Sealed Double Glazed Units And Why They Fail Or Condensate With Moisture.

Many previously installed double glazed windows from 10 to 20 odd years ago will inevitably end up having failed or misted units. There are various reasons for this failure and the first can simply be down to the construction of the double glazed units.


If the quality of the DGU's( double glazed units) is poor to start off with then the life of them will not be very long. Many companies expect this to happen and only give 2 to 5 years on the glazed product , but give a full 10 years on the PVC frame.

The DGU's are designed to sit within the PVC frame held in by a PVC glazing bead and sit on top of glass packers therefore allowing any water that manages to get in behind the gaskets to easily drain away around and below the DGU emptying through purposely drilled drainage holes directly beneath .
The water then works its way through the lower chamber and on to the either the PVC cill which the window sits on, or through the front of the lower part of the PVC window which is face drained.

If there is any blockage of either of these drainage holes it will cause a back up and the frame will begin to fill up.

It is not unusual to have a trace of water inside the PVC frame at the bottom as it designed to be self draining.Any water being held will mean a blockage somewhere.

The installation cause

Many older DGU's may have been sat directly in the window frame without any sort of glass packer beneath. This can be the cause of many failed units as 1)  they are sat in there own water and 2)  the hot melt that units are sealed with when heated in the sun can stick to the inside lower of the PVC frame and 3) if they are not sat on glazing packers correctly they can slip in the warmer months.

Glass Packers

The Glass packers should be suitable ones which are sat square on the lower part of the frame and be the correct width of the overall thickness of the DGU so neither of the 2 panes of glass can slip at any point when heated up on a sunny day. 

Packers should be square and touching both the inner and outer panes. The packer should be seen to reach the very outer of the glass edge and inner glass edge to give peace of mind it doesn't slip even when glazing with the beads are knocked in.

Many failed units are down to this installation overlook and even though it doesn't seem as if one of the panes has slipped, it only takes a pin hole and a drop of moisture to condense a whole double glazed unit. and ruin the look and effect of a PVC window or door.

The right way

Be sure to use a company that gives 10 years on the Double glazed units as well as the PVC windows and doors.

Make sure the glass units are sat correctly on the correct width glazing packers.

Make sure the drainage holes are all clear for water to run through.

Interesting links:

New Rockdoors | PVC windows  | ultimate conservatories | high security door products 

More advice and posts on :

What is PVC ? and what does it stand for ?

combating condensation

Tips on how to re seal your own windows and get a professional guide

The professional view on how to sell your house and what areas to address

What benefits are there with PVC windows

Are the locks secure in your doors? check this post to see what we mean


  1. This trapped air acts as a barrier to heat, sound or anything else trying to escape through the windows. This means that you'll need far less energy to heat your home than before, as the heat generated will be staying where it should be - in your home and not outside!

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  3. its way through the lower chamber and on to the either the PVC cill which the window sits on,

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