A sleeping bag liner is a thin sack you sleep inside your sleeping bag. It does two jobs. It keeps sweat and skin oil off the bag, which matters because washing a down bag is a chore and every wash costs it a little loft. And depending on the fabric, it adds a few degrees of warmth for a fraction of what a warmer bag would weigh.
The two jobs pull against each other. A silk or nylon sheet weighs four or five ounces and adds almost no warmth. A Thermolite or fleece liner can push a summer bag into a cold shoulder-season night, but it weighs closer to a pound. So decide which job you're buying it for first. The eight below are sorted with that in mind.
One warning about the numbers on the boxes. Makers advertise warmth boosts of up to 25 degrees Fahrenheit, and independent testers on Trailspace, Packstack and the Backpacking Light forums keep finding about half that. We have read every claim on this page as a best case, and ranked on the warmth a liner is likely to add, not the figure on its label.
My dad and I have not slept in any of these eight, and I won't write as though we have. The order comes from each maker's published weights and warmth claims, what reviewers found in testing, and what Amazon buyers report after a season of use. A few cottage-made liners are sold only through their makers, so they aren't here.
