Heavyweight base layers (260–350 g/m²) are built for extreme cold—think days well below 0°F. They're thick, plush, and designed to trap maximum heat against your skin. If you're standing around a lot (like ski patrol) or sitting on long, slow lifts in brutal conditions, that extra insulation can be a lifesaver.
The catch: heavyweight is overkill for most resort skiers. If you're actively making turns, you'll generate plenty of body heat, and a heavyweight base layer will leave you sweating through your kit by mid-morning. That sweat then freezes when you sit still, making you colder—not warmer. For most skiers, a lightweight or midweight base layer paired with proper mid-layers is the smarter choice.
Save heavyweight for the coldest days, sedentary riding, or if you genuinely run very cold. Otherwise, you're paying for warmth you can't actually use.