JR> Hi again Folks,
... recognize you from the Model RR Echo. 
JR> I have a patio door by Therma-Tru that is about 4 years old now,
JR> and I had to replace the double glazed window twice now (...)   Has 
JR> anyone else had this problem or knows a solution?  Seems like I will have 
JR> a problem every other year and the warranty will be over soon.  (...)
    Haven't had the problem because I haven't had patio doors, but sure do 
understand it and can suggest the solution.
    Two forces are at work on your window, both based on the factor of direct 
sunlight.  The first is the bonding agent used to seal the two panes of glass 
together.  At the very least it''s some form of silicone seal, and quite 
possibly a hot melt time.  Repeated direct exposure to sunlight will soften 
and weaken the bond of the glue.
    The other factor is the space within the two panes.  Allthough it is 
supposed to be devoid of oxygen and generally is replaced with an insulating 
gas (usually argon), not only can there be as much as 5% oxygen still trapped 
in the space between the panes, a property of most all gases is that they 
expand when heated.  The glass will expand and contract slightly when heated 
and cooled as well.
    When you've got an area as large as a patio door, you've got a very large 
thermal collecting area and it's much more susseptable to these forces than 
a normal sized window.  Between the glass moving, the gasses inside pushing 
in and out on the panes and the bonding agent being softened, you have forces 
that are eventually going to causes the panes to separate somewhere and a 
leak will form that allows moisture into the space within.
    Most good windows have some form of desicant, usually silica, in the 
channel that forms the separating frame between the panes, but the ability of 
the silica to absorb moisture is limited and eventually condensation will 
appear.
    The solution is pretty easy.  Find a way to shade the door from direct 
sunlight during the hottest seasons of the year.  It doesn't need to be 
totally shaded, just enough to reduce the amount of heat that the window is 
forced to absorb.
    One solution would be an awning.  You'd want one long enough so that the 
sun is above the awning's horizon during the hottest summer months (in my 
region that would be end of June through middle of September), but short 
enough that the sun will hit the window during your coldest seasons in order 
to enjoy the benefits of ambient heating.
    Another solution would be to build some form of pergola or trellis over 
the area just outside patio door.  The structure could be somewhat of an 
open nature so that some sunlight gets through.  Letting grape vine or 
wisteria (or some other perenial vine) grow over it would provide additional 
chade during the summer months, but the vine dying back during the winter 
months would allow shade.
    Either solution would incur some cost to install and there would be the 
added need to sweep or rake leaves with the trellis solution, but both would 
outweigh the eventual cost of having to replace the window every couple of 
years.
    Hope I've helped,
        Byrd Mann
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