How do these Granite Gear sacks deal with wet conditions? Are they waterproof at all?

To add to that, the material used in the Granite Gear Round Rock Solid Compression Sacks is waterproof (PU coating), but the seams are not. They are taped (not the same taping as in a waterproof jacket) which helps them from fraying and allows them to be more water resistant. However with prolonged water exposure, the seams will draw in moisture. Granite Gear does offer a more waterproof compression sack that is fully seam taped (waterproof taping). These are the Granite Gear EVent Compression Dry Sacks. These are significantly more waterproof and feature a roll-down closure, eVent material on the bottom of the sack to allow for purging of the air, and every seam is taped for maximum waterproofness. I would recommend using these if going on a trip that will likely involve large amounts of water exposure.

I wouldn't recommend using them for a canoeing or rafting trip down the river like a dry sack, but they will work fine in any other damp or humid environments. They are water resistant not waterproof.