For true winter warmth, prioritize fabric weight over just "wool": lightweight merino or lambswool turtlenecks (like Uniqlo's) work fine as a next-to-skin base layer, but for real cold you want a heavier Donegal-style or Aran fisherman's wool knit over the Cowichan, not instead of it. RMRS note: that bulkier wool also traps more air, so it's warmer per layer than a fine merino crew. Check texture/weight before buying, not just fiber content.