Any discussion of layering has to start with it being somewhat of a misnomer; the point of a good layering system is to provider a few solid pieces which cover as great a range of conditions as possible. If you have to swap, add, or remove layers often, you chose poorly for the day. That said, technical outdoor clothing is both better and more expensive than ever, so if you pick well a small arsenal can work on almost any day of the year.
Base layers should move moisture quickly, and keep abrasion and sun off your skin. We’re into the second decade of merino being full fashion in the outdoor realm, and I’m ready to call it ready to die insofar as backcountry performance is concerned. The virtue of merino was always the extent to which it controlled evaporative cooling by absorbing some of the moisture burden as it moved from skin to the garment surface, with odor control and heat of sorption* being tangential benefits. As the paradigm of all outdoor garments has shifted towards faster moisture transport and greater air permeability, moisture buffering in base layers has made less and less sense. There is no point in effectively storing moisture in your base layer when the mid and/or wind layers are not working at full capacity. Thin wool minimizes this disadvantage, but the continued experiment with introducing synthetics into merino is all the evidence required that merino durability is below 180 grams/meter substantially problematic.
Where wool continues to shine is as lifestyle wear for the urban jungle. A mildly sweaty 3 mile bike commute makes the odor control of merino highly relevant, and while the aforementioned hybrids don’t hold up to brush and backpack abrasion, they do hold up to repeated laundering. All my merino shirts have been shunted into daily use under dress shirts, which keeps the -10 degrees mornings a bit more at bay, and allows me to chase kindergarteners at will without being stinky for 3pm meetings. Insofar as growth in the “outdoor industry” is largely confined to lifestyle or near lifestyle segments, merino has a bright future.
I revisited merino this winter, and quickly got sick of the outer layer of my hats and shirts frosting up, something which was under similar circumstances eliminated by going back to synthetics. The last six years trend in synthetic baselayers has continued, with fabrics getting ever thinner, and mechanical/structural variations making for increased performance. Polartec High Efficiency, popularized by the now-classic Capilene 4, has remained the touchstone cold weather baselayer, while sub-100 grams/meter garments have made this the new cutoff for light weight baselayers. Old tech is also making comeback, in the form of the original synthetic baselayer, polypro. If poly has won out in baselayers most significantly due its low moisture regain (.5%, roughly), poly ought to do far better with a moisture regain many times less (.06%). This fall I’ve spent a lot of time in a nylon/polypro/spandex knit, and have been impressed that a not prodigiously light fabric could wick so efficiently, even under difficult (high humidity, near freezing) conditions. The blend and modern anti-odor treatments seems to have dealt with the old polypro issues (stink, shrinkage at normal laundry temps) quite nicely, with the only downside of this particular piece being the emphatically euro styling (size up).
For the time being my recommendation from two years ago, that with a Cap 4 (now thermal weight) shirt and a Sitka Core LW shirt I could do everything, anywhere holds true. The Sitka fabrics changed a bit and the features got more complex, but plenty of good options (e.g. the OR Echo series) has come on the scene since. Most significantly, I continue to recommend that newcomers building a technical wardrobe sink money into quality baselayers, not so much because the performance gain over cheaper ones is massive, but because the gain/$ ratio is the best, and you’ll notice the refinements in function and fit most often, as your baselayers get used more often than anything else (save socks, another area to not skimp).
Next, the fraught world of mid and wind layers.
*Of sorption is the phenomenon where energy is given off as liquid water (in this case within the wool fibers of a garment) passes into a gas during evaporation. It is popularly cited by wool boosters, and I remain skeptical that the effect is for one significant enough to be relevant in the field, and for another, consequential enough to overcome the downsides of having a bunch of water on board your garments. The largest number I’ve found associated with of sorption in wool is 1 gram of water generating 277 joules (which is roughly .26 BTUs). That would be a compelling figure, but the study was of carpets in residential settings, where the thermal load and mass presumably made the findings of limited generalizability. I assume anyone who claims of sorption as a benefit of merino baselayers, without significant caveats, has not given the subject much thought.