Sea to Summit Reactor Midweight Sleeping Bag Liner

The Sea to Summit Reactor Midweight Sleeping Bag Liner is a technical sleep-system liner built for backpacking, cycle touring, and paddle touring, adding up to 6°C / 11°F of warmth to a sleeping bag through Thermolite Pro hollow-core fibers and infrared-absorbing ceramic pigments. Crafted from 100% post-consumer recycled polyester and carrying bluesign APPROVED certification, the Reactor Midweight is available in Short, Regular, and Long fits to suit a range of user heights.

Warmth Without the Weight

Thermolite Pro fabric works by combining hollow-core fibers (which trap a layer of insulating air) with ceramic pigments that absorb the body's infrared radiation and reflect it back toward the sleeper. The result is meaningful warmth added to a sleep system without a corresponding increase in bulk or packed size. For backpackers and cycle tourers who have already optimized their sleeping bag rating, a liner that adds up to 6°C / 11°F can extend a bag's usable season or serve as a buffer on unexpectedly cold nights, without requiring a heavier bag in the pack.

Freedom to Move Through the Night

The Reactor Midweight uses a stretch-knit construction and shaped arm openings so the liner moves with the sleeper rather than bunching or restricting. A fitted hood adds warmth at the head without extra material. Snap connectors at the shoulders integrate directly with Sea to Summit sleeping bags, making entry and exit straightforward rather than a nightly wrestling match with tangled fabric.

Easier Care, Longer Bag Life

The Reactor Midweight is machine washable without special soaps, though fabric softeners and dryer sheets should be avoided. Washing the liner instead of the sleeping bag after each trip reduces cumulative wear on the bag's insulation and shell fabric. HeiQ Fresh odor control, a biocide-free technology using a bio-based amino sugar polymer, keeps the liner fresher between washes during extended trips. The 100% post-consumer recycled polyester shell carries GRS certification, and the Thermolite EcoMade fibers are made from 100% textile waste.

Features

• Thermolite Pro insulation: hollow-core fibers and ceramic pigments trap and reflect body heat, adding up to 6°C / 11°F to a sleep system
• Stretch-knit construction with arm openings: allows unrestricted movement and fits a wide range of sleeping bag shapes
• Fitted hood: ergonomic hood adds warmth at the head without extra bulk
• Shoulder snap connectors: integrate with Sea to Summit sleeping bags for easy entry and exit
• HeiQ Fresh odor control: biocide-free amino sugar polymer technology neutralizes body odor between washes
• 100% post-consumer recycled polyester: bluesign APPROVED and GRS-certified materials

Specifications

Who This Is For and Isn't

The Reactor Midweight suits backpackers, cycle tourers, and paddle tourers who want to add meaningful warmth to an existing sleep system, extend a sleeping bag's seasonal range, or reduce how often the bag itself needs washing. The snap connectors make it a natural pairing for Sea to Summit sleeping bags, and the stretch-knit fit accommodates mummy and semi-rectangular bag shapes. In summer temperatures and still air, the liner can also function on its own for sleepers wearing long base layers.

The Reactor Midweight is not windproof and is not designed for use as a standalone shelter in cold or exposed conditions. The shoulder snap connectors are designed to integrate with Sea to Summit sleeping bags; compatibility with other brands' snap systems is not stated. Sleepers taller than 198 cm / 77.9 in exceed the stated fit range of the Long size.

Common Questions

How much warmth does the Reactor Midweight add?

The Reactor Midweight Sleeping Bag Liner adds up to 6°C / 11°F to a sleep system. The actual gain depends on how well the sleeping bag retains heat and how well the sleeping mat insulates, so a quality bag and mat are needed to realize the full benefit.

How does the Reactor Midweight compare to other Reactor liners?

Sea to Summit makes three Reactor liner weights. The Reactor Fleeceweight is the warmest option in the lineup, the Reactor Midweight sits in the middle, and the Reactor Lightweight is the least insulating of the three.

Can the Reactor Midweight be machine washed and dried?

The Reactor Midweight Sleeping Bag Liner is machine washable without special soaps. Fabric softeners should not be used. The liner can be air dried or machine dried on a cool-to-warm cycle; dryer sheets should be avoided.

Can the Reactor Midweight be used without a sleeping bag?

In summer temperatures and still air conditions, the Reactor Midweight can function as a standalone sleep layer for sleepers wearing long base layers. The fabric is not windproof, so the liner is not suited for exposed or cold conditions on its own.

Which size Reactor Midweight should I choose?

The Short fits sleepers up to 170 cm / 66.9 in, the Regular fits up to 185 cm / 72.8 in, and the Long fits up to 198 cm / 77.9 in. All three sizes pack to the same 19 × 11.5 × 11.5 cm / 7 × 5 × 5 in packed size.