Yep, the trick is not to put COLD water in your pack.
Took me a while to figure it out .. my problem was summer hiking. I'd fill the water bladder and put it in the fridge 'cause cold water is nicer to drink than warm water. Mistake. Just drink the water warm.
If you're out in cold weather, you might need to pre-heat the water up near body temp. Should (knock on wood) fix the issue.
Tom
