In article ,
"Hugo writes:
"Any original filling has been lost and replaced by air many
times over long before the condensation became visible."

That's after the seal has failed (to put it back in context).

I am looking at making a set of patio doors - but I don't like the idea of
sealed DG units that will probably or definitely go off in 5 years time or
even less.. didn't windows used to last for a century?

When properly manufactured and properly fitted, sealed units will
last for a long time. I replaced some first generation double
glazing after about 25 years, and only one out of probably some
30 panes failed in that time, and that was due to someone having
removed the unit at some point in the past and damaged it in doing
so. The patio windows in my current house are 20 years old, with
no sign of failing seals.

I was thinking of installing two panes of toughened glass to each door as
with a DG unit, but with the panes not sealed. The interior facing pane on
its own frame with spacer fixings. This would be to give ventilation between
the panes to reduce condensation, but be able to close them up tight when it
gets really cold during the winter. Are there reasons why this wouldn't work?

You will always get condensation at some point if the unit isn't
sealed. The temperature differential, temperature variations, and
time to diffuse replacement air in just won't keep the inside dry.

If you are designing your own doors to take sealed units, it's
vital to understand the environment in which the units must be
fitted, as it's incorrect fitting that results in most sealed
unit failures over time. The main cause of failure is that the
unit ends up sitting in a puddle in the bottom of the frame.
In plastic frames, the unit should sit on a number of plastic
spacers along the bottom, which hold it above the drainage
channels in the frame. The drainage channels must actually
work and drain the water away from the bottom channels. The
rubber seals against the glass are not designed to be completely
waterproof (they're designed for speedy fit by less skilled
fitters instead) and water which gets through must drain out.

I don't know how it's supposed to work with timber frames.

With first generation metal frames, the outer glass was sealed
to the frame to prevent any water ingress, and where this was
done properly, those units at now over 30 years old are still
fine. The snag is that fitting those units was a much more
skilled job than fitting today's PVC units is, which made them
both relatively more expensive and/or more prone to being
fitted wrongly.

--
Andrew Gabriel
[email address is not usable -- followup in the newsgroup]