Sea to Summit Reactor Lightweight Liner
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Reactor Lightweight Liner
+4°C / 7°F Warmth Boost · Thermolite® Pro · Ultralight
Optimized for minimal weight and maximum warmth, the Reactor Lightweight enhances sleep system performance with advanced Thermolite® Pro fabric. Hollow-core fibers and infrared-absorbing ceramic pigments trap body heat for ultralight expeditions, adding up to 4°C / 7°F with minimal pack size.
Adds Up to 4°C / 7°F
When paired with a quality bag and mat, a Reactor Lightweight can add up to 4°C / 7°F of warmth — significant heat retention at a fraction of the weight and bulk of a heavier 3-season bag. The actual boost depends on the bag (how well it retains heat) and the mat (how well it insulates from the ground).
Thermolite® Pro Technology (NEW)
Thermolite® Pro with hollow-core fibers AND infrared-absorbing ceramic pigments — engineered hollow fibers trap warm air like standard Thermolite, but the permanent ceramic pigments absorb the body's infrared rays and reflect them back to the wearer. Effectively turns body heat into insulation.
Stretch-Knit + Articulated Arm Openings
Stretch-knit fabric and articulated 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 Fitted Hood
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 the bulk of a draft collar.
Fits Any Sleeping Bag Shape
Fits any sleeping bag — stretch fabric means the Reactor Liner adapts to whatever shape sleeping bag is underneath: mummy, semi-rectangular, rectangular, even a quilt. The liner conforms to the bag's shape rather than dictating it. One liner across multiple bags in a household.
HeiQ Fresh FFL Odor Control
HeiQ Fresh FFL odor control technology keeps the liner fresh after extended use — biocide-free, highly effective, uses a bio-based Amino Sugar Polymer to neutralize body odor (vs silver-ion treatments that wash out faster and have 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 for quick, hassle-free entry and exit. Connected: liner stays attached as one unit. Disconnected: liner doubles as a stand-alone summer sleep, hostel sheet, or fleece blanket.
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 without repeated exposure to body oil.
Lower-Impact Materials (Recycled + bluesign®)
Crafted with certified recycled fibers and bluesign APPROVED materials — Thermolite EcoMade (made from 100% textile waste, GRS-certified) with the same quality and durable lightweight warmth as polymer-virgin fibers, but lower carbon footprint.
Stand-Alone Use (Still Air, Summer)
In summer temperatures and still air conditions, a Reactor Lightweight can function on its own for sleepers wearing long base layers — the hood + the +4°C warmth boost is enough for hot summer nights in mild climates. Note: the fabric is NOT windproof, so any breeze significantly reduces the stand-alone warmth.
Materials & Construction
Insulation Fiber
Thermolite® Pro with infrared-absorbing ceramic pigments (hollow-core fiber)
Recycled Version
Thermolite® EcoMade (100% recycled textile waste, GRS-certified)
Construction
Stretch-knit fabric (articulates with the body)
Hood
Ergonomic, fitted (added head warmth without bulk)
Arm Openings
Articulated, 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 +4°C / +7°F (depends on bag + mat insulation quality)
Reactor Warmth Ranking
Lightest option: Fleeceweight > Midweight > Lightweight (this)
Stand-Alone Use
Summer temperatures, still air, long base layers — NOT windproof
Dimensions & Weight (per length)
Short
SKU 053201 · 178 × 80 cm / 70 × 31 in · 235 g / 8.3 oz · 18 × 10 × 10 cm / 7 × 4 × 4 in packed · recommended up to 170 cm / 5'7" · $64.95
Regular
SKU 053202 · 193 × 80 cm / 76 × 31 in · 255 g / 9.0 oz · 18 × 10 × 10 cm / 7 × 4 × 4 in packed · recommended up to 185 cm / 6'1" · $69.95
Long
SKU 053203 · 206 × 80 cm / 81 × 31 in · 270 g / 9.5 oz · 18 × 10 × 10 cm / 7 × 4 × 4 in packed · recommended up to 198 cm / 6'6" · $74.95
Use
Best Use
Ultralight Backpacking, Thru-Hiking, Bikepacking, Trekking, Camping, Lightweight Mountain Expeditions, Hostel/Travel, 3-Season Use
Activity Fit
Fits ANY sleeping bag shape (mummy, semi-rectangular, rectangular, quilt)
Pairs With
Sea to Summit Spark / Ascent / Trek Down Sleeping Bags (snap connectors link shoulders)
Sister Products
Reactor Fleeceweight Liner (+8°C / 14°F, bulkier) · Reactor Midweight Liner (+~6°C / 11°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 fibers and reduce wicking and warmth-retention properties. Skip bleach (yellows synthetic fibers and breaks down stretch-knit).
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 and infrared-absorbing ceramic pigments. Hanging to dry is the safest option. The Thermolite Pro hollow-core fibers dry quickly, typically taking only 2-3 hours.
Inspect Snap Connectors Before Each Trip
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 (7 × 4 × 4 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 Pro fibers 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 stretch-knit faster than you'd expect. The ceramic pigments survive normal use but degrade in direct flame.
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 Lightly to Preserve Fiber Loft
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 Lightweight is the most packable of the Reactor line, so easy packing is part of its design.
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.
