Sea to Summit Reactor Fleeceweight Liner
- Free Domestic Shipping On Orders Over $100
- In stock
- Backordered, shipping soon
- Add To Cart For Final Price
Reactor Fleeceweight Liner
+8°C / 14°F Warmth Boost · Thermolite® · 3 Lengths
Boost your sleep system's warmth with the Reactor Fleeceweight. Thermolite® hollow-core fibers and grid fleece trap and reflect body heat, adding up to 8°C / 14°F with minimal weight, while the stretch-knit construction and shaped arm openings allow natural movement through the night.
Adds Up to 8°C / 14°F
When paired with a quality bag and mat, a Reactor Fleeceweight can add up to 8°C / 14°F of warmth. The exact boost depends on the bag (how well it retains heat) and the mat (how well it insulates), but in a typical 3-season sleeping bag + air mat setup, this liner effectively turns a 5°C bag into a -3°C bag.
Thermolite® Hollow-Core Fibers + Grid Fleece
Thermolite® hollow-core fibers trap warm air in the fiber core for insulation, and the grid fleece construction creates air pockets that reflect body heat back — the combination delivers the warmth of heavier fleece at a fraction of the weight. Thermolite EcoMade version uses 100% recycled textile waste (GRS-certified).
Stretch-Knit & Shaped Arm Openings
Stretch-knit fabric and shaped arm openings allow unrestricted movement — natural comfort through tossing, turning, and reading on your side. The stretch keeps the liner close to the body (necessary for warmth) without feeling constrictive, unlike a sleeping bag's non-stretch shell.
Ergonomic Hood for Warmth Without Bulk
Technical shape and hood — ergonomic design with a fitted hood delivers optimal warmth and comfort without extra weight or bulk. The hood retains heat around the head where significant body heat is lost (much like a sleeping bag hood), but doesn't add a draft collar or heavy draft tube.
HeiQ Fresh FFL Odor Control
HeiQ Fresh FFL odor control technology keeps the liner fresh after extended use — biocide-free, highly effective, and uses a bio-based Amino Sugar Polymer to neutralize body odor. Unlike silver-ion anti-odor treatments, HeiQ Fresh doesn't wash out as quickly and doesn't have the same skin-sensitivity concerns.
Snap Connectors for Sea to Summit Bags
Sleep system integration — snap connectors at the shoulders easily pair with Sea to Summit sleeping bags (Spark, Ascent, Trek) for quick, hassle-free entry and exit. Snap in and the liner stays attached to the bag as one unit; unsnap for use as a stand-alone fleece blanket or warm-weather sleep.
Machine Washable — Reduces Bag Washes
Machine washable construction simplifies care and reduces wear on your sleeping bag over time — using a liner means body oils and sweat stay on the liner, not the bag's face fabric. Bags stay cleaner, loft is preserved longer, and the bag's DWR finish lasts longer when not exposed to body oil every wash.
Lower-Impact Materials (Recycled + bluesign®)
Crafted with certified recycled fibers and bluesign APPROVED materials to minimize environmental impact — the Thermolite EcoMade fiber is made from 100% textile waste, GRS-certified, with the same warmth as polymer-virgin fibers but a lower carbon footprint.
Usable Stand-Alone in Warm Weather
Can be used on its own as a lightweight sleeping bag in warm-weather conditions — the hood, snap connectors, and full-coverage construction make it a functional summer sleep system for hostels, warm nights, or summer alpine bivys. Adds warmth without the commitment of carrying a sleeping bag.
Materials & Construction
Insulation Fiber
Thermolite® EcoMade (100% recycled textile waste, GRS-certified, hollow-core)
Construction
Stretch-knit grid fleece (traps + reflects body heat)
Hood
Ergonomic, fitted (added head warmth without bulk)
Arm Openings
Shaped, stretch-knit (unrestricted movement)
Sleep System Snap
Shoulder snap connectors (pair with Sea to Summit sleeping bags)
Odor Control
HeiQ Fresh FFL (biocide-free, Amino Sugar Polymer)
Certifications
GRS (recycled content) · bluesign APPROVED · non-PFAS
Thermal Performance
Warmth Boost
Up to +8°C / +14°F (depends on bag + mat insulation quality)
Reactor Warmth Ranking
Warmest in the Reactor line: Fleeceweight > Midweight > Lightweight
Stand-Alone Use
Lightweight sleeping bag in warm weather; summer hostel use
Dimensions & Weight (per length)
Short
SKU 053401 · 178 × 80 cm / 70 × 31 in · 365 g / 12.9 oz · 25 × 15 × 15 cm / 10 × 6 × 6 in packed · recommended up to 170 cm / 5'7" · $84.95
Regular
SKU 053402 · 193 × 80 cm / 76 × 31 in · 400 g / 14.1 oz · 25 × 15 × 15 cm / 10 × 6 × 6 in packed · recommended up to 185 cm / 6'1" · $89.95
Long
SKU 053403 · 206 × 80 cm / 81 × 31 in · 430 g / 15.2 oz · 25 × 15 × 15 cm / 10 × 6 × 6 in packed · recommended up to 198 cm / 6'6" · $94.95
Use
Best Use
Winter Camping, Cold-Weather Backpacking, Mountaineering, Hostel/Travel (warmer climate), Trekking, Glamping, Lightweight Sleeping Bag Substitute (warm weather)
Backpacking Note
Bulkier than Midweight or Lightweight Reactor — requires larger winter backpacks to fit; too bulky for bikepacking or ultralight backpacking
Pairs With
Sea to Summit Spark / Ascent / Trek Down Sleeping Bags (snap connectors link shoulders)
Sister Products
Reactor Midweight Liner (+~4°C / 7°F) · Reactor Lightweight Liner (+~3°C / 5°F) · Thermolite Reactor Liners (silk-weight)
Machine Washable (Without Special Soaps)
Reactor liners can be machine washed without special soaps — regular household detergent is fine. Wash in cold or warm water on a gentle cycle. Skip fabric softeners entirely, as they coat the fleece fibers and reduce the wicking and warmth-retention properties.
Tumble Dry Low or Air Dry
Air dry or machine dry on a cool to warm cycle without dryer sheets — high heat can shrink the stretch-knit fabric or damage the snap connectors. Hanging to dry is the safest option. The Thermolite hollow-core fibers dry quickly, so line-drying typically takes only a few hours.
Snap Connectors — Inspect Before Trips
Before each trip, inspect the shoulder snap connectors for damage or deformation — if the male/female snap doesn't engage cleanly, the liner won't stay attached to the bag. A loose snap results in the liner bunching at the bottom of the bag, which compromises the warmth boost.
Store Loosely Folded, Not Compressed
Store the liner loosely folded in a drawer or on a shelf — don't leave it compressed in a stuff sack for months, as compression reduces the loft of the hollow-core fibers. The Packed Size listed (10 × 6 × 6 in) is for travel; for storage, fold flat and let the fibers recover their spring.
HeiQ Fresh Refreshes With Each Wash
HeiQ Fresh FFL odor control re-activates with each wash — the biocide-free Amino Sugar Polymer neutralizes new odors as it dries. For a quick refresh between full washes, hang the liner outside for a few hours; UV exposure helps break down any residual body oils in the fibers.
Avoid Direct Flame & High Heat
Thermolite and grid fleece are synthetic and will melt on direct contact with a stove flame or candle. Keep the liner well away from campfires, camp stove flames, and any high-heat source. A spark burns a hole in fleece faster than you'd expect.
Repair Holes with Tenacious Tape
Small holes can be field-repaired with Gear Aid Tenacious Tape patches (cut a round patch 1/2 inch larger than the hole, round the corners, apply to clean dry surface, press firmly). The stretch-knit holds stitches poorly, so patch-and-go is faster than sewing in the field.
Pack Properly to Avoid Bulk
When packing the liner for a trip, stuff it loosely into a compression sack or use the included stuff sack — never use a tight roll compression, as it can crush the hollow-core fibers over multi-day trips. The bulk is the trade-off for warmth in the Fleeceweight.
Shipping Policy
Components:
All component items (wheels, drivetrains, suspension, brakes, etc.) ship within 2-5 business days of purchase.
Bicycles:
Complete bikes typically ship within 2–3 weeks due to build, tuning, and inspection times. You will receive tracking information as soon as your order is ready to ship.
Riding Gear & Apparel:
Riding gear and apparel orders (helmets, jerseys, gloves, etc.) ship within 2-5 business days.
Returns & Exchanges:
If you received the wrong size or wish to exchange an item, it must be returned in original condition—unused, uninstalled, and with all packaging intact.
For full details, please see our Refund Policy before initiating a return.
