Misted up double glazed sealed units in wooden frames.

Some causes of premature failure of double glazed sealed units in wooden frames.

(PVC-U and Aluminium have other different problems)

When a double glazed sealed unit fails...

It is often said that a sealed unit "is blown" or "has gone misty", etc. This means that a fault, and it may only be a pinhole to start with, like a small puncture, has developed somewhere in its perimeter, and moisture is getting inside and between the two panes of glass. At different times of the year there will be different amounts of moisture in the atmosphere, and even in the hottest of balmy summer days the atmosphere that we breathe has a moisture content (humidity). With changes in sonic and atmospheric pressure being put upon the 'sealed' unit, moisture will be drawn in to mix with the otherwise arid interior of the unit through this breach. As temperatures change the moisture will condense into a liquid, which will continue to build up and up, as the liquid cannot escape anywhere as easily as the moisture that is being drawn in. I have seen sealed units that have had several inches of water laying at the bottom of them because the unit is acting like a tank! When sealed units are manufactured they are not designed to be taken apart again in the future, and therefore in practice they cannot be economically cleaned and put back together. When a sealed unit has failed it will need replacing, and the old glass is usually just scrapped.

The biggest problems with wooden frames arises because (unlike PVC-U and Aluminium) the sealed units are not drained, and the perimeter of the sealed unit is not ventilated.

Sorry if I seem to go on about double glazing sealed units not lasting in wood windows, but this is what I see in real life almost every other day!

It does not have to be like that - in a nutshell, here's why:

Sealed units fitted in wooden frames MUST in my opinion have near a quarter inch gap between the glass and the wood at the bottom of the frame AND slotted holes to allow both drainage and ventilation. If sealed units are fully bedded and puttied into wood frames (even with non setting butyl putty) this is what you can expect, usually showing signs of failure between 6 and 8 years at most from new. Unfortunately many of the wooden frames manufactured and installed today (and in my opinion wrongly fitted with sealed units with an inherent probability of premature failure of the sealed unit) often have an 12mm (half inch) or so height of rebate, which is wholly insufficient to accommodate this glazing technique.

To go into the most common causes of premature failure in wooden frames in more detail:

Most double glazing sealed units are fitted by fully bedding them into the framework, and to make matters worse, often with the wrong 'putty'. If the wooden rebate is not primed, then the wood will absorb some of the ingredients of the putty, which will dry out and go hard, and allow water ingress to the perimeter of the sealed unit. If a double glazed sealed unit is fully bedded into a wooden frame then only 'non setting' Butyl bedding compound should be used for this. Unfortunately this in itself will not guarantee the life of a 'good' sealed unit exceeding, say, much more than half a dozen years, as there are other crucial factors conspiring against the sealed unit in a wooden frame.

Wooden frames, although aesthetically appealing, can sometimes be little more than frames designed for single glazing, and are often just softwood stained up to look like hardwood. The rebate height is very important, which should be a minimum of 15mm for double glazing in a wooden frame, and the fitting a double glazed sealed unit into a standard wooden frame, with a 12mm rebate height designed for single glazing, is asking for trouble.  Also the rebate depth in wooden frames is often insufficient to allow for beading, and if a double glazed sealed unit is puttied in like single glazing, then again you are asking for trouble.

I see so many double glazing sealed units fitted into wooden frames that have prematurely failed, usually in a timeframe of between 6 - 8 years from new, and as a direct result of a combination of the above design and installation shortcomings. What does not help in this situation is the homeowner not realising the great importance of maintenance to the wooden frames, and it simply is not good enough to ignore the frames because they are looking pretty good to the eye. It is of the utmost importance with all wooden frames to ensure that water penetration to the sealed unit perimeter is reduced to the barest minimum, and I suggest that one straightforward way to achieve this is to apply a clear silicone  sealant around where the glass and wood meets outdoors. Do not skimp and use a cheaper 'mastic' it will not be as good as silicone.

There are other points I could make, such as the practice of using double sided tape against the indoor rebate, and about the natural movement of wood through the seasons putting pressure on the glass, but I think the above covers what most people want to be informed about in as much as what is affecting their wooden frames.

Loadsa stuff about hardwood and softwood and wooden windows.

You cannot avoid the fact that double glazed sealed units in wooden frames cannot be expected to last anywhere as long as they "should" do, at least unless the frame design is one which includes positive drainage and ventilation to the perimeter of the glass, and a dry gasket type of glazing system with no putty.

I can get you a quote for a new windows, doors or a conservatory, anywhere UK