“…under duress the most important characteristic of your clothing system is not the ability to keep external moisture off you, but the ability to allow internal moisture to escape efficiently without chilling you excessively.”
In the ~five years since I wrote the above post, and since Sitka popularized the concept of the rewarming drill. In that time a number of people have produced trials, and a few significant advances in gear have become widespread. It is worth taking a look at both.
Rokslide recently published a static rewarming drill trial; jump in a lake, get in a sleeping bag, use hot drinks and hot water bottles to see how your insulation manages moisture. A useful exercise for the unfortunate but inevitable scenario of having to go to bed damp or wet with no other way to dry out. This can happen in the alpine, or just because of rainy weather without respite. The lessons from the Rokslide article are mostly old hat: the lightest possible layers (especially against the skin) with the least possible spandex are best. Anything beyond mid single digits spandex should be categorically out for backcountry stuff in damp climates, as should merino wool. Synthetic bags and insulating garments provide a significantly larger margin for error, though in the case of the former weight goes up enough that you can almost buy a bigger margin with a premium down bag. It’s also worth highlighting that women, especially those who require more support than a basic shelf bra/tank provides wear a significant handicap when it comes to eliminating moisture islands from undergarments.
There are also a few versions of the various rewarming drills, static and active, that might be worth watching if you really care to geek out on specifics. Subtle but significant lessons here are just how much redundant fabric layers (e.g. pockets) can trap moisture, along with how one poorly conceived layer in the system (most often an inartfully selected mid layer, such as a second heavy baselayer) can slow the whole system down. This performance during a for-video trial is one thing. The cost lagging dry time can exact on metabolism and morale on day 3 of 5 or 7 quite another.
The most important development in this area, in the last five years, has been in active insulation (Alpha Direct, left; Full Range, right). The virtues over fleece are in no small part the much lighter fabric (not necessarily garment) weight relative, which vastly increases dry time when internal heat is driving the process. The advances in fabrics used for shells here also makes a big difference, as they both preserve internal warmth (and thus, temperature gradient) without too far inhibiting moisture transport. Being able to get wet, be it by falling in a river or sweating too much on a skin track, throw on an active insulation jacket, and then work yourself dry without too much attention to detail has been a game changer.
Lately I’ve been revisiting classic pieces, like the Rab Windveil and Patagonia Capilene 4, that firmly prioritize not only dry time not very low moisture accumulation even under poor circumstances. And I’ve been impressed, all over, with how well you can do with a system whose ceiling for error is small. Heavier baselayers, esepcially wool, can in theory do more and better than Polartec HE, just as a softshell windshirt can breath better than the Windveil and peers. But it is darn nice to just not have to faff much, to leave the second layer on for that extra 20 minutes up the hill with minimal penalty. If there is any alteration I’d make to these thoughts, it would be that.