Out of Black Diamond’s entire collection of ski shells, the Men’s BoundaryLine Insulated Jacket is by the far the warmest option for the coldest days of the season. Black Diamond’s BD. dry membrane boasts high waterproof, breathable ratings to keep comfy when you’re getting after it on powder days. Thermolite insulation provides performance-ready warmth fit for demanding skiers. There are ventilation zippers under each sleeve to amplify airflow, and the powder skirt keeps you covered on the deepest days of the year.