The source tube insulator the insulator keeps your drinking tube and drinking valve insulated in extreme cold weather matches all source hydration systems as one of our cutomers put it recently the source tube insulator is the most user friendly adaptable and effective insulator out there the full length zipper makes it easy to fit it on your hydration pack the small pocket added to the mouthpiece end of the insulator is designed to hold a small hand warmer device adding the hand w