A sleeping bag liner is one of those quiet workhorses of a camp kit item, and the Therm-a-Rest Synergy Sleeping Bag Liner earns its place by doing several useful things at once.

The stretch-knit polyester fabric feels like a soft base layer against skin, with flatlock perimeter seams that eliminate any pressure points or bunching inside your bag. It's shaped to fit mummy bags properly, so it conforms rather than bunches, and fits bags up to 6'6" (198 cm). Beyond comfort, the liner acts as a protective barrier between your body and your sleeping bag's insulation, absorbing sweat and body oils that would otherwise work their way into the fill and gradually reduce its loft over time. When it needs cleaning, you pull the liner out and toss it in the washing machine rather than putting your whole bag through a wash cycle.

The warmth contribution is a genuine bonus, adding up to 5°F/3°C to your sleep system, verified in Therm-a-Rest's own cold chamber testing. That's a meaningful buffer for a chilly night when you're pushing the lower limit of your bag's rating.

• Weight: 12.1 oz (340g)
• Dimensions: 31 x 80 in
• Fits mummy bags up to 6'6" (198 cm)
• Fabric: Stretch-knit polyester with flatlock seams
• Adds up to 5°F/3°C of warmth to existing sleep system
• Protects bag insulation from body oils and sweat
• Machine washable
• One size