But how much water is going to get on or into a bag through that route? Not much, really. Meanwhile, each night the average person exudes one or two pints of water. That's a lot. So you do NOT want that water to become trapped in a bag. And face it, ANY water-resistant barrier is going to slow the transpiration of that moisture. So my view is that the best sleeping bag shells err toward the breathability side. I like polyester microfiber for a shell. It's light, durable, breathes extremely well, and polyester is naturally water-resistant.
So, in the case of products such as Dryloft and Epic, which are now often found as bag shells (Dryloft in particular was largely designed by the Gore folks as an insulated garment/sleeping bag shell), I'm not convinced there's a problem that they solve. I'm particularly skeptical of Epic, which uses silicon-encapsulated threads. It became very popular because it was much cheaper than Gore Activent, a fabric that Epic pretty much shoved out of the marketplace. But while Epic is light, water-resistant, and wind-resistant, my experience is that it breathes very poorly. I've had conversations about this with the folks at Nextec, makers of Epic, and they think I'm full of it and that their tests support the position that Nextec makes a very breathable fabric. But, one of the designers at a major outdoor gear maker that uses Nextec agrees with me.
Ultimately it's a case-by-case thing. If I camped a lot with a down bag in a wet climate in the 20- to 40-degree range, then maybe I'd consider a more water-resistant shell. But if it's warmer than 40 degrees, it doesn't matter. Colder, same story-cold weather is dry weather (Don't believe me? Try getting water from a pot full of packed snow on Denali). And when it's cold, condensation in the shell is even more of a concern, so the nod really goes to a breathable shell.
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