I use multiple coats of varying design and insulating materials, and insulation levels. An oversize parka like a Carhartt, with synthetic insulation, that has a very durable outer shell that can be used alone, or with a down vest, and even a fairly bulky down coat underneath it. The Carhart alone is ok for down to about 30F if you are not undertaking any physical activity. They don't make the one I own anymore, but this is similar:
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Under that, or on its own, a down jacket that does not extend far below the waist, and does not have an attached or removeable hood. The one I have is a Mountain Hardware, that they called a sweater. It is 800 fill power down, and the down is in "boxes", or "tubes" meaning it s not sewn through like a quilt, which results in cold spots where the stitching creates thin spots.
Another step up, is a thicker down jacket that goes below the waist, and has a non-removeable down hood.
A removeable hood is usually not as warm as a permanently attached hood.
The next level is not using the Carhartt as an outer parka, but using an oversized Columbia down insulated parka over either of the above listed types of jackets. This parka has a down insulated "snorkel" hood, with a removeable synthetic fur ruff on it. Removing the ruff increases visibility when using the hood in the snorkel mode.
I own an older version of the Mountain Hardware Absolute Zero Parka, that is very warm, and very well designed.
Invalid Link Removed This parka is an example of box rather than quilted construction that makes a much warmer parka. This is an example of the difference between a Sears parka, and a real parka. It costs more to make but you get what you pay for. Mine fits under my oversized Columbia down parka for incredible warmth.
Canada Goose makes some of the best cold weather coats you can buy. Not cheap, but very warm, and very well made.
For extended time outside in the coldest temperatures, (twenty below F or colder), I break out the Canada Goose Snow Mantra Parka:
https://www.canadagoose.com/us/en/snow-mantra-parka-9501M.html This is the parka you need if you will be going to Antarctica. When I bought my Snow Mantra, I discovered that there are counterfeiters in China selling fake Canada Goose parkas on eBay, so shop carefully if you buy a Snow Mantra. I have a pair of their down insulated bibs. With just this parka, and bibs, wearing only a pair of cotton briefs underneath, I stood out on a frozen lake in -20F temperature, with a 15mph wind, and did not have to batten down the parka to its warmest setup. I actually had to unzip the parka after a while to verify that it was very cold out.
Mountain Hardware, Marmot, The North Face, and Columbia all make great cold weather gear.
Many down coats have a percentage of fill that is not down. Read the label of any coat you are considering. You don't want waterfowl feathers. 100% goose down is very good.
Understand fill power:
Fill power - Wikipedia
Baffin makes some of the warmest boots I have ever owned. Like this:
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Merino wool is a very soft and fine wool that comes in a few grades. The finer the better. It does not cause itching, it insulates even when it it is wet, and has natural anti microbial properties that keep it from getting smelly. I have bought merino wool sweaters at Goodwill second hand stores for only a few bucks apiece. I have bought new as well. It is expensive but is very warm. I also buy regular Woollrich shirts too.
The base of my cold weather dress is a pair of polypropylene liner socks, under wool socks. Silk top and bottom long underwear, under 100% merino wool long underwear.
If you will be physically active in very cold weather, you have to ensure you don't saturate your insulation with water vapor from your body. Wool insulates when it is wet. Most synthetics, don't absorb moisture. Polypropylene is used for cold weather base layers because it does not easily absorb water, and it dries quickly. Cotton clothing does not dry easily once it is wet, and if you are active enough to make your cotton clothing damp, and then you cease the activity, you will become cold very quickly.
