Sea to Summit Reactor Fleeceweight Liner

Sea to Summit Reactor Fleeceweight Sleeping Bag Liner

A versatile, lightweight liner that adds meaningful warmth to your existing sleep system or stands alone as a summer bag for ultralight travelers. The Reactor Fleeceweight combines Thermolite hollow-core fibers with grid fleece to add up to 14°F of warmth at 13.6 ounces without meaningfully affecting your packed size.

Why We Carry It
It's the liner we recommend for backpackers who want to extend the range of their current sleeping bag into colder conditions without buying a second bag.

Highlights

• Thermolite hollow-core fibers with grid fleece — A dual-material construction that traps and reflects body heat more efficiently than a standard fleece liner. The hollow-core fibers add warmth without the weight of a traditional insulating fill.
• Adds up to 14°F to your sleep system — A meaningful temperature rating boost that can push a three-season bag into shoulder season territory or a summer bag into a legitimate three-season option without carrying a second sleeping bag.
• Stretch-knit construction with shaped arm openings — Moves naturally with your body through the night without restricting movement or bundling up uncomfortably. The shaped arm openings allow a more natural sleeping position than a standard mummy liner.
• Fitted hood with ergonomic design — Adds warmth at the head and neck where heat loss is most significant without excess bulk or an ill-fitting hood that shifts during the night.
• HeiQ Fresh odor control — Keeps the liner fresh through extended use and multi-day trips without requiring a wash after every outing. A practical advantage for long backpacking trips and ski touring where laundry access is limited.
• Snap attachments for Sea to Summit sleeping bags — Connects directly to compatible Sea to Summit bags to prevent shifting inside the bag during the night.
• Easy to clean and quick-drying — Reduces the frequency of washing your sleeping bag by acting as a barrier between you and the bag. Washing a liner instead of a bag preserves loft and extends the life of your sleep system over time.
• 13.6 ounce weight packing to 10 x 6 x 6 inches — Light and compact enough to justify carrying as a dedicated cold weather boost or a standalone summer option without meaningfully adding to your pack weight or volume.

Best for

Backpackers, ski tourers, and cold-weather campers who want to extend the warmth range of their existing sleep system without buying a new bag. A strong standalone summer liner for ultralight travelers and a smart investment for anyone who wants to protect the loft and longevity of a quality sleeping bag.