Ice & Cold
Use a vapor barrier liner in extreme cold
In deep cold, moisture from your body destroys insulation. A vapor barrier keeps sweat out of your down or synthetic layer.
Step-by-step
Choose a non-breathable liner — trash-bag plastic, coated nylon, or a purpose-built VBL sock or shirt.
Wear the liner directly against skin or over a thin base layer, and put your insulation over the liner.
Dress cool on the move — the liner traps moisture, so you'll feel damp inside but your insulation stays dry.
At camp, remove the liner and dry your base layer against your body inside your sleeping bag.
Use only when temperatures stay below about -15°F (-26°C). Above that, sweat management outweighs insulation protection.
Warning: Vapor barriers can macerate skin during heavy exertion. Watch for hot spots and stop to dry if any skin surface feels sodden.
Related outdoor skills
Educational reference only. Wilderness conditions change fast — practice in low-stakes settings, take a certified wilderness first-aid course, and confirm regional regulations before you rely on any of these skills in the field.