Sleeping Bag Liners
Sleeping bag liners either add extra warmth to your sleeping bag or they help keep your bag clean and save you washing it too regularly. This is especially handy for down bags, which are more difficult to clean. They are also a requirement in many Youth Hostels around the world.
Sea to Summit's range of silk liners are the best for providing extra warmth at very low bulk. One of their cotton liners will provide some warmth, but they are a bit heavier and bulkier to carry.
Bag liners fulfill a variety of uses.
Firstly they help keep your bag clean, which helps retain the lofting and thermal properties of the bag for longer. This is especially important for down sleeping bags, which are less wash-friendly. Silk or cotton liners are the best for this, silk having lower weight and size but greater cost. Check out the silk and cotton range from Sea to Summit.
Liners can add thermal properties to a bag. As a rough rule of thumb, a good quality silk liner will add a couple of degrees to a bag's comfort rating. Fleece liners such as those from Outdoor Designs can add an extra season to the bag.
They can be used alone in very hot climates, and here cotton comes into its own. It's cooler and less sweaty than silk and fleece. A liner can be used with a cheap, light bag in temperate regions and also to improve your usual sleeping bag for extreme cold use.
They are also handy for use in huts and youth hostels. Look at the Traveller series in particular.