February 5, 2001


Why is my sleeping-bag insulation freezing?

I own a Marmot Xanadu bag that is rated to -30 degrees Celsius. The last time I used it, I encountered a little problem: The insulation of the bag was full of ice when I woke up. This is the second time this has happened, so I'm searching for both the cause and a solution. I don't think that it's because I got too warm as it was -25 degrees one night and -15 degrees the other. Can you explain that phenomenon to me? Thanks,

— Mathieu Darsigny St-Hyacinthe, Quebec

What you have come across is a common-enough winter problem: Freezing condensation inside the bag. Even if you're not "warm," and therefore not sweating, your body still gives off a pint or more of water during the night. If the outside cold has penetrated the bag's insulation —- as it surely would on a -25 Celsius night (-13 Fahrenheit) —- that moisture is going to freeze. Will Steger, the famed ultra-Arctic explorer, tells of sleeping bags that never have a chance to defrost and end up weighing something like 60 pounds or more.

The good news, I suppose, is that the Xanadu is a Polaguard 3D-filled bag and as such will still give good insulation when damp and even half-frozen. But of course you don't want a bag that's full of ice, either to sleep in or lug around. So what you need to get is a vapor-barrier liner. That's simply a nylon shell that is coated so as to be waterproof. You stick in inside the bag, then crawl into the VBL. All that moisture you exude stays inside the VBL, keeping your bag drier and warmer. It's definitely for cold nights—otherwise you'll sweat. But I should think anything below —17 Celsius (0-degrees Fahrenheit) or so would qualify. Besides, a VBL weighs only 10 to 11 ounces, so it's not a big deal to lug one. Cost also is reasonable: Usually about $22 U.S.

When using a VBL, make sure you pack an extra set of long underwear (assuming that's your sleeping layer). That's because your clothes may trap some of the moisture that otherwise would go into the bag, so when you wake up you might be a little damp. That will dry quickly enough, but in very cold conditions may make for a bit of discomfort when you first are out of the bag.

Keep warm!