Insulation in outdoor clothing can be confusing.
The common question is “will ___ keep me warm during ____”, which is as understandable as it is naive (and bluntly, stupid). Clothing does not make you warm, clothing keeps you warm, and neglecting metabolic training (ex: burning fats), proper fueling and hydration, and technique (ex: slow down in last 30 minutes before camp to minimize sweat) will always result in failure no matter how fancy your duds.
That said the confusion is quite understandable. Outdoor marketing hardly ever emphasizes technical details, and many companies outright hide the relevant specifications (and their CS folks often don’t know them). Appealing solely to “core” outdoor users may not be a sustainable business model for outdoor clothing even under ideal circumstances, but even if it is companies and trade industry groups have made clear that their growth strategy is not growing core users, but in making the tent bigger by bringing in marginal outdoor sports like running and area skiing. There’s little need for the generic runner or skier to have intimate knowledge of their insulation; they can just go inside, and so long as this is the case knowledge will remain too often at arms length.
To evaluate how warm a piece of clothing might be you need three things: what sort of and how much insulation is in the piece, what shell and liner fabrics (if any) are in play, and how warm comparable pieces have proved for you in the past. This last requires getting out a decent bit, and buying at least a few pieces of insulating clothing. It also requires maintaining a sense of your body composition and metabolism: go from 15 to 10 percent body fat and once stopped you’ll get a lot colder a lot faster. Assuming you know these three things, the following are general principles and suggestions for figuring out how warm a given garment might be, as well as some assorted guidelines for sorting through the noise and hype. In no particular order.
Insulation has as much to do with stopping air flow as it does with trapping volumes of air. The advent of Polartec Alpha and the rush to build synthetic puffies with air permeable fabrics has demonstrated this well, as does the remarkable insulting value maintained by my totally cashed out Rab Xenon, whose shell and liner (both Pertex Quantum GL) are exceptionally air impermeable. High-dollar shell fabrics like Quantum absolutely make a jacket warmer. I’m not sure it will ever be possible to make a fairly air permeable fabric which is downproof, but if it is it’d be interesting to see how breathable (and thus, potentially cold) a down puffy might be.
External moisture is rarely a problem for down garments unless you do something neglectful (read: fall in creek). Ambient humidity and internal moisture are far, far more problematic. I’ve used hydrophobic, treated down in two applications; first when I overfilled the top third of my Feathered Friends Vireo with 3 ounces of 800 FP treated down from Thru-Hiker, and second in my recently purchased Sierra Designs Better Vest. Thus far I am not impressed. A common scenario would have me arriving at a stop (be it to glass, fish, have a snack, or set up camp) with a bit of moisture in my system, especially under my pack. I try to let this vent as far as possible before I get too cold, but my insulating garments inevitably end up over and covering this moisture, and having to let it pass through. Alpha does this very well. Lighter (sub 4 oz fill) down coats generally loose most of their loft over my back and leave me cold. The Better Vest does exactly this, though it does puff back up (dry out) quite a bit faster. So maybe there is something to DriDown. In any case, “tests” like this one are at best misguided and at worst monumentally ignorant of what goes on in the field, and down still has acute weaknesses. It also remains the only practical game in town for serious cold.
The corollary is that synthetic insulation is still a very useful thing. All the major sorts of synthetic insulation are way more alike than they are different, save the form they come in. Climashield maintains loft longer, due to construction, but drapes less well and thus lacks the street appeal of Primaloft. Alpha is far less warm per weight, both due to the insulation itself and because the shells fabrics are thus far much more air permeable. Though as I explained above the practical advantages make Alpha (and the like) a very appealing option for multi-day stuff.
I would put 240 grams/meter fleece, a generic 2 oz/800 fill hoodless down coat, and a 60 grams/meter Primaloft One/Gold jacket as roughly equivalent in warmth, if you assume the fleece has a windshirt over top. Comparing Alpha is as mentioned problematic, but you’d probably need 90 grams/meter to be on equal footing.
Comparing fleece is a complex subject due to the many permutations beyond mere fabric weight. One major trend over the past decade has been in hi-loft fleeces, which seek to provide more warmth for the weight. When new these work, and the ones which are shaggy both inside and out are the faster moisture movers of the really warm stuff (think midlayer for skiing at -30). They do loose loft with use, and there is a lot to be said for the way dense, thin-for-weight old school fleece balances insulation, weight, breathability, and longevity. The Kiwis know this and keep using microfleece in a variety of weights, when US and now even UK companies have largely abandoned them. Grid fleece is superior against the skin, generally, but for all-around use microfleece hasn’t really been improved upon in over two decades. The lycra content in so much of the new stuff gives it a severe handicap.
Variations in clo rating less than .25 are generally less significant than the inter-rater variability when discussing garment warmth. Put another way; quantifying garment warmth can only get you so far (not very).
So what’s good out there? The lightest versions of grid fleece, like Capilene 4 or the new Sitka Core LW, make fantastic baselayers. For midlayers for moving, microfleece around 160 grams/meter is hard to beat. Rab just discontinued their Micro Pull-On, and if you don’t already I’d highly recommend snagging one. I’d like to see them make a vest version, too. There are many shirts like it but in typical Rab fashion the evolved cut puts it above the rest. There is still a lot to be said for the versatility of a lighter synthetic jacket, and again Rab lead the field with the Xenon X, which has all the right features and light, windproof fabric. I’d love to see a 80 or 100 gram/meter Xenon, in the old Quantum GL fabric. It took me a while, but I’ve really warmed up to the Rab Strata, and their new 120 gram/meter Alpha jacket will likely be excellent for ski touring and the like (weight notwithstanding).
I’ve never been a fan of the down sweaters, as especially once loft is degraded they have too little warmth for the weight. So I’m a fan of the recent trend to make sewn through jackets with 4 or so ounces of fill. Much more practical. Some folks will need a massive, baffled parka, but most can do fine with a 4 oz/fill down hoody in addition to the fleece or synthetic they already have.
So, good luck cutting through the nonsense.