Sleeping bag liner

Sleeping bag liners are lightweight cloth sacks usually fitted inside sleeping bags to provide extra comfort, insulation, and help keep the sleeping bag clean.

A thin liner may feel softer against the sleeper's skin, add 5 °F warmth to the bag (not pad), and be easily washed after use (unlike the thicker sleeping bag). A thick, fleece-like liner can increase warmth by 10-15 °F. Liners also allow a sleeper to use the liner alone, without the bag in hot conditions. Optionally, bug-repellents, pockets, and a pillow holder can be added to liners.

A vapor barrier liner (VBL) is special type of liner that blocks the sleeper's moisture from reaching the bag, thus stopping evaporative heat loss. Usually a sleeping bag liner is used inside a VBL.

Sleeping bag liners are issued as basic kit by various militaries including the US and British armed forces.

Fabrics
Liners are made of silk, cotton, nylon and polyester among others;
 * Silk liners: light, soft and strong and retain much thermal insulation even when wet
 * Cotton: heavier and bulkier than silk and absorbs water
 * Synthetic [mix]: usually light, strong, and do not absorb water