Frame Materials:
Woods

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 their own, different problems.)

In a year, there are different times in which there will be different amounts of moisture in the atmosphere, and also in hot summer days the atmosphere that we breath has a moisture content humidity. With the 'sealed' unit is subjected to changes in sonic and atmospheric pressure, moisture will be drawn in to mix with the otherwise dry interior of the unit through this breach. As temperature changes the moisture will compress in to liquid and this will go up and up, as the liquid cannot escape anywhere as easily as the moisture that is being drawn in. There are sealed units that have several inches of water lying at the bottom because the unit acts like a tank. Sealed units are not designed to be taken apart again in the future, hence, practically they cannot be cleaned and put back together. Whenever sealed units fails it requires replacements and the old glass is usually thrown away.

The biggest problems with wooden frames arise 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 nearly a quarterinch (0.6cm) 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), you can expect failure to manifest itself between six and eight years from new at most. 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 a half-inch or so (1.2cm) height of rebate, which is wholly insufficient to accommodate this glazing technique.

Here is some more detail about the most common causes of premature failure in wooden frames.
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 to enter 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. Unfortunately, this in itself will not guarantee that a 'good' sealed unit will have a life exceeding, say, more than six 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: it should be a minimum of 15mm for double glazing in a wooden frame; 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 within six or eight 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 a homeowner who doesn’t realise the importance of maintenance to the wooden frames. 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. One straightforward way to achieve this is to apply a clear silicone sealant around all the places where the glass and wood meet outdoors. Do not skimp and use a cheaper 'mastic', which will not be as good as silicone.

Loadsa stuff about hardwood and softwood and wooden windows

You cannot avoid the fact that double-glazed sealed units in wooden frames do not generally last nearly as long as you would like them to, at least unless the frame design includes positive drainage and ventilation to the perimeter of the glass and a dry-gasket type of glazing system with no putty.

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More specific information about: 
Hardwoods
Softwoods
Wooden Windows
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