The Complete Handbook of Sleeping Bag Liners

Chapter 1: Understanding Sleeping Bag Liners

A sleeping bag liner is a removable inner shell that sits between your body and the sleeping bag's insulation. It is not an accessory or an afterthought—it is a functional layer that changes how heat, moisture, and mechanical stress move through your sleep system. To understand what a liner actually does, you have to understand what happens inside a sleeping bag on a cold night, not just what a liner looks like in a product photo.

Imagine you are sleeping in a down bag rated for 20°F (-7°C) on a night that is actually 30°F (-1°C). You are not cold, but you wake up with damp underwear and the back of your neck feels clammy. That dampness is not sweat from exertion; it is insensible perspiration—your body releases roughly 15 to 30 grams of water vapor per hour during sleep, even when you are not visibly sweating. Over an eight-hour night, that is 120 to 240 grams of moisture. A sleeping bag's insulation, whether down or synthetic, works by trapping still air in small pockets. When those pockets fill with water vapor that condenses as the temperature drops, the air is displaced by water, which conducts heat about 25 times faster than air. The insulation loses its loft, the temperature rating shifts several degrees, and you feel cold despite the bag being "rated" for that condition.

A liner's first job is to intercept that moisture. A non-absorbent or low-absorbency liner—typically polyester, nylon, or tightly