In article ,
Tony Williams writes:
In article ,
Grunff wrote:

Or have I misunderstood you totally?

I think you have (or I may have). The little demo window one
salesman had had the two glass sheets sandwiched onto a plastic
frame (and the whole sandwich then dropped as a complete assembly
into the main window frame). I'm fairly certain I saw drain slots,
along the base of the sandwich, opening up to the airgap between the
glass sheets.

There are holes in the separator which allow moisture through to
a dissicant in it, but the outside edge of the sandwich is sealed.
The dissicant absorbs the moisture in the unit when it was built,
and might absorb a small amount which leaks in, but when it's
all used up and more gets in, then the window mists up.

Some reasons the outside seal might break a

o poor drainage from the channel the window sits in, so it sits
in a puddle, which might freeze in winter,

o seal damaged during installation,

o unit just not well made (in extremes, they have to handle
perhaps a 40º temperature difference and differential
thermal expansion between the two panes).

--
Andrew Gabriel