Do water bladders sweat inside a daypack?

A: Michele Hermansen Chandler, Arizona Not really, but they do leak sometimes. I know this because three weeks ago I was out mountain biking in the Olympics while carrying a hydration pack. I got hung up when crossing a rocky dry streambed, tipped over, and ended up on my back. The hydration bladder's zippered end opened, dousing me with about two quarts of very cold water.

What you're thinking about, of course, is not the same thing—condensation is what you have in mind. And in theory it's quite possible. If you have a bladder full of cold water, warm, moist air from around you will inevitably cause some condensation, or "sweating." Enough to be a problem? Not likely. Besides, most bladder makers sell an insulated bag that the bladder fits into. Cascade Designs makes a product called a Hydration Insulator, which does exactly that: $18, and no sweat