I used a fairly large commercial dessicant pack in a letterbox on
placed on the 13th of September. It held probably near two
tablespoons of "dehumidifier".

A few days later, we were inundated with rain by the then-remnants
of Hurricane Ivan. That's a LOT of moisture! The creek just 40 feet
away from the box's position was 2 to 3 feet above normal.

Then we had a drastic temp change, down to the low 50s at night from
a humid 80 during the day. A good chance for lots of condensation.

Well, good news... I checked the box on Friday, and though there was
condensation on the OUTSIDE of the box, there was NONE INSIDE.

There may be one important thing to consider when choosing a
dessicant: what the material was designed for...
The packs you find in shoeboxes, electronics, and things like that
are designed to pull moisture from the air; they don't do squat when
in direct contact with liquids. Kitty litter, especially the new
crystal stuff, works when it is in direct contact with liquids; it
does nothing (except maybe make your litterbox--I mean, your
letterbox--smell nice) as far as pulling moisture from the air!

Now I'm curious to see how the box I placed two days earlier
faired... it doesn't have a dessicant pack!