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Posted by Bill S on October 17, 2006 at 5:31:20 pm EDT
In reply to Re: Synthetic Vs. Down posted by Gear Selection (Guest) on October 17, 2006 at 3:18:56 pm EDT

Ian -
As noted in a previous similar thread here, a waterproof shell has the problem that it allows (actually, promotes) condensation of body moisture on the inside of the outer shell of the bag, thus collecting the moisture in the down. This also happens in waterproof bivy sacks. In part, this is why Steiger's North Pole ski and dog sled trip found they gained about a pound a day in their sleeping bags, so that by the time they reached the Pole, they had 75 pound bags that could not be stuffed for carrying (they had to strap the "boards" onto the tops of the sleds).

Waterproof/breathable shells are better. By personal experience, I found that Goretex and the more breathable version DriLoft do not breathe well enough in Arctic conditions (or even Sierra and Tetons winter conditions), allowing accumulation of a fair amount of moisture (people sweat off about a pint of water overnight and breathe out an additional pint). Microfiber, such as Pertex, is water repellent, eVent (wp/b), and Epic (wp/b) shed spills well enough, yet breathe well enough to not accumulate much moisture in the down in cold conditions. Synthetic is necessary for a fill in above-freezing, humid, or outright wet conditions, with a water repellent or wp/b shell. Or just use it in a tent or wp/b bivy sack.

In really cold conditions, meaning below about 20F, you need lots of breathability to vent the sweated-off moisture (or use a VBL), to prevent the condensation and build-up in the insulation fill of the bag (down or synthetic, either one). My statement is based, not just on reading about Steiger and crew, but on personal experience in the Alaska Range, and winter treks in the Cascades, Rockies, Tetons, Sierra, NH Whites, and elsewhere, with temperatures down to and below -40 deg. You won't notice it on trips less than 3 or 4 days perhaps, but you sure do on trips of a week or more, with month-long trips really showing whether you know how to prevent the moisture buildup.

To emphasize, in really damp conditions, whether wet snow above 20F or rainy conditions, you will still need a tent or bivy sack to keep your bag dry and warm. So you will still have the weight of that cover in addition to your sleeping bag as long as there are damp conditions.

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• Re: Synthetic Vs. Down - SteveTheFolkie 14:19:18 11/13/2006

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