Sea to Summit Reactor Extreme Liner

Published · Researched 2026-09-21

SKU: UNVERIFIED as an official Sea to Summit manufacturer part number. Retailer-level code seen: SEAT0055 (BlackOvis). Bass Pro manufacturer model #: A3208. Research date: 2026-09-20. All prices USD, observed through indexed retail sources on that date — re-verify live before purchase; indexed crawl ages vary.

1. One-line summary

The most famous bag-booster in camping: a stretchy Thermolite Pro mummy liner Sea to Summit claims adds up to 25°F (14°C) of warmth — the standard answer to "how do I push my 20°F bag into the teens without buying a winter bag."

2. 2026 lineup note

Current as of 2026-09-20. The Reactor family: standard Reactor (+14°F claim, lighter) and Reactor Extreme (+25°F claim, 110 g/m² fabric) — the Extreme is the warmest knit liner Sea to Summit makes. Also offered in Compact and Long sizes. Sibling-track items (Sea to Summit Flame II bag, also new in this catalog) are covered separately; the liner integrates with Sea to Summit's Free-Flow Zip footboxes.

3. Lineage note (2023–2026 changes; buying old/clearance/used)

• The Reactor Extreme is a long-running product (originally plain Thermolite, +25°F claim since launch).
• ~2024–2025: Thermolite Pro update — hollow-core fibers plus infrared-absorbing ceramic pigments, HeiQ Fresh odor control, EcoMade recycled content (50% Thermolite Pro / 50% Thermolite EcoMade yarn per current retail copy). The old claim structure shifted from pure temperature claims to ASTM-tested thermal-boost percentages (+7% for 3-season bags, +15% for 1-season bags, per Ascent Outdoors' ASTM D1518-2014 reference). Exact year of the update is UNVERIFIED — treat as approximate.
• 2025–2026: no verified redesign; street price ~$85.
• Buying old/clearance/used: older Thermolite (non-Pro) liners are functionally similar — fine at a discount. Liners are intimate gear; buy used only if you can hot-wash it. Check the drawcord footbox and shoulder openings for stretched-out elastic.

4. Detailed specifications (Standard)

Weight conflict note: listings show 342 g (Sojourn/locally), 364 g (Tom's AU), 376.4 g (Bushcraft Equipment AU "Standard Drawcord"), and 399 g/14 oz (Bass Pro/BlackOvis). The AU "Standard" may be a larger cut. Treat trail weight as ~12–14 oz and confirm the size.

5. Pricing in USD (observed 2026-09-20, not checkout-verified)

• Realistic street: ~$85; older $70-era pricing is gone. The $114.99 listing looks anomalous.
• MAP status: UNVERIFIED for this product.

6. What's in the box (and what's needed)

• Included: the liner + its own Ultra-Sil stuff sack.
• Nothing else needed. Machine washable with no special soap (avoid fabric softeners and dryer sheets).

7. Warranty

• "Sea to Summit Guarantee is offered for the Lifetime of this product" (per Sojourn Outdoors' listing of the current generation).
• UNVERIFIED: exact current official terms, exclusions, and proof-of-purchase requirements. Check seatosummit.com before relying on this.

8. Support & parts

• UNVERIFIED: Sea to Summit's official repair program and spare-parts availability (replacement stuff sacks, drawcords) for the Reactor Extreme as of Sept 2026. Liners are simple enough that field repair is rarely needed.

9. Community verdict (aggregated, sourced, balanced)

• Why people buy it: it's the default liner recommendation across Reddit, OutdoorGearLab (long-time top pick), and guide services — the "+25°F" claim is the most quoted number in sleep-system discussions. Owners praise the stretch knit (no twisted-liner wrestling match), the footbox you can open to walk around camp, and the bag-hygiene benefit (wash the liner, not the $500 down bag). As a standalone summer bag it genuinely works into the 60s°F.
• Honest negatives:
  • The +25°F claim is widely considered optimistic — experienced users report a realistic boost of 10–15°F, and even that depends on the host bag. Treat the claim as marketing headroom.
  • 12–14 oz is heavy for what it is — UL hikers often skip it in favor of wearing their puffy to bed.
  • The mummy cut with drawcords can feel fiddly; some users prefer a simple rectangular liner.
  • At ~$85 it's the priciest liner in its class — the standard Reactor or a Cocoon (also in this catalog) cost less.
  • Adds warmth but also adds condensation risk inside the bag for sweaty sleepers.
• Bottom line: the category-defining bag booster — bought by 3-season campers stretching one bag across shoulder seasons and by anyone who wants to wash something other than their down bag. Discount the warmth claim by half and it's still worth it.

10. Images

1. [research workspace] — details in [internal image manifest — not shipped]
2. [research workspace] — details in [internal image manifest — not shipped]
3. [research workspace] — details in [internal image manifest — not shipped]

• [research workspace] (2560×1571 JPEG) — labeled Reactor Extreme stuff sack in the snow, packed angle. Source: The Good Ride Reactor Extreme review (https://thegoodride.com/snowboard-accessory-reviews/sea-to-summit-reactor-extreme-liner-review-by-steph/)
• [research workspace] (1500×1500 JPEG) — hood drawcord and press-stud detail with S2S tag. Source: Bergfreunde Reactor Extreme listing (https://www.bergfreunde.es/sea-to-summit-reactor-extreme-sleeping-bag-liner-mummy-saco-de-dormir-de-viaje/)
• [research workspace] (645×611 JPEG) — retail packaging of Reactor Extreme liner. Source: ShortGirlOnTour Everest packing list (https://shortgirlontour.com/everest-base-camp-packing-list/)

11. Sources

• https://www.basspro.com/shop/en/sea-to-summit-reactor-extreme-mummy-sleeping-bag-liner-3825924 ($84.99, 12.8 oz, specs)
• https://sojournoutdoors.com/collections/sleeping-bags/products/sea-liner-reactor-ex-mummy-com (Thermolite Pro detail, 342 g, lifetime guarantee)
• https://www.locally.com/product/1816018/sea-to-summit-reactor-extreme-sleeping-bag-liner ($47.97–$50.97 stale MSRP, feature list)
• https://ascentoutdoors.com/products/sea-to-summit-reactor-extreme-sleeping-bag-liner ($84.95, ASTM thermal-boost percentages)
• https://www.blackovis.com/sea-to-summit-thermolite-reactor-extreme-liner (older Thermolite generation specs, $56.21)
• https://www.overtons.com/sea-to-summit-reactor-extreme-thermolite-sleeping-bag-liner-211382.html ($69.99 older evidence)

Long-term durability & warranty-claim reality

Knit liners age gracefully: the Thermolite Pro knit resists pilling better than fleece, and owners report multi-year service with regular washing. The drawcord footbox elastic is the first thing to tire — a cheap fix. HeiQ Fresh odor control washes out gradually (all such treatments do). The main "failure" is thermal: as the knit compresses permanently with use and washing, the real-world warmth boost declines — which is why the +25°F claim was optimistic even when new. No pattern of seam failures surfaced in this pass.

Head-to-head vs closest rivals

• vs Cocoon CoolMax TravelSheet (catalog): the Cocoon is the summer liner (10.8 oz, +~9°F, $60, rectangular, moisture-wicking); the Reactor Extreme is the winter liner (+25°F claimed, mummy, $85). They don't compete — they cover opposite ends. Own the Cocoon for hostels and summer, the Reactor for shoulder season.
• vs Sea to Summit standard Reactor (same brand): the standard Reactor claims +14°F at lower weight and cost — the honest choice if you only need a modest boost. The Extreme is for genuine cold.
• vs wearing your puffy to bed: the UL counter-argument — free, no extra weight, but less comfortable and no hygiene benefit. Liner loyalists value the washability.

Care & maintenance

Machine wash warm with regular detergent — no special soap needed. Avoid fabric softeners and dryer sheets (they coat the fibers and reduce wicking). Tumble dry low or line dry. The knit stretches when wet — dry flat if possible to preserve shape.

Buying traps

• The +25°F claim: real-world boost is closer to 10–15°F — size your system on the conservative number.
• Standard vs Compact vs Long: three sizes exist — confirm before ordering; the AU "Standard" 376 g listing suggests regional sizing differences.
• Old Thermolite (non-Pro) stock: fine at a discount, but the current Pro generation is the $85 product — don't pay full price for old inventory.
• The $114.99 listing: anomalous — street is ~$85.

Sources

• thegoodride.com — retrieval date not recorded
• www.bergfreunde.es — retrieval date not recorded
• shortgirlontour.com — retrieval date not recorded
• www.basspro.com — retrieval date not recorded
• sojournoutdoors.com — retrieval date not recorded
• www.locally.com — retrieval date not recorded
• ascentoutdoors.com — retrieval date not recorded
• www.blackovis.com — retrieval date not recorded
• www.overtons.com — retrieval date not recorded