Sea to Summit Reactor Fleeceweight Sleeping Bag Liner

The Sea to Summit Reactor Fleeceweight Sleeping Bag Liner is a mummy-style fleece liner built for cold-weather backpacking, ski touring, and three-season camping. Thermolite hollow-core fibers and grid fleece construction can add up to 8°C / 14°F of warmth to a sleep system when paired with a quality sleeping bag and insulating mat. The liner is available in Short, Regular, and Long sizes to match a range of user heights.

Warmth That Earns Its Weight

Thermolite hollow-core fibers are engineered to trap and reflect body heat rather than simply adding insulating bulk. The grid fleece construction creates air pockets that hold warmth close to the body, so the liner contributes meaningful thermal performance without the weight penalty of a heavier synthetic fill. For backpackers who want to extend a three-season bag into colder conditions, or who want a margin of safety on unpredictable nights, that warmth-to-weight ratio matters more than a raw temperature number.

Using the Reactor Fleeceweight liner also reduces how often a sleeping bag needs washing. Fewer washes mean less compression of the fill, which preserves loft and extends the bag's effective lifespan.

Freedom of Movement and System Integration

The Reactor Fleeceweight is cut with stretch-knit fabric and shaped arm openings so it moves with the sleeper rather than binding at the shoulders or hips. A fitted hood adds warmth at the head without excess material. Snap connectors at the shoulders attach directly to Sea to Summit sleeping bags, making it easy to get in and out of the combined system without the liner bunching or twisting. The liner can also be used on its own as a standalone sleep layer in warm-weather conditions.

Materials and Care

The shell and fill are 100% post-consumer recycled polyester, certified under the Global Recycled Standard and made with bluesign approved materials. The stuff sack is 100% polyester. HeiQ Fresh odor control technology, a biocide-free amino sugar polymer treatment, neutralizes body odor during extended use between washes. The liner is machine washable without special soaps; avoid fabric softeners and dryer sheets, and air dry or machine dry on a cool-to-warm cycle.

Features

• Thermolite hollow-core fill: traps and reflects body heat, adding up to 8°C / 14°F when paired with a quality bag and mat
• Stretch-knit construction with arm openings: allows unrestricted movement throughout the night
• Fitted hood: ergonomic shape adds warmth at the head without bulk
• HeiQ Fresh odor control: biocide-free technology neutralizes body odor between washes
• Shoulder snap connectors: integrate with Sea to Summit sleeping bags for easy entry and exit
• Recycled and bluesign approved materials: GRS-certified post-consumer recycled polyester throughout
• Machine washable: simplifies care and reduces wear on a primary sleeping bag

Specifications

Who This Is For and Isn't

The Reactor Fleeceweight suits cold-weather backpackers and ski tourers who want to extend a sleeping bag's warmth range, three-season campers who want a buffer against unexpectedly cold nights, and anyone who wants to reduce washing frequency on a high-quality sleeping bag. The liner can also serve as a standalone sleep layer for warm summer nights. Larger winter backpacking packs generally have enough volume to carry it.

The Reactor Fleeceweight is not suited to bikepacking with frame bags or ultralight backpacking setups where packed volume is tightly constrained; it is likely too bulky for those applications. The snap connectors are designed to integrate with Sea to Summit sleeping bags; compatibility with other brands' bags is not stated. The warmth addition of up to 8°C / 14°F is conditional on the quality of the paired sleeping bag and insulating mat, not a guaranteed absolute rating.

Common Questions

How much warmth does the Reactor Fleeceweight Sleeping Bag Liner actually add?

The Reactor Fleeceweight Sleeping Bag Liner can add up to 8°C / 14°F of warmth to a sleep system. The actual gain depends on how well the paired sleeping bag retains heat and how effectively the sleeping mat insulates from the ground. A quality bag and mat are needed to reach that upper figure.

How does the Reactor Fleeceweight compare to other Reactor liners in warmth?

Among Sea to Summit's Reactor liner range, the Reactor Fleeceweight is the warmest option. The Reactor Midweight sits in the middle of the range, and the Reactor Lightweight is the least insulating of the three. Choosing the Fleeceweight makes sense when maximum warmth addition is the priority over minimum packed weight.

Is the Reactor Fleeceweight suitable for bikepacking or ultralight backpacking?

For bikepacking with frame bags or ultralight backpacking where pack volume is tightly limited, the Reactor Fleeceweight Sleeping Bag Liner is likely too bulky. It is better suited to larger winter camping packs that have the volume to accommodate it, or to car camping and base-camp setups where packed size is less critical.

How do you wash and dry the Reactor Fleeceweight Sleeping Bag Liner?

The Reactor Fleeceweight Sleeping Bag Liner is machine washable without special soaps. Do not use fabric softeners or dryer sheets. After washing, air dry the liner or machine dry it on a cool-to-warm cycle. This care routine keeps the liner fresh and maintains the integrity of the Thermolite fill over time.