I routinely use a silk sleeping bag liner inside all my sleeping bags for both comfort and warmth.
A sleeping bag liner adds warmth. It can add several degrees of warmth to your bag depending on the fabric, which allows one to buy a lighter bag but still get the temperature rating of a heavier bag.
For customers with old sleeping bags with compressed insulation that has lost warmth, a liner allows a boost to the warmth and delay the purchase of a new bag.
A liner can act as a draft barrier keeping users warmer and can fill up the excess room in a mummy or rectangular bag, boosting warmth.
A liner keeps a sleeping bag clean and minimizes the need for laundering. Washing a liner after a trip is way easier (and cheaper) than going to a Laundromat and washing a whole sleeping bag in a large commercial machine. Washing a sleeping bag is the fastest way to ruin it. Most manufacturers recommend using a liner and just spot-cleaning the sleeping bag.
A liner adds comfort. A liner made of Silk or CoolMax is more comfortable against the skin than the linings of many bags.
Many liners will help wick away moisture, keeping users drier to help them sleep more comfortably. And a liner helps avoid the initial shock of climbing into a cold sleeping bag.
A liner adds versatility. Often three-season bags are too hot to sleep in on summer nights, when 90 percent of people do 90 percent of their camping. A liner allows customers to use their sleeping bag like a big quilt. Climb in the liner, unzip the bag all the way and pull it over as much or as little as needed. No more zipping and unzipping the bag all night long because they’re too hot or too cold.
One can use the liner alone or as a warm-weather bag. It can be used as a sleep sack for traveling or for nights on friends’ couches, in youth hostels, airports, trains or even hotels.