If you want something to keep you dry while you’re working up a sweat cross-country skiing, opt for a lightweight, high-output pair. Long johns designed for high-output activity are slim and generally fit similarly to compression activewear. Since moisture would sit close to your body, you’ll get cold fast if your long johns get wet and stay that way. Thermolite is a popular synthetic blend common in long johns because it’s insulating and moisture-wicking. WoolThough it’s the most expensive, merino wool is considered the best base layer for its natural properties, including odor control, heat retention, and moisture-wicking.