Sunday 18 November 2012

Making Single Glazed Windows Into Double

I'll be honest - this wasn't a job I was looking forward to!
The customer wanted the 5 windows on the exposed face of his property to be changed from single glazed to double. As the windows were hardwood, of good quality and thick enough, we decided that it would be best to increase the size of the rebate (or rabbet for the Americans) where the glass sits so it could take the thicker units.
Working on the wrong side of the heater!
 This is a bit of a dirty job really every step involves dirt and dust! First we had to remove the old glass from the windows, this was done with a combination of chisels, knifes, the multimaster and a hammer! Cleaning up any broken glass before moving on to the next step.
Luckily I had Tim with me to help or it would have seemed a bit of a daunting job on my own as you couldn't rust it for fear of damaging the frames.
Removing all the old putty
Once we'd removed all the glass and cleaned up the old putty from the edges I could set about making the rebate deeper. My new little Bosch router (bought mainly for this job) was ideal as it has such a small base it could get round all the edges without hitting the walls. It was tricky to find a rebate cutter for the router with the right sized bearing to give us the correct width of cut, the ones in the set were either too big or too small but luckily I had a bearing from an old router bit set that was just the right size (shows you should throw nothing out!).
My new little router was ideal for this job - still had to have a steady handso as not to wobble
 Once I routed round each window, three of them having 16 panes of glass (this was a dusty job - I had sawdust everywhere - and I mean everywhere!) it was just a matter of tidying up the corners with a sharp chisel. I had to be careful when routing as a lot of the time the router was just resting on the thin mullion or transom and a wobble would have been really noticeable.
Cleaning up the waste in the corners
I then made some templates for the new glass and boarded up the windows.
A dirty job, but one that will make the house feel a lot warmer on this exposed site when the new glass is fitted.


  1. Kev, just stumbled across your blog and this post in particular. Am very interested as have just bought a 1930s place with original single glazed bay windows which just WATERFALL with condensation. How thick/deep do windows have to be in order to pull off this nifty trick (and which bit should I be measuring)? Would love to be able to do it. Sadly I think Cornwall is a bit far to ask you to travel......

    Many thanks

    1. Hi Sue,
      Cornwall is a bit far but I'll try to help you the best I can!
      The problem of condensation can be helped with vents at the top of the windows (found on most mondern windows now) they keep the air moving slightly and reduce the condensation.
      As for routing out the windows and fitting double glazing it's difficult to say without seeing them. Before you think about it make sure your windows aren't rotton or damaged, if they are you might as well go for new once with the effort and time it will take to put them right and alter the glass.
      You need to make sure you can fit the new glass and the beads/putty to hold them in, the total thickness will depend on what glass units you fit but I think the narrowest we could find was around 14mm thick, plus a 9mm bead - you're looking for around 23-25mm deep rebate.
      Have a look at where the glass sits in the frame and then measure forward from that. The glass will probably about 3-4mm thick so add that to your measurement. If you haven't got what you need then you need to think about routing them out, go inside and look at the mullion and transoms (narrow vertical and horizontal bits) and the frame Then measure the thickness from the glass back until you get to the decorative moulding (if there is one). You'd need to leave at least 2mm on here (more if there isn't a moulding).
      If you've got enough space then you can do it. Any more questions feel free to ask. Hope this helps.


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