If you really are going to stand or sit around in -20F weather, you'll need something in the TNF Baltoro, Marmot 8000 Meter, or Feathered Friends Rock and Ice class of baffled down parka. It's that cold--I have had hot coffee in a styrofoam cup freeze sold in those temperatures in less than ten minutes. -20F is the usual summit temperature of Denali during the peak climbing season, and no one goes there (and makes it to the top) with just a down sweater.

Don't worry about down getting wet in the winter--it won't happen at the temperatures you're talking about.

I have plenty of down jackets, and some synthetic filled ones--including both a Patagonia down parka and the Patagonia DAS parka. When considering the DAS parka, it helps to understand what it was designed for: to be a belay parka that could be worn over all other layers and be relatively immune to the damp and sharp metal items found in ice climbing. Synthetic insulation is good at keeping its loft when wet and not having the insulation fly away when the shell is torn. But the DAS parka isn't as warm as even a TNF Nuptse down jacket, primarily because its loft is about half as much as the North Face jacket.

The Patagonia down parka is an excellent piece of kit, but it has its limitations--it's polyester shell and lining is quite light, so care is required to keep all that down where it belongs. It's a full-fledged parka (both baffled and with an integral hood), but not as thick or warm as the big three I mentioned in the opening sentence. I'd say it's good to about 0F when immobile. The DAS parka is pushing it at 20F.

If you're on a budget, look for TNF Baltoros on eBay: I bought one once for $125. Stay away from the ones that have the reflective liner or any of the newer iterations which have far less down than the one shown in the photo of Tom above.

Here's some other tips for staying warm in below zero weather.

First and foremost, stay well-feed and hydrated, which means having a reliable cold weather stove.

Don't forget your extremities. Wear a hat and protect your neck with either a scarf or by wearing a balaclava. Definitely get a parka with a hood, but don't drive a car with the hood of the Baltoro up (the insulated muff kills any chance for peripheral vision). Put zipper pulls on the sliders so you can manipulate the openings even with mittens on.

Wear mittens. TNF himalayan down mitts are very nice, but if you're on a budget, thick wool or fleece mitts are available. If you insist on gloves, don't have them too tight, including reefing down on the wrist closure and the gauntlet drawstring. Warm softshell gloves are very rare, but worth it (Hestra makes some). No need for GoreTex when there's no water, but many of the other WP/B gloves are far less breathable than GoreTex. And yes, there are eVent gloves. Wear thin liner gloves under everything so that your fingers don't freeze to anything metal you touch.

A common mistake is to forget about the legs. Down pants (they don't need to be baffled) are wonderful, and synthetic filled ones (such as Patagonia's micropuff pants) aren't bad. Barring that, get the thickest fleece pants you can find. For shell pants, go with something that isn't waterproof.

Don't have your boots too tight (and that includes cramming more socks on). Deep lugs on the soles are good for putting some distance between your feet and the snow. Double plastic boots have the great advantage of allowing you to warm the liners in your sleeping bag. Wear down booties around camp.

Sit and stand on a closed cell foam pad--you'll be amazed at what a difference this makes. Sorel type pac boots are hell to walk in, but wonderful for standing around.

People in the Arctic do actually wear cotton underwear, but get merino wool or polyester--I do like merino wool the best, but a Patagonia Wool 4 shirt is about the most expensive t-shirt you'll ever come across.

In general, try to keep the number of layers to a minimum--there's less binding and constriction (what you want to do is to keep your blood circulation flowing), and it's easier to do things like go to the bathroom. But make sure that everything can be closed off at the wrist, waist, face, etc.

Having said all of that, I once met an old trapper here in Montana who wouldn't go out in the winter in anything but wool and cotton. And his reasons were sound--he often was in thick brush that would shred nylon, and his means of staying warm was a fire, which meant that inevitably there would be sparks on his clothes.

But perhaps the best tip of all is to learn to do most everything while in your sleeping bag: cooking, eating, pissing, playing cards.