Originally Posted by sailingdog
Dessicant packets, like silica gel, really need an airtight environment in order to function properly. Otherwise, their limited ability to draw moisture out of the air is easily overwhelmed, and then they're effectively doing nothing.
You also need to make sure you occassionally either replace or "desorb" the silica packs from time to time. It amazes me how often you will see something large like a guitar shipped in a cardboard box with a single silica pack.