Repairing a failed Double-glazed Window Unit

Our living room sealed double-glazed unit has failed.  The sealing has been breached and allowed moist air to enter which has become trapped and resulted in mist patches and water droplets condensing on the inside faces of the glass panels.

you'll need to look very closely to see the water droplets between
 the panes at the bottom, and the line of misting near the top...

close-up of the bottom right corner ...

and in the centre...

Last year we also had a failure of a similar sealed double-glazed panel in our bedroom window.   In this case, we ordered a new glazed unit from a manufacturer around 80 miles away, and picked it up ourselves in the back of the car.  We then swapped it out with the failed panel, which we took to the local tip.  The new panel was less than £100, so a reasonably cost-effective DIY repair.

However, this same process wasn't an option this time around because the failed window in the living room is much larger, so it wouldn't fit in the car and the manufacturer we used last time won't deliver so far from their base.  Other online glaziers wanted ridiculous sums of money to manufacture a new unit to my dimensions, and anyway only a very few offered a delivery service.

We don't want to spend too much money on this window repair, because I think all the windows on the house will need replacing within the next few years anyway.  They're at least thirty and maybe even forty years old.

So I decided to repair this failed window unit myself, or at least to improve its appearance, by trying to remove the trapped water from between the panels.   

Assuming we don't break the glass, then the worst that can happen is the repair doesn't work, so it's definitely worth a try.


Summer is a good time to attempt a repair.  The air is warm enough for the water to evaporate without using hairdryers etc.

I first had a look around online for the tools and materials I needed.   There are several vendors on eBay and Amazon offering repair 'kits', but they all want around four times more cash than the cost of simply buying all the stuff individually.   

So I ordered what I wanted from different vendors and just waited for them all to be delivered.

assembling my own repair kit ...

There's a 4.0 mm diameter drill bit for glass & ceramics, a length of 4.0 mm o/d nylon pneumatic hose and 200g of desiccant beads.  For a total cost of £10.99, deliveries included.

There were cheaper drill bits available, but the Bosch bit is designed to be used dry whereas diamond-dusted bits usually need a continuous supply of cooling water during drilling.

The desiccant is a premium grade 'molecular sieve' material, as used in newbuild sealed glazing units and also for drying of pipelines and other industrial applications.  When used in glazing units, it's poured inside the perimeter aluminium spacer bars that keep the two glass panes apart.  These hollow spacer bars are perforated to allow an exchange of air with the main space between the glass - you can see these spacer bars with perforations in your own windows.

In our windows, there are two 4 mm thick panes of glass fixed to either side of 20 mm wide aluminium spacer bars, giving a total glazed unit thickness of 28 mm, which is a fairly standard size here in the UK.

The desiccant material I bought was supplied as beads of between 1.0 and 1.5 mm diameter, and delivered in a vacuum-sealed bag to stop them absorbing any moisture from the air before they're ready to use.

The nylon hose has a 2.5 mm bore and the idea is to insert it into the upper holes drilled through the glass and use it to pour the desiccant beads down into the side spacer bars.

So, onto the actual repair.

I didn't want to remove the glass panel although I've done this before - it's very heavy - so I decided just to remove the uPVC beading from both sides, leaving the top and bottom beads in place.  I used a beading removal blade I'd bought previously, which makes it really easy to remove them, just a couple of minutes each side.

So now I'd exposed the vertical edges of the sealed unit.   I drilled into the glass in line with the centres of the aluminium spacer bars, i.e. through the glass and directly through the side wall of the spacer bar into its hollow cavity.  In these locations, the holes will not be visible when the uPVC beadings are replaced.

I drilled four holes, near the top and bottom of each side.  You need to ensure you're not too close to the corner, because the spacer bars are usually fixed together by a solid right-angle plastic joining piece, and drilling into this won't allow access to the cavity.

hole in the upper right side, through the
glass, sealing mastic and spacer wall ...

I took my time with the drilling, using a cordless drilling machine and applying only minimal pressure on the drill bit.   Each hole took more than 10 minutes to drill, stopping regularly to let the drill bit cool.

drilling the glass in progress at the bottom left ...

After drilling the four holes, I inserted an old piece of bicycle brake cable into the spacer bar cavities from both the top and bottom, to ensure that they were clear all the way down their lengths.  I then used the vacuum cleaner to clean up all the drilling dust and debris etc.

So, Part 1 of the repair is complete.    We'll wait to see how much of the water between the panes will clear naturally now there are holes to allow the air to enter and escape.  

This is a north facing window so it never sees the direct sun which might have accelerated the process.  So we'll give it at least several days, and maybe even a few weeks - there's no great hurry. 

We'll then add the desiccant to the side spacers and seal the holes etc to complete the repair, and provide an update.



Update 17-Aug-20

It took around six weeks for the visible water to disappear from between the glass panes, and it was a further two weeks before I managed to get around to finalising the repair.

This was just a matter of pouring the desiccant through the drilled holes into the side spacers in the unit, using a small funnel.  The tubing was not necessary, the material flowed well enough directly from the spout of the funnel.

The drilled holes were closed-off with a dab of silicone sealant and the fixing beads replaced to secure the glass.

So now it's just a question of waiting to see if the repair has been successful or not.  Maybe I'll post another update next spring ...