Moreso than with most categories I feel sympathy for beginners trying to come to terms with understanding insulated garments for the outdoors. Staying warm outside, on the face, shouldn’t be so complicated, and while the nuance and especially implementation of staying warm outside can be hard to hew closely with, having warm enough clothing shouldn’t be much of a mystery. If action layers are for staying warm during various permutations of on-the-go, pure insulating layers are for keeping you warm while you’re still. Plenty of concerns usually associated with action layers almost always bleed over into pure insulation, and indeed one of (in two more years I expect to be able to way the) the major recent developments is using lessons from active insulation in warmer garments designed for static activities.
To know how warm a jacket will be, you need to know what kind of insulation it has, and how much is there. I tempted by the analogy of no one buying a new house from a builder who couldn’t give you the R in the roof without thought, but I’m sure there are plenty of folks who have and will do just that. Don’t be that person, don’t assume attributes into a jacket based on marketing copy or a pinch test. I’m loath to buy any insulation from a company that doesn’t list numbers front and center, but if they don’t (looking at you First Lite) they should at least be able to dig them up readily. If a company can’t do that, run away quickly.
Knowing fill type and weight doesn’t help too much without a half dozen well worn garments in your closet to which you can compare. Even so, knowing you need more then 4 ounces of down and less than 10 doesn’t help much if you’re in the market for synthetic.
I haven’t used that many insulated jackets; the above is my best approximation of functional and equivalent warmths. You can dive into seemingly more precise detail with clo ratings, but shell fabrics bias these rating significantly, and lab findings do not generalize well to the field (below).
Even if you have the necessary amount of insulation along on a trip, implementation can easily sabotage your plans. Being tired, poorly hydrated, and badly fed can when combined take a huge percentage off the value of your insulation by robbing your body of the ability to make heat. Bad technique, such as allowing yourself to get too cold during the day and/or letting your base and action layers to take on too much moisture can similarly kill the practical value of insulation. Not maximizing your mental state also makes it hard to stay warm, both insofar as good psyche is linked to not making the aforementioned mistakes, and in that mental and physical well being cannot in my mind be meaningfully separated. Embracing discomfort makes a cold, wet night seem warmer.
All that said, some moisture in your action layers is inevitable, and for this reason I now highly prize insulating layers which can not only not degrade significantly when damp, but move moisture. The Hyperpuff seems like the way forward here. I’ve had plenty of synthetic jackets which dried fast, but none which dried so well from body heat alone. How much of this is the insulation, and how much the liner and shell fabric? My hope is that continued development here will provide more answers.
In summer, I can make do with a lighter insulating layer. Often this is down, as longer and warmer days make drying out easier, or just an active insulating jacket, as long days make it easy for ones sleeping bag to be the only actual static insulator. Outside the warmest months of the year I rarely regret something as warm as the Hyperpuff, with a rarely used massive down parka the only other thing needed for those below 0 days.