the install is long forgotten.

The cost.

This saves a great deal of money on replacement sash. And as we’ve covered, they aren’t aesthetically as good as new windows, but they can still look nice and tidy – well in keeping with your traditional features.

So to sum up double glazing existing sash windows- if you like my ethos, think we can work together, (if you’ve read this far I know we are on the same wavelength 🙂 so give us a call or email – when we are busy response takes a while but we get there. Brian, Dan (Brian’s best mate), Samantha (my sister and Brians wife), Lauren (family friend), Natalie (Brian’s sister), or Georgia (Brian’s sister) will be the other end of the phone or email. We are a true family run business and only operate with family and friends on site too. Thanks for reading to this point – the next best place to go would be the contact us page or view our sash window prices. Or if you like a story or more interesting installs, I’d take a look at this. I loved the finish on this!