Fogged/Failed Thermal Windows
Thermal windows have two and sometimes three glass panes with a space between
the glass that is typically filled with argon or has a vacuum.  A vacuum provides one of
the best forms of thermal insulation, but it is harder to maintain.  A properly functioning
thermal window is sealed at the edges and will not allow the gas inside to escape or let
outside air to enter the space between the panes.

Over time the window seal and frame materials can start to deteriorate, due to exposure
to sunlight and air.  Also, with changes in temperature the window components (glass,
framing, seals) expand and contract at different rates.  This, combined with the
differences in air pressure between the space inside the window and the outside air can
cause the seal between the window panes to fail and leak.   The 'failed seal' can now
allow air to move in and out of the space between the windows.  This exchange of air is
what results in fogging.   Often times if you have a fogged window, it is referred to as
having a failed seal.  This does not imply that the window will leak water into the house.
Dual Pane Window
Dual Pane Window Cut-Away View
Fogged/Failed Thermal Windows
cause air to be forced out from between the panes when the pressure is higher inside the window (for example when the sun is shining
on the window, the air between the panes heats up and expands, creating a higher pressure).  As the air cools down it contracts resulting
in lower pressure and outside air enters the space between the window panes.   This new air being drawn into the window cavity has
moisture in it.  When the window surface cools down the water vapor in the air can condense on the inside of the window (just like the on
the outside of a glass of ice water).  The condensation that forms is water but it also has salts, minerals and other contaminants which
can be left behind when the water evaporates.  This cycle can be repeated multiple times every day.  Eventually the salts and minerals will
create a film and powder can even build up near the bottom of the window.   If enough air moves in and out of the space between the
windows due to the leaking seal the condensation can get so bad that water will accumulate and run down the inside of the window.  
Between condensation itself and the film left behind from the salts and minerals, the window can become unsightly and even hard to see
Most windows, if properly manufactured and installed, should last 10 to 20 years before the first signs of fogging begin to show up.  It can
take years between the time the first hints of fogging show up and the window becomes unsightly.  Larger windows and windows
exposed to direct sunlight are more likely to experience failed seals sooner.  The fogging and condensation is mostly a cosmetic issue.  
The loss of the vacuum or argon gas that was originally between the window panes will decrease the thermal properties of the window,
however as long as the seal between the panes is mostly intact, the double panes will still provide significantly better thermal insulation
verses a single pane window.

Most window frames can be taken apart and a new dual pane assembly installed, which should be much cheaper than replacing the
entire window.  However if the windows frames are worn or if you want to change from aluminum framed to vinyl or wood framed, then you
can change the entire window assembly.   Some companies offer a "window a month" plan, where they just replace one or two windows
every month or so to make it more affordable.