Double Glazing

Today we will look at double glazing existing sash windows. Firstly before we go anywhere with our tools, we must use our tape measure. To double glaze an existing sash we require at least an inch and a half sash. For a beginner it might be worth starting on inch and three quarter thick sashes because the tolerances at 38mm are 2-3mm only.

If your sashes are 40mm + then remove them, cutting cords, and removing from frame. The old cast iron weights are no longer of use because we require heavier, more dense material to maintain full and smooth travel of the window when rebuilt.

Sashes on workbench, carefully remove the original glass. It pays to be careful and gentle here. Slowly break the glass up and throwing it away into a bin as you go. The last thing you want is to be working in a pile of broken glass! With glass removed we need to hack out the old putty. Putty will dull the edge on your router cutter too quickly and since a specialised rebate cutter costs nearly £100 it is important to look after it well.

So now the putty is neatly removed we are ready to start the process of double glazing our existing sashes.

Firstly take your half inch router, and the specialised rebate cutter. Then cut to a depth of 20mm. This will allow for a (4-6-4) double glazed units. Run your router around the inside edge to deepen the rebate. The corners will need neatly squaring up. Once cut back, with a quality silicone bed the new sealed unit into the old sash, and with a 6mm timber glazing bead finish the glazing process. It’s important to use a quality sealant here, we don’t want the double glazed unit breaking down.

Now you’ve double glazed the existing sashes it’s time to counterbalance and re-cord appropriately. Weigh out the sash and lead weight required. Then record with the correct balance. Rebuild the window and admire. Professionals will always install a draught proofing system with the double glazing as it is pointless in terms of energy efficiency to do one without the other and is part of any typical sash window refurbishment programme. After you’ve rebuilt the window then you will want to decorate the sash internally and externally. Make sure that prior to decoration there is a week of dry weather. This will ensure the moisture content in the timber is low enough that a quality paint like Dulux Weathershield will last well. Basic sand preparation will be fine, enough to key the original timber. No special requirements from decoration to produce a brilliant finish.

Here’s a little look inside a double glazed original sash window. As you can see the unit is hidden nicely and from straight on the window would certainly not appear double glazed which is exactly how it should be.

Double glazed existing sashes London

A look inside a single glazed sash window that has been rebated to allow for double glazed unit. Image credit to

Perhaps this is a little beyond the level you’d like to go to. Why not give these double glazing existing sash windows experts a call and let them take care of the process for you. They’ll know all the pitfalls and all the tricks to make your windows secure, safe, warm, and draught free. Prices range from £500 upwards and include new window furniture, new timber beads, five year guarantee, and a reliable service from a market leader.


  1. Does this really work? It seems like a considerable saving. Has anyone else ever double glazed the existing sashes in this way?

    • Yes I think it works brilliantly but I had all my top sashes sealed in as well to reduce noise through the gaps. The cost was around £600 per window + decoration and I would definitely do it again but it all comes down to are your sashes thick enough to handle it.

      • Hi Terry,

        Thanks for your response, and Nathan my apologies I didn’t get here more quickly. Exactly as Terry mentions we must be careful because the mortice and tenon is going to be seated centrally on an original sash, and we must avoid cutting into the mortice and tenon as it will weaken the sash. On the lower rail that is thick and chunky, it’s really no problem, more the meeting rail of the upper sash which takes all the weight. Done properly it’s really effective and a great way to save money.

  2. I’m also super interested in the idea of double glazing my existing sash windows, does anyone have a supplier I can go to for the glass and double glazed units. I’ll admit I am a little daunted by the process and considering maybe just getting the pros in if it’s cheap enough. Can anyone help me please? Thanks John

    • Hi John,
      Glass is so well priced in London that I would honestly walk into the three nearest glass merchants, hand over sizes and ask for prices. Because glass is so cheap and heavy it must be bought locally. No need to hunt, Pilkington deliver to all the local glass shops. I use Lee Green Glass because I drive past them frequently and it’s convenient, they might be more expensive marginally than your local shop!

  3. I’m thinking about sash window double glazing myself. I had a guy round and he’s told me if I have the top sashes fixed he can do it for only £500 per window. This seems crazily cheap. What would be the best way to ensure I am getting the job I had hoped for? Any advice will be well received.

    • This sounds very reasonable indeed and my best guess is that the quote is cheap based on the condition and size of your windows, and of course having the top sash fixed means no additional cost in weighting as the originals can be tied together to create the balance you need. This sounds a little rough but it’s perfectly acceptable. We tie the weights anyway, its just two ties like this. Go for it – ask for references and then visit a previous customer to ensure yourself.

  4. I’d like some help sourcing sash window weights for double glazing. My carpenter has said the old cast iron are too light and they are not a dense enough material to provide full travel of the sash window. I went to a scrap yard but they all melt down any lead weights they get. Do you have the number for a reputable supplier? Also do you have any idea of price at the moment so I can budget for it as well. Thanks

    • Hi Tony,

      I am not sure that my supplier would like me publicly publishing his details so I will email you with a contact I have. The price fluctuates madly with the markets which the lead man has no control over, the good news is prices are quite good at the moment.

  5. Hi, I live in Bethnal Green and wondered if you could take a look at an installation I’ve had done recently. The guys have installed double glazed units and the room is warmer but I can see the edges of the sealed units and it just looks so bad I can’t stand how badly it’s ruined the aesthetics. When I look at the image of your unit double glazed above, even undecorated it’s a million times better than what I’ve got. I don’t trust them in my home now because they told me this is the best that can be done and the finish I want is not possible. The problem is for them i have seen a picture of your work! Any help would be greatly appreciated.

    • Hi Emily,

      I feel sorry for your situation, I’ve once mis measured units myself. I forgot to add for the rebate both sides and as a result had the same situation your facing, however I would not of dreamed of fitting these units, I simply ordered again and lesson learned the expensive way, measure twice, fit once. There are all kinds of problems with this and I don’t enjoy berating other peoples work and I certainly am not interested in getting involved in this legally, or even having my word used to challenge them, however authoritative I am in the subject, so my sincere apologies if that’s what your looking for, and I am not even available for hire on that basis either. I hope you understand. If you’d like to have the work done properly then I can certainly help with that. Please feel free to email greater detail and I’ll provide a written quote online since you know the process and the work involved. Regards

  6. Hi there,

    My husband has decided that he’s going to try one window and we live in Forest Hill. If we get this wrong do you have an emergency services to come by and maybe board up the window until you could repair or make two new sashes to fit the original frame. He’s pretty confident because he’s a carpenter but I appreciate how specialist this is and want to prepare for the worst. Also he has been quoted £70 per sealed unit – does this seem reasonable to you?


    • Hi Mandy,
      For such a specific request I would call into the office and ask them what the options are. You ought to book a slot on standby just in case it goes wrong, meaning that we can arrange a site visit the same day as you do not want to leave your home insecure overnight! Boarding on lower floor is not very nice.

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