Conduit SL Bivy
Conduit SL Bivy (courtesy, Mountain Hardwear)
Gear Guy

How can I stop condensation ruining my sleeping bag?

I recently purchased an Integral Designs Endurance Bivy made of Pertex Endurance fabric, as I wanted a lightweight bivy sack to protect my down sleeping bag from drips and condensation in tents, tarps, and snow cave situations. So I've used my bivy sack in humid Pacific Northwest conditions inside a tent with temperatures in the 30s and 40s, and each time have been disappointed to find my sleeping bag glistening from head to toe with condensation. My question, then: Does Pertex Endurance live up to its hype as a highly breathable waterproof fabric? My experience suggests that it doesn't, but I'm curious to know what others think about this fabric. John Seattle, Washington

Conduit SL Bivy

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Well, I hate to be a wise guy (no, that’s not true…), but I will quote a line from the Integral Designs Web site. This is in reference to the aforementioned Endurance Bivy. Ahem:

Conduit SL Bivy Conduit SL Bivy

“You will experience some condensation build-up in humid conditions.”

“You” in this case would be, well, you. The “condensation build-up in humid conditions” no doubt covers your own experience—to recap, inside a tent on a damp night in the 30s, you got soaked.

The “why” is easy enough. While it’s true Pertex Endurance finds its way into sleeping bags, in the case of the Endurance Bivy ($120; it coats the fabric—rendering the bivy totally waterproof and totally non-breathable. The moisture on your bag in the morning was actually exuded by you. It got through the sleeping bag cover just fine, but hit the waterproof shell, chilled, and condensed. Generally speaking, the minor drips and splashes you encounter even in winter camping don’t do much to affect a down bag. Modern bags all have shells that are treated for water repellency, so they do a pretty good job of shedding incidental moisture. True, a week spent outside in the damp, chilly Pacific Northwest is bound to take a few degrees out of any down bag, but you should still be OK without having to resort to a bag cover.

If you do, be sure it’s one that is breathable. Mountain Hardwear makes a decent, inexpensive light bivy called the Conduit SL Bivy, which uses that company’s proprietary waterproof-breathable fabric. It’s only $105 ( Or, Outdoor Research makes a bag shell specifically designed for use as a light bivy or inside a tent as a bag cover. It’s simply called the Sleeping Bag Cover, and sells for $120 (

Stay dry!

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From Outside Magazine, April/May 2021 Lead Photo: courtesy, Mountain Hardwear