The very best thing along with layers is to keep your blood warm and flowing. I don't know why UA works being so tight, but it does at keeping you dry. I even bought the boxers this year and they did well. That stuff though doesn't keep the heat in. Kind of like sitting in front of a fan and layers that hold heat outside of those is a must. UA is just a name brand and others are around at less cost.
Having said that chem heat over your kidneys works amazingly well at keeping your blood warm and they stay put under UA. You don't even notice what's going on until it dawns on you your hands and feet are not cold. Keep your circulation going and add in the warm blood, your good.
Got some Cabela's extreme silk a few seasons ago. That stuff is something else. Funny stuff, doesn't dry well at all but some how it keeps me warm. Look at that stuff and their smart wool socks are tops too.
Heat over the kidneys is the ticket.