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 bag 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.
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 are light, soft and strong and retain much thermal insulation even when wet.
Cotton is heavier and bulkier than silk and absorbs water.
Synthetic [mix] liners are usually light, strong, and do not absorb water.